Ground-level ozone standards, 1979-2015

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Ground-level ozone standards are part of the National Ambient Air Quality Standards (NAAQS) established under the Clean Air Act. The Clean Air Act requires the U.S. Environmental Protection Agency (EPA) to set national air quality standards for six pollutants, including ground-level ozone (also known as smog). Ground-level ozone is formed when nitrogen oxide and volatile organic compounds react in sunlight. In October 2015, the EPA lowered the maximum amount of ground-level ozone from 75 parts per billion (ppb) to 70 ppb. The ozone standard was last lowered in 2008.

Background

Ozone is a colorless, inorganic gas that exists at ground level and in the Earth’s upper atmosphere. Naturally occurring ozone in the stratosphere (a layer from six to 30 miles above the Earth’s surface) shields the Earth from the sun’s ultraviolet radiation. Ground-level ozone is formed in the troposphere (the lowest layer of the Earth’s atmosphere) when nitrogen oxide and volatile organic compounds react in sunlight to produce smog, which may damage crops and vegetation. Smog can also cause health problems depending on concentration levels of ozone, duration of exposure, the average amount of air breathed per minute, and the length of time between short-term exposure to ozone. Ground-level ozone concentrations in general peak during the afternoon, and average concentrations are typically highest during summer months.

There are natural and human-caused contributors to ground-level ozone. Volatile organic compounds (VOCs) are carbon-containing reactive compounds that are emitted naturally by plants and vegetation and from the burning of gasoline, coal, natural gas, and wood as well as solvents, paints, glues, and other household products. Natural sources of nitrogen oxides, which are contributors to ground-level ozone, include anaerobic plant respiration, lightning, and volcanic activity. Human-caused sources of VOCs and nitrogen oxides include motor vehicles, chemical and petroleum facilities, dry cleaners, electric power plants, and other industries.

National ozone standards

The Clean Air Act established nationwide air quality standards for six air pollutants known as criteria pollutants: ground-level ozone, sulfur dioxide, particulate matter, nitrogen oxide, carbon monoxide, and lead. These standards, known as National Ambient Air Quality Standards (NAAQS), set ceilings for the six air pollutants, and states enact state implementation plans outlining enforceable, source-specific emissions limits for these pollutants. Each state plan must show that the state will meet and maintain the NAAQS.

In setting NAAQS, the EPA sets two standards for criteria pollutants—primary and secondary standards. Primary standards limit pollution to protect human health, and secondary standards limit pollution to protect against visibility impairment and damage to animals, vegetation, and buildings. If a geographical area exceeds the NAAQS for one or more of the six criteria pollutants, the area is considered by the EPA as a nonattainment area. Geographical areas with pollutant concentrations below the NAAQS are known as attainment areas.

In 1979, the EPA set the primary and secondary ozone standards at 120 parts per billion (ppb). The standards establish the maximum hourly average of ground-level ozone in a geographical area. Areas were considered in compliance with the standards if they have one day or less per calendar year of ozone concentrations greater than 120 ppb. In 1997, the EPA replaced the one-hour standards with eight-hour standards and set the maximum 8-hour average of ground-level ozone at 80 ppb. The standards were lowered to 75 ppb in 2008.

2015 standard

See also: Implementation of the Clean Air Act

In October 2015, the EPA lowered national ground-level ozone standards from 75 parts per billion (ppb) to 70 ppb. The EPA and other proponents of the standard argued that the 75 ppb standard was inadequate to reduce the long- and short-term risks of ozone exposure in children, older adults, and individuals with asthma or lung disease. The agency concluded in 2015 that the standard should be lowered to 70 ppb to reduce the effects of short-term ozone exposure in the above individuals—such as decreased lung function and respiratory inflammation that may lead to emergency room visits or missed days of work or school.

Proponents of the 2015 standard argue that the 2008 standard of 75 parts per billion was outdated and in need of an update to reduce smog levels. Proponents argued that recent clinical studies had justified lowering the standard to 70 parts per billion in order to protect public health and reduce negative health effects in healthy adults. Further, proponents argue that the standard would reduce premature deaths, asthma attacks in children and older adults, and missed days of school and work.

Opponents of the 2015 standard argue that the 2008 standard is adequate to reduce the negative effects of ozone exposure. In addition, opponents argue that the costs of the 2015 standard are greater than its benefits because the EPA included reductions in particle pollution in addition to a reduction in ozone to calculate health benefits. As a result, opponents argue that the EPA already regulates particle pollution through the NAAQS for particulate matter and that few benefits would result specifically from further reductions in ozone.

Implementation delay (June 2017)

In June 2017, the EPA announced it would delay for one year the implementation of 70 parts per billion (ppb) standard. EPA Administrator Scott Pruitt (R) notified state governors that the EPA will determine which areas and regions do not meet the 2015 ozone standard by October 2018. Under the original implementation plan, the EPA was slated to determine nonattainment areas in June 2017 and to finalize its decisions in October 2017. In a statement, Pruitt said that the delay would allow the EPA to work with states and localities to implement the standard in a way that improves air quality and does not interfere with local decisionmaking or negatively affect economic growth. Pruitt further stated that the delay would provide flexibility for states and localities to avoid being designated as nonattainment areas, which Pruitt argued result in reduced infrastructure spending, more regulatory requirements, higher business costs in these areas.

Recent news

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See also

• Clean Air Act
• Implementation of the Clean Air Act
• Ozone
• National Ambient Air Quality Standards
• Air pollutants
• Glossary of environmental terms