Background

In this document, the term “ventilation” refers to the flow of outside air indoors, which is always accompanied by an equal flow of indoor air outdoors. Ventilation removes and dilutes indoor airborne pollutants, brings outdoor air pollutants into buildings, and often removes or supplies heat and water vapor. Ventilation is also needed to maintain oxygen concentrations inside buildings, although the quantity of ventilation needed to supply oxygen is very small relative to other ventilation requirements.

Increasing the rate of ventilation generally leads to overall improvements in indoor air quality; however, the indoor concentrations of some pollutants from outdoors, such as outdoor particles and ozone, can increase with the ventilation rate. Indoor humidity can increase or decrease with ventilation rate. When it is cold and dry outdoors, increased ventilation usually reduces the indoor humidity.

While increased ventilation rates are usually considered beneficial for health and for improving perceived air quality (e.g., odors), ventilation air must often be heated (and sometimes humidified) or cooled and dehumidified. Consequently, the ventilation rates selected for buildings must strike a balance between the benefits of energy savings with reduced ventilation and the known or suspected benefits to health with increased ventilation.

Several metrics are used to specify the rates of building ventilation. Generally, these metrics are flow rates of outside air normalized by the number of occupants, floor area, or indoor volume. Corresponding units of ventilation rates include the following: liters per second per person (L s^–1 per person); liters per second per square meter of floor area (L s^–1 per square meter); and air changes per hour (h^–1).

Municipalities typically adopt one of the several building design codes used in the United States. These codes, or state energy codes, include building design provisions intended to maintain ventilation rates above a minimum rate that varies with the building type. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) publishes a minimum ventilation standard that is the basis for the ventilation specifications in many codes. The current version of the ASHRAE standard is Standard 62-1999—Ventilation for Acceptable Indoor Air Quality (ASHRAE, 1999). Standard 62-1999 lists 0.35 h^–1 as a minimum ventilation rate in residences,³ 10 L s^–1 per person (20 cubic feet per minute [cfm] per person) as a minimum ventilation rate in offices, and 8 L s^–1 per person (15 cfm per person) as a minimum ventilation rate in schools. Due to a paucity of scientific data on the relationship of building ventilation rates with the health and well-being of occupants (Seppanen et al., 1999), the minimum ventilation rates in the ASHRAE standard are based substantially on professional judgment and on studies performed in laboratories with conditions quite different from those encountered in real buildings.

Building design codes and ASHRAE's minimum ventilation standard do not ensure that all buildings maintain the specified minimum ventilation rates. In most states and municipalities, there are no legal requirements to actually maintain ventilation rates at or above the levels in building design codes. Additionally, building ventilation rates are difficult to measure accurately, infrequently measured, and as discussed later, poorly controlled.

Ventilation systems, although intended to remove indoor pollutants, can also become sources of pollutants. Portions of ventilation systems, particularly components that become wet, can become colonized by microorganisms and produce bioaerosols that are transported by the airflow to the occupied space. In addition, particles, fibers, and odorous and potentially irritating volatile organic compounds (VOCs) may be emitted from synthetic materials, including fibrous insulation materials, from residual oils used in component production, from deposited dusts, and from microorganisms. Ventilation also affects the indoor humidity which in turn influences the growth or survival of microorganisms within buildings.

Heating, ventilating, and air conditioning (HVAC) is the more general process of thermally conditioning and ventilating buildings. In commercial buildings, these functions are usually integrated. The HVAC process employed in commercial buildings is reviewed here because HVAC features may influence exposure to pollutants that are known or thought to be associated with asthma.

Methods and Rates of Ventilation in U.S. Single-Family Residences

Diamond (1999) has summarized many of the basic physical characteristics of the U.S. residential building stock. In 1997, detached single-family units and row houses constituted 73% of the U.S. housing stock, 6% of the housing stock was mobile homes, and the remainder was apartments. The average heated floor space in all U.S. housing stock was 181 m² (1,950 square feet) and air conditioning was installed in 70% of these dwellings. The average conditioned floor area of mobile homes was 87 m² (940 square feet) and 70% of mobile homes had air conditioners. Fourteen percent of all housing units used humidifiers, and nine percent had dehumidifiers.

When windows are closed, the ventilation of single-family residences in the United States is almost exclusively an uncontrolled process. In air infiltration (or infiltration and exfiltration), air leaks through unintentional cracks and holes in the building envelope. The infiltration rate is driven by small pressure differences across the building envelope that are typically less than a few pascals in magnitude. These pressure differences arise due to the differences between the indoor and outdoor air temperatures, resulting in different indoor and outdoor air densities, and also as a consequence of wind. Unintentional air leakage in the ductwork of forced-air heating and air-conditioning systems located in attics and crawl spaces, also causes large increases in air infiltration. Even if the ducts do not leak, forced-air systems can pressurize or depressurize specific rooms relative to the outdoor pressure, forcing air leakage through the building envelope.

U.S. homes often have intermittently-operated exhaust fans in bathrooms and kitchens. When operated, these fans draw outdoor air into the building. Window and door opening by occupants, predominantly during mild weather, also has a large influence on residential ventilation rates.

A very small portion of single-family dwellings in the United States have mechanical ventilation systems (i.e., fans operating continuously or intermittently to provide ventilation). Mechanical ventilation is most common in the State of Washington because the state energy code now requires mechanical ventilation. The technologies used to mechanically ventilate residences are described in Roberson et al. (1998).

Ventilation rates in residences vary considerably over time. The lowest ventilation rates occur during mild weather with windows and doors closed. When weather is more severe, windows remain closed but ventilation rates are higher due to increased indoor-to-outdoor temperature differences and increased use of forced-air heating and air conditioning. The highest ventilation rates generally occur when windows or doors are open.

Present data on ventilation rates in U.S. single-family residences are limited and possibly not representative of the building stock. One source of information is measurements of the airtightness of building envelopes with windows and doors closed. Ventilation rates are predicted with semiempirical models, using measured values of building airtightness⁴ combined with climate data and indicators of a building's shielding from wind, as model inputs. When annual average ventilation rates are desired, the predictions may also include terms to account for natural ventilation via windows; however, the current knowledge of window use and effects on ventilation is cursory. The second source of information on residential ventilation rates is measurements made using a tracer-gas procedure. Although considered more accurate than predictions based on airtightness, the measured data are more sparse than airtightness data. Therefore, we presently have only crude estimates of residential ventilation rates.

Based on airtightness and climate data for about 12,000 houses, Sherman and Matson (1997) estimate that the arithmetic average effective ventilation rate of houses in the United States is 1.1 h^–1. This average reflects ventilation rates when windows are closed and also the higher ventilation rates that occur with open windows during mild weather. Airtightness normalized by house size is highly variable (Sherman and Dickerhoff, 1994), with a standard deviation that is approximately 50% of the mean. The mean of the airtightness data from individual states varies among states by more than a factor of three. In the available data, there is no trend in airtightness with severity of climate. The available data indicated that houses constructed after 1980 are more airtight (by ~50%) than older houses (Sherman and Dickerhoff, 1994); however, there was no trend evident in airtightness with age for houses constructed after 1980.

A set of 2,844 measurements of residential ventilation rates in U.S. houses was analyzed by Murry and Burmaster (1995). The measured data from 66 research projects are not from a representative sample of residences; however, this analysis is probably the best available information on the distribution of ventilation rates in U.S. houses. When considering all climate zones and seasons, the arithmetic and geometric mean ventilation rates were 0.76 h^–1 and 0.53 h^–1, with a geometric standard deviation of 2.3. There are large variations in ventilation rates with season and climate zone. The winter and summer arithmetic means, for all climate zones, are 0.55 and 1.50 h^–1. Approximately one-third of the measurements in the winter season are less than the 0.35 h^–1, the rate in the current ASHRAE ventilation standard. In the coldest climate zone, approximately 55% of the measured ventilation rates, from all seasons, are less than 0.35 h^–1.

Methods, Patterns, and Rates of Ventilation in U.S. Multifamily Apartment Buildings

In 1997, 21% of U.S. housing units were apartments. The average conditioned floor area of apartments was 85 m² (920 square feet) and air conditioning was installed in 65% of apartments (Diamond, 1999). Published information on the methods and rates of ventilation in multifamily apartment buildings are extremely sparse. Based on the limited information available,⁵ older lowrise (i.e., less than ~three stories) apartment buildings usually have no mechanical supply of ventilation air. Much like single-family dwellings, these buildings are ventilated primarily by uncontrolled infiltration and natural ventilation windows that can be opened. Intermittently operated bathroom and kitchen exhaust fans cause temporary increases in ventilation rates. Leakage in the ductwork of forced-air heating and air-conditioning systems and pressurization or depressurization of individual rooms can drive infiltration and exfiltration in apartments, just as it does in single-family houses. Newer low-rise apartment buildings are ventilated similarly to older low-rise buildings; however, a larger portion of these buildings have continuous mechanical exhaust ventilation from the bathrooms and/or kitchens of each apartment.

Older apartment buildings with more than approximately three stories typically have no mechanical air supply or some mechanical supply to the interior hallways. The air supply system, when present, is frequently not functional (Shapiro-Baruch, 1993). Apartments within these buildings sometimes have a system for continuous exhaust ventilation from bathrooms and kitchens, although it is not always operational. Some portion of these older high-rise buildings have a vertical ventilation shaft that functions much like a chimney and passively draws air from the apartments.

In new apartment buildings with more than three stories, exhaust ventilation is usually drawn continuously from the kitchen and bathroom(s) of each apartment. The exhaust fans may serve groups of apartments or individual apartments. Outside air enters either from unintentional leaks and vents at windows or via ventilation systems that supply air continuously to each apartment. When a mechanical air supply is present, often this air is supplied to a single room of each apartment from a duct system in the building's interior hallway.

The airflow in heated multistory apartment buildings without mechanical ventilation often occurs in an upward direction from lower-level to upper-level apartments (e.g., Diamond et al., 1986; Modera et al., 1986). Cool outdoor air leaks into the lower apartments; flows upward, picking up moisture and pollutants; and exfiltrates through the walls and ceilings of upper-level apartments. Due to this airflow pattern, the lower-level apartments tend to have more fresh air supply, lower humidity, and more cold drafts. Humidity and pollutant levels are often increased in upper-level apartments because a portion of the air entering these apartments comes from lower levels of the building. As moist air exfiltrates out of the upper-level apartments, water vapor may condense within cold walls and ceilings. Possibly, the higher pollutant levels and humidity in upper-level apartments could contribute to asthma symptoms.

This same upward-flow phenomenon occurs to a variable degree in all heated multistory buildings. When the building is air conditioned (i.e., cooled), the airflow direction can reverse; however, the downward airflow in air-conditioned buildings will be less pronounced because the indoor-to-outdoor temperature differences are typically much smaller during air conditioning than during heating of buildings. By reducing the openings between floors, the vertical airflow between floors can be reduced. Mechanical ventilation can also reduce or overwhelm the upward buoyancy-driven airflow.

In addition to the buoyancy-driven upward airflow in apartments, other unintentional flows of air between adjacent apartments are reported commonly from case studies. These flows occur through unintentional openings in walls and floors, and may be driven by mechanical ventilation systems, buoyancy, and wind.

The vertical and horizontal airflows between apartments transport pollutants; therefore, occupants may be exposed to pollutants such as tobacco smoke, cooking odors, and pet allergens from the apartments of neighbors. These airflows also transport indoor moisture, potentially causing moisture problems.

The ASHRAE ventilation standard has the same minimum ventilation requirements for multifamily and single-family residential buildings, 0.35 h^–1, with exhaust capacity required in kitchens and bathrooms. Measurements of ventilation rates in apartments are extremely limited. Based on case studies in a small number of buildings, the reported ventilation rates are 0.5–1.5 h^–1 for low rise apartments with frame or brick construction and 0.2– 1.0 h^–1 for high-rise apartments with a more airtight concrete construction (Diamond et al., 1999). Based on the available information, we can expect ventilation rates to vary a great deal with time, among apartment buildings, and among apartments within a building.

Heating, Ventilating, and Air Conditioning in U.S. Commercial and Institutional Buildings

Ventilation Rates in Commercial and Institutional Buildings

Commercial and institutional buildings are ventilated mechanically with the HVAC system, via air infiltration and through the openable windows present in 40% of the commercial floor space. The ventilation provided through each of these mechanisms varies over time. In warm climates, the mechanical supply of outside air is minimized when the outside air exceeds approximately 20–24^oC, hence, the building may be operated with a minimum ventilation rate for much of the cooling season. Minimum ventilation is also provided during cold weather when the outside air temperature falls below a set point that varies among buildings. During periods of minimum ventilation, concentrations of indoor-generated pollutants will tend to be highest. During mild weather in the spring and fall, the indoor-to-outdoor concentration ratio for some outdoor air pollutants, such as outdoor particles (e.g., molds) and ozone, will tend to be at a maximum.

Ventilation rate data from United States commercial and institutional buildings are very limited and come primarily from convenience samples of buildings and modest-size research projects. Consequently, only very rough estimates can be provided on the distribution of ventilation rates in U.S. commercial buildings. Table 10-2 provides summary information from the largest data sets. The measured ventilation rates vary over a large range and usually exceed the minimum ventilation rates specified in ASHRAE Standard 62-1989. However, the ventilation rates measured when the HVAC systems provided the minimum amount of outside air were less than the corresponding minimum rate specified in the current ASHRAE standard⁶ in a majority of buildings (i.e., in 8 of 13 buildings studied by Turk et al., 1989; 20 of 49 buildings [including 13 out of 14 schools] studied by Lagus Applied Technologies, 1995; and 8 of 8 buildings studied by Persily and Grot, 1985).

TABLE 10-2

Summary Information from the Three Largest Surveys of Ventilation Rates in U.S. Commercial Buildings.

In commercial and institutional buildings that are not occupied continuously, HVAC systems are often not operated when the building is vacant. The concentrations of indoor-generated air pollutants may increase when the building is vacant and there is no mechanical ventilation, and then decrease over a period of a few hours after mechanical ventilation starts. Therefore, occupants' exposures to indoor air pollutants depend on the operating schedule for the HVAC system.

Even less is known about the proportion of commercial building ventilation that results from air infiltration; however, the existing data suggest that infiltration is appreciable, particularly in the smaller buildings (Lagus Applied Technologies, 1995; Persily, 1999). The ratio of infiltration to total ventilation may be important for asthma because the air that infiltrates a building is not filtered to remove outdoor particles before it enters the building.

Pollutant Sources in HVAC Systems

HVAC systems can become sources of indoor air pollutants, possibly increasing asthma symptoms. Pollutant emissions from certain types of HVAC systems constitute one of several potential explanations for the consistent association between the type of HVAC system and the prevalence of nonspecific health symptoms (called sick building syndrome) experienced by office workers (e.g., Mendell, 1993). In particular, almost all studies have found a statistically significant increase in symptom prevalence among occupants of office buildings with mechanical ventilation and air conditioning (Mendell, 1993) relative to occupants in naturally ventilated offices. There is also some evidence that humidifiers are associated with increased symptoms (Mendell, 1993). The subjectively assessed (i.e., via questionnaire) nonspecific health symptoms in these studies usually include a few symptoms potentially indicative of asthma, such as wheezing, tight chest, and difficulty breathing. Therefore, the evidence that HVAC systems can become sources of pollutants is relevant for asthma. The available information about pollutant sources in HVAC systems is quite limited and comes predominately from commercial buildings; however, similar pollutant sources, or risk factors for sources, may be present in HVAC systems of other types of buildings. This section provides a brief review of the issue.

Liquid water is often present at several locations in or near commercial building HVAC systems, facilitating the growth of microorganisms that may contribute to asthma. The outdoor air is often drawn from the rooftop or from a below-grade “well” where water (and organic debris) may accumulate. Raindrops, snow, or fog can be drawn into HVAC systems with the incoming outside air, although the systems are usually designed to prevent or limit this moisture penetration. Moving along the supply airstream in the direction of airflow leads to the cooling coil where moisture condenses and ideally drips from the surfaces of the coil into a drain pan with a drainage pipe connected to the sewer or to outdoors. Frequently, drain pans contain stagnant water because they do not slope toward the drain line. The drain also may be plugged or nonfunctional because air pressure differences prevent drainage, sometimes causing the drain pan to overflow with water. If the velocity of air passing through the cooling coils is too high, drops of water on the surface of the cooling coil can become entrained in the supply airstream and deposit in the HVAC system downstream of the cooling coil. Air exiting cooling coils is frequently nearly saturated with water vapor, and the high humidity of this air increases the risk of microbiological growth. The HVAC system may have a humidifier that uses steam or some evaporation process to add moisture. Humidifiers, used predominantly in the colder climates, may have reservoirs of water, surfaces that are frequently wetted, or water drops that do not evaporate. Thus, there are many sources of liquid water and very high humidity in HVAC systems.

Microbiological contamination of HVAC systems has been reported in many case studies and investigated in a few multibuilding research efforts (e.g., Batterman and Burge, 1995; Bencko et al., 1993; Martiny et al., 1994; Morey, 1994; Morey and Williams, 1991; Shaughnessy et al., 1998). Sites of reported microbiological contamination include the outside air louvers, the mixing box where outside air mixes with recirculated air, filters, cooling coils, cooling coil drain pans, humidifiers, and the surfaces of ducts. The porous insulating and sound-adsorbing material called duct liner, which is used inside some HVAC systems may be particularly prone to contamination (Morey, 1988; Morey and Williams, 1991). Supporting the evidence of microbiological contamination from field studies are numerous laboratory-based studies demonstrating that fungi can grow on various HVAC materials at a wide range of temperatures and humidities. The flow of air through HVAC systems to the occupied spaces is a means of transporting bioaerosols from contaminated sites inside HVAC systems to occupants. Thus, microbiological contamination of HVAC systems is theoretically a risk factor for asthma. However, relatively little is known about the extent to which microbiological contaminants within HVAC systems actually increase bioaerosol exposures or affect asthma symptoms.

HVAC systems can also be sources of other pollutants such as fibers, nonbiological particles, and VOCs. Sources of these pollutants inside HVAC systems include duct liners, gaskets, oil left on surfaces after manufacturing, dust deposited on surfaces including dusts generated during building construction and renovation, and the wear of fan belts. It is clear that HVAC systems and components, particularly dirty filters and the oily surfaces of ducts, can release odorous compounds that significantly degrade the perceived acceptability of the air supplied to the occupied space (e.g., Pasanen et al., 1995; Pejtersen, 1996; Seppanen, 1998); however, the relevance of these pollutants for asthma is not known.

Causes of HVAC Problems

The rates of ventilation provided by HVAC systems and the contamination of HVAC systems with pollutants depend as much on system construction, maintenance, and operation practices as on system design. Proper operation and maintenance of HVAC systems is hindered by system complexity, inaccessibility of components, and lack of training of construction and maintenance staff. The absence of economic incentives for proper HVAC operation and maintenance may be even more important. When buildings are leased, their owners and operators are usually unaffected economically by the adverse health of building occupants, unless the health problems are severe and clearly linked to the building. In speculative new construction, the designer is similarly unaffected. However, these designers, owners, and operators have an incentive to reduce the highly tangible costs of building construction, maintenance, and operation.

Costs of Ventilation and Associated Carbon Dioxide Emissions

Ventilation is one of the most energy-intensive methods of reducing indoor pollutant concentrations primarily because of the need to thermally condition ventilation air. For several reasons, including great uncertainty about average ventilation rates in buildings, the available estimates of the energy used for ventilation are relatively crude. Orme (1998) estimates that ventilation accounts for about 30% of the energy used in U.S. residential buildings. A rough estimate of the average annual cost per residence of energy for ventilation is $400.⁷ In the U.S. service sector (e.g., commercial, institutional, and government buildings), the estimated energy consumed for ventilation (Orme, 1998) is approximately one-quarter of total service-sector building energy use. The cost of energy for ventilating commercial buildings depends highly on climate, building size, occupant density, and type of HVAC system. If the average ventilation rate is 10 L s^–1 per person, Emmerich and Persily (1998) have estimated that 46% of the total heating and cooling load in the stock of office buildings is attributable to ventilation. With nearly 50% of the U.S. work force, or 57 million workers, in offices (U.S. Department of Commerce, 1997; Table 645) and average energy prices, the annual cost of ventilation per office worker is roughly $25.⁸

Carbon dioxide, a greenhouse gas, and other air pollutants are generated in the production of the energy used for ventilation. The annual CO₂ emissions attributed to ventilation are approximately 1000 and 800 million tons for the residential and service sectors, respectively (Orme, 1998).

Influence of Ventilation Rates on Indoor Concentrations of Pollutants

Direct Impacts

Measured data quantifying the influence of ventilation rates on indoor concentrations of indoor-generated pollutants are surprisingly limited, particularly for pollutants associated with asthma. Data from surveys with ventilation rate and pollutant concentration measurements have emphasized measurements of pollutants that are generated indoors and also present in outdoor air—reducing the influence of ventilation rates on indoor concentrations. None of the large cross-sectional surveys have monitored concentrations of indoor-generated particles associated with asthma, such as environmental tobacco smoke (ETS) particles and indoor-generated bioaerosols. Indoor particle mass concentrations have been measured in some surveys, but these measurements reflect both indoor-generated particles and particles from outdoors. In a study of 150 residences located in Riverside, California (Özkaynak et al., 1996), there were statistically significant, but small, increases in indoor particle concentrations with increased ventilation rates. The values of PM₁₀ and PM_2.5 increased about 12 and 5 µg m^–3, respectively,⁹ for a 1 h^–1 increase in ventilation rate.

In data from surveys, there is sometimes no clear association of indoor pollutant concentrations with ventilation rates. In a survey of 26 houses near Spokane, Washington, and 35 houses near Portland, Oregon, the indoor formaldehyde concentration decreased 40 and 300 parts per billion (ppb), respectively, per 1 h^–1 increase in ventilation rate (Turk et al., 1987b). However, in surveys of 38 commercial buildings in the U.S. Pacific Northwest, there were no clear associations of ventilation rates with indoor concentration of respirable particles, nitrogen dioxide, or formaldehyde (Turk et al., 1987a, 1989). Apparently, variations in indoor pollutant emission rates and other factors are large enough to obscure the effects of ventilation in a survey of this size.

Few experiments have been performed with changes in ventilation rates as the only intervention. Again, most of the experimental data involve pollutants generated indoors but also present in outdoor air. Offermann et al. (1982) used mechanical ventilation systems to increase wintertime ventilation rates in nine houses from an average of 0.35 to 0.83 h^–1. The average indoor formaldehyde¹⁰ concentration decreased 21% and average indoor relative humidity decreased 13%. Indoor nitrogen dioxide concentrations increased slightly because concentrations were higher outdoors than indoors. In a set of two houses, inhalable particle concentrations decreased 30%. Using a mechanical ventilation system with a filter, Turk et al. (1997) increased ventilation rates in two classrooms from about 0.7 to 2–3 h^–1 and indoor particle concentrations decreased by approximately 50%. Berglund et al. (1982) increased the percentage of outside air in the air supply of an office building from 20 to 50%, and then to 80%, and their measure of total indoor-generated VOCs in detector-response units decreased from 120 to 50 and then to ~20. Menzies et al. (1993) increased ventilation rates in a set of four office buildings and found that formaldehyde and total VOC concentrations were 40 and 60% lower, respectively, during periods with a higher ventilation rate. Hodgson et al. (1999) measured concentrations of 24 VOCs in a house with ventilation rates of 0.14 and 0.32 h^–1. At the higher ventilation rate, on average for the 24 compounds the indoor concentrations was 42% lower, with a standard deviation of 11%.

Some experimental studies did not detect clear reductions in indoor pollutant concentrations when ventilation rates were increased. Shaughnessy et al. (1997) increased ventilation rates from ~ 0.2 h^–1 to 3 h^–1 in one elementary classroom and from ~ 0.7 h^–1 to 3 h^–1 in another classroom. There were no clear changes in indoor PM₁₀ concentrations reportedly because of large natural temporal fluctuations. Indoor total volatile organic compound (TVOC) concentrations decreased by about a factor of two, but natural temporal variations precluded firm conclusions about the effects of ventilation. Nagda et al. (1990) increased ventilation rates in an office building from 0.84 h^–1 to 1.08 h^–1 and indoor concentrations of formaldehyde, nicotine, respirable particles, and carbon monoxide did not change significantly.

Neither the cross-sectional nor experimental data provide us with a clear indication of the influence of ventilation rates on indoor concentrations of indoor-generated pollutants. The cross-sectional findings are subject to many confounders. Experimental data are complicated by temporal variations in indoor pollutant emissions. Neither type of study has focused on pollutants with only indoor sources. Therefore, Equation A1 in Appendix A has been used to provide theoretical predictions for particles of various sizes as well as for some types of gaseous pollutants.¹¹ For predictions, ventilation rates have been varied from 0.2 to 4 h^–1, and 0.75 h^–1 has been used as a reference point for which the concentration is arbitrarily set equal to one. The low end of this range (0.2 h^–1) is equivalent to the lowest ventilation rates commonly reported in buildings. Because of energy costs and equipment requirements, increasing ventilation rates to greater than 4 h^–1 during periods of heating or cooling seems unlikely except in spaces with a high occupant density such as classrooms. A ventilation rate of 4 h^–1 is a factor of five greater than the arithmetic mean ventilation rate of residences reported by Murry and Burmaster (1995) and corresponds approximately to the total rate of supply of recirculated plus outdoor air in U.S. commercial buildings. The reference ventilation rate of 0.75 h^–1 is typical of the ventilation rates in residences (Murry and Burmaster, 1995) and also roughly equivalent to the minimum ventilation rates in schools and commercial buildings (Table 10-2).

The influence of ventilation rate on pollutant concentrations depends on the magnitude of pollutant removal by air cleaning. Figure 10-1 provides some predicted relationships for a space with no air cleaning. In this figure, relative concentrations are used, which are defined as the actual concentration divided by the concentration at the reference ventilation rate of 0.75 h^–1. One clear observation is that the relative concentrations of some indoor-generated pollutants increase dramatically as ventilation rates become unusually low (e.g., below a few tenths of an air change per hour). Because ventilation rates are poorly controlled in buildings, a portion of the building stock will have these low ventilation rates. The airflow requirements and associated energy penalties of avoiding these particularly low ventilation rates are modest, but low ventilation rates cannot be prevented with current ventilation technologies and processes (e.g., natural infiltration) that control ventilation rates poorly.

FIGURE 10-1

Predicted trends in the relative concentrations of indoor-generated pollutants with ventilation rate.

As illustrated in Figure 10-1, the predicted change in pollutant concentrations with ventilation rate is greatest for an “ideal” gaseous pollutant that is not removed by deposition or sorption on surfaces and that has an emission rate unchanged by ventilation rate. In this highly ideal case, often used to illustrate the effects of ventilation, the relative concentration in Figure 10-1 decreases from 3.0 to 0.2 as the ventilation rate increases from 0.2 to 4 h^–1. For real pollutants, ventilation rates have a smaller predicted impact on concentrations—in many cases, a much smaller impact. The range in predicted relative concentrations of nitrogen dioxide¹² is 1.6 to 0.3—much less than the range for the ideal pollutant because of losses of NO₂ via sorption on surfaces. If outdoor NO₂ concentrations are significant relative to those indoors, ventilation rates will cause an even smaller change in indoor NO₂ concentrations. For indoor-generated VOCs, the dependence of indoor concentration on ventilation rates is more difficult to predict and will vary among compounds, but concentrations will usually be less affected by ventilation rate than the ideal gaseous pollutant.

For indoor-generated particles, the predicted effects of ventilation rate depend highly on particle size because the depositional losses of particles increase dramatically with particle size. Practical changes in ventilation rates have a substantial predicted impact on indoor concentrations of small particles, such as ETS particles and the portion of airborne cat allergens associated with particles smaller than a couple micrometers. Ventilation rates are not likely to have an appreciable direct¹³ impact on indoor concentrations of the larger particles associated with dust mite and cockroach allergens and many indoor-generated fungal spores. For example, in Figure 10-1 the predicted range in relative concentrations of 10-µm indoor-generated particles is only 1.14 to 0.57 as the ventilation rates varies from 0.2 to 4 h^–1. If there is some form of air cleaning within the building, ventilation rates will change indoor pollutant concentrations by a smaller amount.

The previous discussion has focused on indoor-generated air pollutants. Outdoor pollutants are not substantially addressed in this document; however, the influence of ventilation rate on indoor concentrations of outdoor pollutants is a relevant issue for asthma. Increasing ventilation rates will increase the rates of entry of outdoor pollutants into buildings. For pollutants that are not removed indoors by deposition on surfaces or air cleaning (e.g., outdoor carbon monoxide), a change in ventilation rate will not substantially affect the time-average indoor concentration, although peak concentrations may be affected. However, an increase in ventilation rate will increase the indoor concentrations of other outdoor pollutants such as particles and ozone from outdoor air.

Indirect Impacts

Ventilation rates may have a very significant indirect impact on indoor concentrations of some pollutants because they affect indoor humidities, which in turn modify indoor pollutant sources. Potentially most important for asthma is the influence of indoor humidity on indoor dust mites, molds, and bacteria.

Two humidity parameters are necessary to characterize the relationship between indoor and outdoor humidity. The humidity ratio is the mass of water vapor divided by the mass of dry air. The relative humidity (RH) is the partial pressure of water vapor divided by the partial pressure of water vapor in air at the same temperature that is saturated with water vapor. The amount of water vapor required to saturate air decreases with decreasing air temperature; hence, 0^oC outdoor air with a 100% RH will have a low humidity ratio relative to room-temperature indoor air with a RH of 50%.

Due to indoor moisture generation, the humidity ratio indoors will exceed that outdoors,¹⁴ unless the indoor air or incoming outside air is dehumidified. In cold winter climates, outdoor air is very dry and ventilation rates have a pronounced impact on humidities indoors, with the humidity decreasing as the ventilation rate increases. In warm humid climates, air-conditioning systems must remove moisture from the indoor air to maintain acceptable indoor relative humidities. Increasing the ventilation rate in warm humid climates will increase indoor humidity ratios and relative humidities unless the rate of water removal by the air-conditioning system increases accordingly. This relationship between ventilation rate and humidity places constraints on the ventilation rates in buildings. Particularly high ventilation rates during cold weather causes indoor RHs to decrease to a point (less than ~25%) that causes discomfort and dryness symptoms. Particularly high ventilation rates in warm humid weather over-load the moisture-removal capabilities of typical HVAC systems, leading to excessive indoor relative humidities.

Recognizing the relationship of ventilation rate to RH, a few studies have investigated whether indoor dust mite levels or allergy symptoms in dwellings can be controlled by increasing ventilation rates with mechanical ventilation systems (Table 10-3). In cold climates, mechanical ventilation appears to be associated with decreases in indoor humidity and dust mite levels (Emenius et al., 1998; Harving et al., 1994) or allergic symptoms (Aberg et al., 1996). In more humid climates, the results are mixed. Mechanical ventilation was associated with significant reductions of dust mite levels in the small study by McIntyre (1992) in the United Kingdom. However, Fletcher et al. (1996) did not measure significant reductions in dust mites in England and Aberg et al. (1996) did not find significant reductions in allergic symptoms in the more humid areas of Sweden.

TABLE 10-3

Investigations of the Association of Residential Mechanical Ventilation Systems with Indoor Humidities, Dust Mite Levels, and Allergy Symptoms.

Summary

Ventilation rates are poorly controlled in residential and commercial buildings. Measured data indicate that ventilation rates vary widely among buildings of the same class (e.g., residences, schools, offices) and that rates of ventilation are frequently below or well above the rates specified in the current ASHRAE ventilation standard. Insufficient data are available for conclusions about changes in building ventilation rates during the past few decades; however, there are some indications that ventilation rates have decreased.

Concentrations of indoor-generated air pollutants associated with asthma may be influenced by ventilation systems via three primary mechanisms. First, changes in the rate of ventilation modify indoor concentrations of indoor-generated air pollutants by changing the rates of pollutant removal and dilution with outdoor air. Measured data and model predictions indicate that the changes in indoor pollutant concentrations associated with the typical range of indoor ventilation rates vary dramatically among pollutants. Because of the sparseness of empirical data, model predictions are the best available means of estimating the typical changes in indoor pollutant concentrations that result from changes in ventilation rates. However, there are considerable uncertainties in the applicability of these predictions to actual complex indoor environments, with rates of pollutant loss by deposition on indoor surfaces being one of the largest sources of uncertainty.

Based on these predictions, the indoor concentrations of some pollutants increase dramatically as ventilation rates become unusually low (e.g., <0.25 h^–1). The energy costs associated with avoiding these particularly low ventilation rates are modest; however, current methods and technologies of ventilation do not consistently prevent these low ventilation rates.

Increases in ventilation rates are one obvious approach for reducing exposures to some pollutants associated with asthma. Increasing ventilation rates from a typical value (0.75 h^–1) to a high value (4 h^–1) should decrease concentrations of indoor-generated particles 1 µm or smaller by up to 80%. Indoor-generated particles in this size range that are relevant for asthma include those associated with ETS, portions of airborne cat allergen, and most of the droplet nuclei produced during coughing and sneezing. Some grass and birch allergens from outdoors are also smaller than 1 µm. As particle size increases, changes in ventilation rates have a diminishing predicted impact on indoor concentrations. For particles 10 µm in size, increasing the ventilation rate from 0.75 to 4 h^–1 would be expected to decrease indoor concentrations by about 50%. For 20- and 30-µm particles, an even smaller change in concentration with ventilation rate is expected. The limited data available indicates that much of the airborne dust mite and cockroach allergen is associated with particles that are 10 µm or larger in size.

The impact of ventilation rates on concentrations of gaseous pollutants associated with asthma will generally be modest. Without a strong indoor source of nitrogen dioxide, outdoor NO₂ concentrations are often higher than indoor concentrations, and increases in ventilation rates will increase the concentrations indoors. Even when strong indoor sources are present, such that the outdoor concentration is negligible, practical changes in ventilation rates should change indoor nitrogen dioxide concentrations by 50% less. The influence of ventilation rates on indoor concentrations of indoor-generated VOCs will vary among compounds and, for many compounds, is not easily modeled with existing data. In general, concentrations of the more volatile compounds are more strongly affected by ventilation rates.

Overall, these predictions and available data suggest that large increases in ventilation rates would be most effective in reducing exposures to ETS, cat allergens, infectious droplet nuclei, and some volatile organic compounds. Such large increases in ventilation rates, applied broadly, would substantially increase building energy use and energy costs and would significantly increase emissions of the greenhouse gas carbon dioxide. Unless ventilation systems with heat recovery were used, increasing ventilation rates to ~4 h^–1 in U.S. residences would typically increase annual energy costs from several hundred to more than a thousand dollars. When possible, eliminating or reducing indoor sources of these pollutants would be a much more effective method of reducing exposures and generally would not increase building energy use.

Microorganisms and other pollutants that are potentially relevant for asthma can contaminate HVAC systems and be transported via the system to occupied spaces. There are many sources of liquid water in HVAC systems, as well as regions with very humid air, that facilitate the growth of microorganisms. The contamination of HVAC systems with microorganisms and other pollutants is one hypothesized explanation for the consistent findings that office workers in buildings served by HVAC systems with air conditioning have more nonspecific health symptoms than occupants of naturally ventilated office buildings. However, the relevance of this association to asthma remains unclear. Preventing HVAC system contamination requires improvements in system design, construction, and maintenance.

Ventilation rates also affect indoor humidities, in turn, influencing indoor levels of dust mites. Higher indoor humidities may also increase indoor molds and bacteria. When it is cold and dry outdoors, increases in ventilation decrease the indoor humidity. When it is warm and humid outdoors, increases in ventilation rate tend to increase the humidity in air-conditioned buildings. The limited data available suggest that using mechanical ventilation systems to increase ventilation rates in residences can result in significantly lower dust mite levels only in cold climates.

Most U.S. houses and many schools rely for ventilation on air leakage through unplanned openings in the building envelope plus natural ventilation through windows and doors. Increased window opening will increase ventilation rates, but not in a controlled or predictable manner. Mechanical ventilation systems would have to be installed in these buildings to achieve controlled increases in ventilation rate. Mechanical ventilation systems are commercially available, but they are often expensive (hundreds of dollars to more than a thousand dollars) and unfamiliar to many homeowners.

Several health effects other than asthma, including nonspecific irritation symptoms, allergies, and communicable respiratory illnesses, are potentially influenced by ventilation rates and ventilation system contamination. All of these health outcomes have to be considered when policies and education programs about ventilation are established.

Conclusions Regarding the Relationship of Ventilation to Asthma

Existing data are inadequate for conclusions regarding the association between ventilation rates or ventilation system microbiological contamination and either the exacerbation of asthma symptoms or asthma development. However, there are both theoretical evidence and limited empirical data indicating that feasible modifications in ventilation rates can decrease or increase indoor concentrations of some of the indoor-generated pollutants associated with asthma by up to 75%.

Research Needs

Additional research is needed on ventilation, but only a small proportion of these research needs are critical to advancing our understanding of the relationship of ventilation to asthma. At the present time, our understanding of the influence of changes in ventilation rates on concentrations of (or exposures to) indoor-generated pollutants associated with asthma is very limited—accordingly, model predictions that have not been adequately evaluated are the best source of information. Experiments in actual buildings, with manipulation of ventilation rates, are the preferred approach for quantifying the direct (i.e., via pollutant removal) influence of changes in ventilation rates on the indoor concentrations of these pollutants. Because indoor pollutant source strengths can vary temporally, experiments should be repeated several times. To assess how changes in ventilation rates affect indoor humidities and, in turn, the proliferation of dust mites and molds in buildings will require either long-term experiments lasting a year or more or large cross-sectional studies with control for confounding factors.

Airtight building envelopes and low rates of ventilation have been cited as factors that may contribute to asthma incidence or symptoms or may explain recent increases in asthma; however, very few relevant data are available. The evidence of a linkage of ventilation rates with asthma is not sufficient to justify large studies intended to resolve only this issue. However, measurements of ventilation rates should be included, when possible, in future asthma case-control studies or cross-sectional surveys. Ventilation measurements in houses can be performed using nonobtrusive tracer-gas methods with passive tracer-gas emitters and samplers (Dietz and Cote, 1982; Stymne and Eliasson, 1991).

Finally, research is needed to advance our very limited current knowledge about microbiological contamination of HVAC systems, its influence on microbial exposures, and its influence on asthma development or asthma symptoms.

Background

Because many of the indoor pollutants associated with asthma are airborne particles, particle air cleaning is considered a potentially beneficial technology for the prevention of asthma or asthma symptoms. Particle air cleaning is any process used intentionally to remove particles from the indoor air. Filtration and electronic air cleaning are the two most common examples. Natural deposition of particles on indoor surfaces, ventilation, and measures that reduce indoor particle emission rates are not considered particle air cleaning.

The magnitude of the reduction in indoor particle concentrations accomplished with particle air cleaning depends on the air cleaner's particle removal rate relative to the particle removal rate by all other processes. The rate of particle removal by an air cleaner varies with particle size and equals the flow rate of air through the air cleaner (Q_ac) multiplied by the air cleaner's particle removal efficiency ε. The product of Q_ac and ε integrated over particle size for a specific type of particle source is sometimes called the clean air delivery rate (CADR). Based on standard test protocols, the American Home Appliance Manufacturers provides CADRs for tobacco smoke, dust, and pollen for many portable air cleaners.

Although particle air cleaning reduces indoor particle concentrations, microorganisms can grow on some air-cleaning equipment such as filter media; thus, air cleaners are also a potential source of indoor pollutants.

Particle Air-Cleaning Technologies

By far, the most common method of air cleaning is to circulate air through a fibrous filter. The filtration media is usually a mat of thin fibers, often glass fibers. The efficiency of the filter is a function of fiber diameter, fiber packing density (e.g., distance between fibers), thickness of the media, velocity of the air as it passes through the media, particle size, and other factors. A pleated (i.e., folded) filter media is often employed to increase the media surface area, reducing the air velocity in the media, and the resistance to airflow through the media. A wide range of filter products are available, with a correspondingly wide range of efficiencies and prices. Based primarily on data from Hanley et al. (1994) and data from manufacturers, Figure 10-2 provides examples of particle removal efficiency versus particle size for filters with a range of efficiency ratings, using an ASHRAE rating method (ASHRAE, 1992). At one extreme are the coarse panel filters, usually called furnace filters, commonly used in residential forced-air furnace systems. This filter has a negligible efficiency for particles smaller than ~0.5 µm and a low efficiency (e.g., < 20%) for particles smaller than 10 µm. The other extreme is a high-efficiency particle air (HEPA) filter, which has a minimum efficiency of 99.97 % for 0.3-µm particles and a higher efficiency for smaller and larger particles. For indoor applications, the efficiency of a HEPA filter is effectively 100% at all particle sizes if all air that flows through the air cleaner actually passes through the filter.

FIGURE 10-2

Efficiency curves for filters and an electrostatic precipitator.

In addition to fibrous filters, a wide range of electronic air cleaners are available. Electrostatic precipitators first produce ions that attach to and electrically charge particles entering the air cleaner. These charged particles then pass through an electric field where they migrate to a surface and attach. The collection surfaces must be cleaned or replaced periodically. An example of an efficiency curve for an electrostatic precipitator is also provided in Figure 10-2. An ion generator is another device that produces ions that attach to and charge particles, but ion generators often have no particle collection surfaces. Instead, ion generators may increase the rate of particle deposition on normal indoor surfaces. A variety of hybrid technologies employ both fibrous filters and particle charging or fibrous filters and electric fields. Electronic air cleaners can produce ozone, sometimes in significant quantities (U.S. EPA, 1999; Viner et al., 1989). Many units have a charcoal filter to remove the ozone produced by the air cleaner.

The efficiency data in Figure 10-2 are from tests with all air passing through a previously unused filter. In practical installations, a portion of the air bypasses the filter, flowing between adjacent filters or between filters and their housing, causing the air cleaner's particle removal efficiency to be lower than the efficiency of the filter media. In commercial installations, gaps of a few centimeters between adjacent filters, even missing filters, are common. Systems of higher-efficiency filters usually have fewer gaps and more gaskets to reduce the quantity of air that bypasses the filter media. Almost no information is available on typical bypass rates. As filters accumulate deposited particles, their efficiency generally increases (sometimes markedly); however, the resistance to airflow through the filter also increases.

The resistance to airflow as air passes through an air cleaner is a common concern of engineers, building operators, and HVAC equipment providers. If airflow rates are constant, more powerful fans and increased fan energy are required when the air pressure drop increases. More efficient fibrous air filters tend to have a greater pressure drop. However, the pressure drop of high-efficiency filters can be diminished by increasing the degree of pleating of the filter matt, which usually increases both the thickness of the filter in the direction of airflow and the filter cost. Additionally, a larger number of filters in parallel can be used to reduce the air velocity and pressure drop. Thus, pressure drop does not necessarily increase with increased filter efficiency; however, the cost of, and space required for, the filters increases unless a greater pressure drop is accepted. Electronic air cleaners tend to have smaller pressure drops than fibrous filters with the same particle removal efficiency.

There are two basic installation options for particle air cleaners. First, air cleaners can be installed within existing HVAC systems and rely on the normal fan-driven airflow through the HVAC system to force air containing particles through the air cleaner. Air drawn from the occupied indoor space flows though a duct system, through the air cleaner, and then back to the occupied space. With this type of installation, often called “in-duct” air cleaning, the rate of air cleaning is limited by the rate of airflow through the HVAC system, and air cleaning occurs only when the HVAC fan operates. For example, an air cleaner in a residential furnace system cleans air only when the residence is heated, unless the furnace fan is forced to operate at other times. One option for improving filtration in buildings is to increase the efficiency of the in-duct filters or air cleaners. The particle removal rate increases only for particles that are removed with a higher efficiency after the filtration upgrade. Hence, substitution of a high-efficiency in-duct air cleaner for a lower-efficiency device may dramatically increase the particle removal rate for small particles but have a small or negligible influence on the removal rate for large particles.

The second option is a supplemental air cleaner with an integral fan that forces airflow through the air-cleaning devices. Normally, these are portable devices placed on the floor, designed to clean the air predominantly in a single room. The addition of portable air cleaners to a building with an in-duct air cleaner increases the total flow rate of filtered air. Portable air cleaners are often a significant source of noise. In some studies, occupants have turned air cleaners off because this noise is annoying. The air flow rate of some portable air cleaners is too small for meaningful air cleaning within a room or house.

Typical Particle Air-Cleaning Practices in U.S. Buildings

In U.S. single-family homes with forced-air heating or air-conditioning systems, a filter designed to remove coarse particles is generally installed in the stream of air circulated through the heating or air-conditioning system. Usually this is a coarse panel filter, such as the furnace filter with an efficiency versus particle size as depicted in Figure 10-2. Recently, filters and electronic air cleaners with a higher removal efficiency for small particles have been designed to replace the standard furnace filter. Standard furnace filters cost only a few dollars or less and should be replaced a few times per year. Higher-efficiency replacements for furnace filters often cost between $10 and $20 per filter.

An unknown proportion of residential heating and cooling systems have an additional in-duct particle air cleaner within the recirculated airstream, often an electronic air cleaner. The price of supplemental in-duct air cleaners for residential applications varies widely (e.g., $500 to $800) among products. The cost of electricity used to operate the electronic components of the air-cleaning system is usually insignificant¹⁵ (e.g., $25 per year). If the recirculation fan is operated only during heating or air conditioning, there is no significant incremental cost for fan energy. Occasionally, home owners will operate the HVAC fan continuously in order to have continuous air cleaning. If 4,380 incremental hours of fan operation (50% of the year), a fan power of 300 W, and an electricity prices of $ 0.098 per kilowatt-hour (kWh) are assumed, the annual incremental fan energy cost would be $130. The rates of airflow through these air cleaners correspond to the rates of airflow through the residential heating and air-conditioning system, with about four indoor air volumes per hour being typical.

In U.S. commercial and institutional buildings with HVAC systems, the system invariably contains a filter in the supply airstream (i.e., in the mixture of outside air and recirculated indoor air).¹⁶ A typical supply air filter has an efficiency rating of roughly 40% (Figure 10-2);¹⁷ however, the efficiency rating for supply air filters varies widely among buildings. Historically, the rationale for these supply filters has been to reduce the deposition of large particles on the heat-transfer equipment inside HVAC systems. In the past few years, more attention has been directed toward filtration to protect human health. The cost of filters in commercial buildings varies with the product used. Using cost estimates from Burroughs (1997) for filters with a 30% ASHRAE efficiency rating, annual costs per person for a typical level of air filtration in a commercial building are on $4 to $8.¹⁸ Fisk and Rosenfeld (1997) estimated that the annual incremental cost of using very high efficiency filters in an office building was $24 per person. ¹⁹

In a typical commercial HVAC system, the filter is upstream of many of the HVAC components, including the cooling coils, coil drain pans, humidifiers (when present), and sections of duct work that can become contaminated with microorganisms. Thus, the supply filters do not prevent particles released from these components from entering the occupied spaces of the building.

Supplemental portable air cleaners are not standard equipment in any particular type of building; however, they are widely available and used in many buildings, particularly residences. Although a very large range of products is available, many of the heavily marketed products incorporate HEPA filters and a multispeed fan, with airflow rates at maximum fan speed ranging from 50 to 200 L s^–1. This range of flow rates corresponds to 8 to 32 room air volumes per hour in a small 22-m³ bedroom and to 0.4 to 1.6 house volumes per hour in a 450-m³ house. In practice, units may often be operated with less than maximum airflow or they may be unused, for example, because they are noisy. These portable air cleaners are sized primarily for cleaning the air in single rooms. When the door to the room is open or a forced-air heating or cooling system operates, room air will often mix with air throughout the building and the air-cleaning system must have a larger capacity to obtain the same reduction of particle concentrations within the room. The retail cost of portable air cleaners varies widely. As an example, the retail cost of one of the most commonly used HEPA room filter units with a maximum flow rate of 165 L s^–1 is about $250 ($1.50 per L s^–1 maximum airflow).²⁰ Periodic replacement of pre-filters and HEPA filters in this unit would cost about $70 annually. At maximum fan speed, this unit consumes 350 W of electrical power, with an annual electrical cost of ~$300 if operated continuously. If a product life of four years is assumed, the annualized cost is roughly $400 ($2.4 per L s^–1 of maximum airflow), with 75% of this cost associated with electricity to operate the fan. The annualized costs per unit airflow of other commercially available products could be considerably higher or lower.

Higher-capacity supplemental air cleaning units, sometimes mounted at ceiling level, are also readily available. These devices are intended primarily for use in health care facilities, smoking rooms, and restaurants, but they could be used in residences. Prices vary widely, and in many instances, there is no obvious relationship between product price and published specifications.

Air Cleaner Standards

Standard test procedures for assessing the efficiency of particle air cleaners are available from ASHRAE (1992). The most commonly cited current test methods yields an “ASHRAE dustspot efficiency,” hereafter called an ASHRAE efficiency, but this standard does not provide an efficiency rating versus particle size. Hanley et al. (1994) have provided efficiency curves for typical products. Air cleaner manufacturers also often provide size-dependent efficiency data upon request.

At present, there are no standards that specify the minimum allowable filter efficiency in HVAC systems in U.S. buildings. A proposed revision of the ASHRAE minimum ventilation standard includes a minimum efficiency specification.

Predicted and Measured Influence of Particle Air Cleaning on Indoor Concentrations of Indoor-Generated Particles of Various Sizes

Measurements

Figure 10-3 presents the results of experimental studies of air cleaners that specify both the rate of airflow through the air cleaner and the reduction in indoor pollutant concentration. Some of the studies summarized later in Tables 10-4 and 10-5 also provide a measured reduction in indoor pollutant concentration but no rate of airflow through the air cleaner. The studies specifying the rate of air cleaning are listed at the top of each table in order of decreasing air cleaner flow rate.²¹ Most experimental studies have used portable air cleaners with HEPA filters in rooms of homes. Ten studies provide some information on the decrease in airborne allergen or particle concentrations, usually within the room containing the air cleaner, associated with air cleaner operation.²² Six of these studies reported large or statistically significant decreases in airborne particles or allergens. The reported percentage reductions in particles or allergens in five studies ranged from 30 to 90% and averaged 60%. Overall, these data indicate that air cleaners can significantly reduce airborne allergens or particles in some applications. However, these findings should be interpreted with caution. Temporal variations in indoor allergen production or resuspension rates and in outdoor allergen concentrations make accurate experimental assessments of the effects of air cleaning quite difficult. Most of the experimental studies have been short term, thereby increasing the potential errors from natural fluctuations in allergen sources. The studies of de Blay et al. (1991) on the effects of air cleaning on airborne cat allergen concentrations serve as an example of changes in allergen sources. The influence of air cleaner use on airborne cat allergen concentration varied widely (Figure 10-3), with large reductions occurring only if the room was previously vacuum cleaned, presumably because the resuspension of allergen from indoor surfaces was enhanced by the higher indoor air velocities that occur with air cleaning.

FIGURE 10-3

Measured reductions in indoor pollutant concentrations with air cleaner operation. NOTE: Solid data points are for pollutants that are generated only indoors. SOURCES: Cat allergen—Wood et al. (1998) and Blay et al. (1991); all pollens and mold (more...)

TABLE 10-4

Influence of Air Cleaner Use on Indoor Allergens and Allergy or Asthma Symptoms of Subjects with Perennial Allergic Disease.

TABLE 10-5

Influence of Air Cleaner Use on Indoor Allergens and Allergy or Asthma Symptoms in Subjects with Seasonal Allergic Disease.

A few of the experimental studies provide information on the type of air cleaner needed to reduce concentrations of various types of pollutants. Van der Heide et al. (1997) used multistage filter systems and found that most of the dust mite antigen was removed by the coarse filters—indicating that HEPA filters are unnecessary for the large particles associated with dust mite antigen. Miller-Leiden et al. (1996) determined that HEPA filter units did not perform significantly differently from non-HEPA units for particles with a mass median diameter of 0.7 µm. Offermann et al. (1991) evaluated the effectiveness of six types of air cleaners installed in a forced-air furnace duct system for removal of environmental tobacco smoke. The highest rates of ETS removal occurred using a filter with an ASHRAE efficiency rating of 95%, followed by an electrostatic precipitator, and then by a HEPA filter. The HEPA filter had a higher airflow resistance and thus, a lower airflow rate and smaller particle removal rate.

Predictions

The influence of various particle air-cleaning options on concentration of indoor-generated particles was predicted using the steady-state mass balance equation and assumptions provided in Appendix A. Figure 10-4 provides predictions of the percentage reductions in indoor particle concentrations versus particle size for various air filtration systems installed within the forced-air heating and air-conditioning system of a house. Figure 10-5 provides predictions of the effects of using portable filter units in isolated bedrooms or entire houses. All predictions are for spaces with perfectly mixed air.

FIGURE 10-4

Predicted reduction in indoor-generated particles in a house with use of various filters in the forced-air heating system if a recirculation flow of four house volumes per hour and a ventilation rate of 0.75 h–1 are assumed.

FIGURE 10-5

Predicted reduction in indoor-generated particles from operation of portable air cleaners in an isolated bedroom or an entire house, where a ventilation rate of 0.75 h^–1 is assumed. NOTE: Isolated bedroom: bedroom door closed and no air circulation (more...)

Several observations based on these predictions follow. First, to obtain a substantial reduction in indoor concentrations of 10-µm particles, the rate of airflow through the air cleaner per unit of indoor air volume must be high. Even with 10 room volumes per hour through a filter with a 100% efficiency for 10-µm particles, the predicted reduction in concentration is only 70%. The effectiveness of air cleaning would diminish rapidly with increases in particle size above 10 µm because of increases in gravitational settling rates with particle size. Thus, air cleaning does not appear to be a very attractive option for reducing exposures to dust mite allergens, which predominantly contain particles larger than 10 µm. The second observation is that increasing the filter efficiency rating from a furnace filter to an ASHRAE 95% efficiency filter, while maintaining a constant rate of airflow through the filter, decreases the predicted indoor concentrations by approximately 40% or less for particles 5 µm or larger. An equivalent reduction in concentrations of particles within this size range is obtained using an ASHRAE 65% filter in place of the ASHRAE 95% filter. Thus, for many of the bioaerosols associated with asthma, high-efficiency filters, which are more expensive and often require larger and noisier fans, are not likely to be superior to moderateefficiency filters. Third, it appears feasible to reduce concentrations of particles smaller than 2 µm, such as ETS particles, droplet nuclei, and the smaller particles of cat allergen, by 70% or more using air cleaners with a moderate to high efficiency rating and a flow rate of several indoor air volumes per hour.

Figure 10-6 provides predicted reductions in indoor-generated particle concentrations from the use of typical filters and higher-efficiency filters in a commercial and institutional building HVAC system. These upgrades could substantially reduce concentrations of submicron particles but have virtually no effect on particles larger than a few micrometers.

FIGURE 10-6

Predicted reduction in indoor-generated particles from air filtration in a commercial building HVAC system with a recirculation rate of three indoor volumes per hour and a ventilation rate of 0.75 h^–1.

Portable air cleaners could be used to reduce concentrations of the larger-size indoor-generated particles in the air within commercial buildings. As in residences, high rates of airflow through the filters would be necessary to obtain substantial percentage reductions in indoor concentrations. Use of portable air cleaners in isolated individual offices of asthmatics is not likely to be effective unless the office door is kept closed, and even with closed doors the reductions in indoor particle concentrations may be quite small.

Effects of Particle Air Cleaning on Allergy and Asthma Symptoms

The influence of air cleaner use on asthma and allergy outcomes has been evaluated in many experimental studies, and was the subject of a 1997 review by the American Lung Association (ALA, 1997). Building on the review articles of Nelson et al. (1988) and de Blay et al. (1997a), these studies are summarized in Tables 10-4 and 10-5 for subjects with perennial and seasonal symptoms, respectively.²³

Many of these experimental studies have important limitations, including a very small number of subjects (e.g., <15), lack of blinding, low or unspecified rates of air cleaning, no information on building ventilation rates, virtually no specification of relevant buildings characteristics,²⁴ short-term study periods, and reliance on self-reports of symptoms. Many studies assessed changes in allergy symptoms rather than clear indications of asthma symptoms. Some studies selected subjects who were allergic to dust mites, and as discussed above, air cleaning is unlikely to be highly effective in reducing exposures to the large particles that carry dust mite allergens. A minority of the studies quantified the reduction in airborne allergen levels. Many studies reported whether changes in outcomes were significant but did not provide relative risks or odds ratios. Thus, the results of these studies are suggestive, rather than a basis for firm conclusions.

Most of the studies involving subjects with perennial allergic or asthma symptoms (Table 10-4) used portable air cleaners, most often portable HEPA filter units, in bedrooms. Large reductions in concentrations of allergens within these bedrooms would not be expected unless bedroom doors were kept closed and forced-air heating or cooling systems did not mix air between the bedrooms and the remainder of the house. Excluding three studies with filtered air delivery over the bed, only 4 of 11 studies reported improvements in symptoms or reduced use of medication, and in 3 of these studies the subjects were not blinded. In one of the four studies (Reisman et al., 1990), symptoms and medication use improved significantly only during the period when patients had no respiratory illnesses. In another of the four studies, only air cleaners used in combination with impermeable mattress covers were associated with significant improvements in airway hyperresponsiveness and eosinophils.²⁵

Three experimental studies involving subjects with perennial allergic or asthma symptoms (bottom of Table 10–4) used aircleaning devices that supplied filtered air over the bed. All three studies reported improvements in outcomes, suggesting that sup-plying cleaned air to the breathing zone may be more effective than attempting to clean the air in entire rooms or buildings.

The results of studies involving subjects with seasonal allergic or asthma symptoms are much more encouraging. Excluding the two studies using only air conditioning as a form of air cleaning, six of seven studies reported improvements in symptoms, and the seventh study showed a borderline significant improvement (p = 0.07). However, subjects were blinded in only two of these studies, and most studies were old and without formal statistical tests.

One additional experimental study (Bascom et al., 1996) performed in an experimental chamber (not included in the tables) assessed the reduction in acute symptoms from ETS exposure when particle air cleaners are used to reduce concentrations of ETS particles. With a 50% reduction in particle concentrations, some symptoms (headache, eye irritation, rhinorrhea) and the minimum cross-sectional area of the nasal passage improved. The relevance of these findings for asthma is not clear.

Overall, these data suggest that air cleaners are probably helpful in some situations in reducing allergy or asthma symptoms, but air cleaning, as applied in the studies, is not consistently and highly effective in reducing symptoms. It is conceivable that a more consistent or larger reduction in symptoms could be obtained using higher rates of air cleaning. The available data provide no information regarding the effects of air cleaning on the development of asthma or the development of sensitization to allergens.

Air Conditioning as a Substitute for Air Cleaning

Previous reviews of air cleaning and asthma have included studies of air conditioning. Air conditioners can remove some particles from the indoor air because they often contain coarse particle filters and because some particles may be removed from the air along with water in the air conditioner's cooling coil. Consequently, Table 10-5 also summarizes the results of two investigations in the 1930s that evaluated the effects of air conditioning on symptoms. One study demonstrated that the use of an air conditioner and closing windows resulted in a large reduction in indoor pollens. Symptoms improved in one of two subjects. In the second study the air conditioners contained filters, and marked improvements in asthma symptoms were reported for 9 of 10 subjects. Neither study was blind, and neither performed a statistical test to assess the significance of the change in outcomes.

Air conditioners are not designed to remove particles from air. Air conditioning will, in general, be less effective than conventional air cleaning in reducing exposures to indoor-generated particles. However, air conditioners enable the occupants of buildings to keep windows and doors closed during warm weather, greatly reducing the rate of entry of pollens and other outdoor pollutants into buildings. Additionally, air conditioners reduce indoor temperatures and humidities that may influence asthma symptoms.

Home owners and medical doctors sometimes consider residential air conditioning a form of ventilation. In the general mode of operation, residential air conditioners do not provide ventilation—they simply remove heat and moisture (and, to some degree, particles) from a stream of recirculated indoor air. However, to save energy, some modern air conditioners will stop the recirculation and cooling of indoor air and instead provide ventilation (i.e., supply outdoor air to the building) when the outdoor air is relatively cool and suitable for cooling the house.

Summary and Discussion of Limitations of Assessment

Many of the indoor pollutants associated with asthma are airborne particles; thus, particle air cleaning has been considered a potentially beneficial technology for the prevention of asthma or asthma symptoms. Technologies for particle air cleaning are well developed. Air filters with a moderate to high efficiency for particles larger than approximately 2 µm are used routinely in the heating and air-conditioning systems of buildings.

The magnitude of the reduction in indoor-generated particle concentrations accomplished with particle air cleaning depends on the air cleaner's particle removal rate relative to the particle removal rate by all other processes including ventilation and particle deposition on surfaces. The rate of particle removal by an air cleaner varies with particle size and is proportional to the flow rate of air through the air cleaner multiplied by the air cleaner's size-dependent particle removal efficiency. The two primary air cleaning options for reducing indoor particle concentrations are to replace the existing filters in heating and air-conditioning systems with higher-efficiency filters and to operate supplemental air cleaners with integral fans in the occupied space.

In field studies, enhanced air cleaning has been associated with reductions in airborne particle concentrations that range from negligible to more than 90%. For the airborne particles associated with asthma, the published data are very limited. Simple model predictions indicate that substantial reductions in indoor concentrations of 10-µm particles can be obtained only when the rate of airflow through the air cleaner per unit of indoor air volume is large, for example, 10 room volumes per hour or more. The predicted effectiveness of air cleaning diminishes rapidly with increases in particle size above 10 µm because gravitational settling rates increase with particle size. Thus, air cleaning does not appear to be an attractive option for reducing exposures to dust mite allergen, which predominantly involves particles larger than 10 µm. However, based on predictions it is feasible to reduce concentrations of particles smaller than 2 µm, such as ETS particles, droplet nuclei, and smaller particles with cat allergen, by 70% or more using air cleaners with a moderate to high efficiency rating and a flow rate of several indoor air volumes per hour.

Both the available experimental data and model predictions indicate that HEPA filters, which are more expensive and often require larger and noisier fans, are not likely to be superior to lower-efficiency filters in reducing concentrations of many of the bioaerosols associated with asthma. Even for submicron-size ETS particles, available data indicate that HEPA filters are not necessarily the preferred option. Thus, the very common recommendation that HEPA filtration, in contrast to lower-efficiency air cleaning, be used by allergic and asthmatic individuals when they choose to employ air cleaning, is not supported by either experiments or theoretical predictions. Unfortunately, the limited performance data available for many non-HEPA residential air cleaners make it difficult to provide alternate recommendations.

The influence of air cleaner use on asthma and allergy outcomes has been evaluated in numerous experimental studies; however, most of these studies have important limitations. Overall, the data suggest that air cleaners are helpful in some situations in reducing allergy or asthma symptoms, particularly seasonal symptoms, but it is clear that air cleaning, as applied in the studies, is not consistently and highly effective in reducing symptoms. The available data provide no information regarding the effects of air cleaning on the development of asthma or the development of sensitization to allergens.

Conclusion Regarding Air Cleaning and Asthma

There is limited or suggestive evidence that particle air cleaning is associated with a reduction in the exacerbation of asthma symptoms. There is insufficient evidence to determine whether or not the use of particle air cleaners is associated with decreased asthma development. Theoretical and limited empirical data suggest that air cleaners are most likely to be effective in reducing the indoor concentrations of particles smaller than approximately 2 µm. Much of the airborne allergen appears to be within larger particles. Relevant particles smaller than 2 µm include environmental tobacco smoke particles, significant portions of airborne cat, grass, and birch allergen, and virus-containing droplet nuclei from coughs and sneezes.

Research Needs Related to Air Cleaning and Asthma

The results of existing experimental studies are inadequate to draw firm conclusions regarding the benefits of air cleaning for asthmatic and allergic individuals. Many of the existing studies have important limitations, such as small study size, lack of blinding, a small or undefined rate of air cleaning, placebo air cleaners that may significantly remove the larger particles associated with asthma, and no exposure assessment or inadequate assessment. Additional research to assess the benefits of air cleaning is clearly warranted, but future studies must overcome as many of these limitations as possible. Because air cleaning is most promising for reducing indoor concentrations of particles smaller than a couple of micrometers, future research should emphasize these agents.

Sensitization to allergens—a critical step in the development of allergic asthma—often occurs early in life. No information is available to indicate whether air cleaning of spaces occupied early in life can reduce the rate of allergic sensitization. Research is needed to address this issue.

As described in Appendix A, particles larger than a few micrometers have a complex and inadequately understood behavior in the indoor environment, including rapid rates of gravitational settling, resuspension from surfaces, and possibly incomplete mixing with the indoor air. Consequently, the influence of air cleaning systems on exposures to particles in this size range is not well understood and the associated benefits from air-cleaning cannot be predicted with a high degree of confidence. A combination of aerosol science and air-cleaning research is needed to fill this gap in our knowledge.

The limited data on the size distribution of many of the bioaerosols and allergens associated with asthma limit our understanding of the benefits of air cleaning. Additional data are needed particularly for pet allergens and pollens.

As stated earlier, HEPA filter units have been widely recommended for allergy and asthma patients who desire to use air cleaners. Air cleaner manufacturers have responded by aggressively marketing air cleaners with HEPA filters and offering few other products. However, experimental data and theoretical predictions indicate that air cleaners with a lower efficiency rating are likely to be equally effective in reducing the concentrations of most, and perhaps all, of the indoor-generated particles associated with allergies and asthma. These lower-efficiency air cleaners could have a lower product cost, less powerful or noisy fans, higher rates of airflow and particle removal, and reduced energy consumption. The scientific and medical community should develop revised recommendations regarding the selection of air cleaners by allergic and asthmatic individuals, and air cleaner manufacturers should respond by providing new air-cleaning products.