Greta Jimenez
The United States' Clean Air Act requires the Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants, also known as criteria pollutants. These pollutants are harmful to human health, the environment, and property. They are found all over the US and include particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Of these, particle pollution and ground-level ozone are the most widespread health threats. While EPA regulations have helped reduce pollutants, certain areas continue to exceed standards, with data from 2021 indicating that 75% of Asian non-Hispanic children lived in counties where air quality standards for any pollutant were exceeded.
| Characteristics | Values |
|---|---|
| Number of criteria pollutants | 6 |
| Names of criteria pollutants | particulate matter, ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead |
| Pollutants with the most widespread health threats | particle pollution and ground-level ozone |
| Number of types of national ambient air quality standards | 2 |
| Name of the two types of national ambient air quality standards | primary standards and secondary standards |
| Purpose of primary standards | public health protection, including protecting the health of "sensitive" populations such as asthmatics, children, and the elderly |
| Purpose of secondary standards | public welfare protection, including protection against decreased visibility and damage to animals, crops, vegetation, and buildings |
| Percentage of Asian non-Hispanic children living in counties in which air quality standards for any pollutant were exceeded in 2021 | 75% |
| Percentage of Hispanic children living in counties in which air quality standards for any pollutant were exceeded in 2021 | 73% |
| Percentage of children living in counties with measured ozone concentrations above the level of the current 8-hour ozone standard on more than 25 days in 1999 | 38% |
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Particulate matter
The Clean Air Act requires the Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six principal pollutants ("criteria" air pollutants) that are harmful to public health and the environment. These criteria air pollutants are found all over the United States and can cause harm to health, the environment, and property damage.
One of these criteria air pollutants is particulate matter (PM) pollution. Particulate matter is a mixture of solid particles and liquid droplets found in the air. Some particles are large enough to be seen, while others are so small that they can only be detected with an electron microscope. PM pollution can be made up of a variety of components, including acids, organic chemicals, metals, and soil or dust particles. The size and composition of the particles vary widely, and they come from a range of sources, both man-made and natural. Some examples of natural sources include wildfires, volcanic eruptions, and dust storms. Common man-made sources include motor vehicles, industrial processes, and power generation.
The EPA has established National Ambient Air Quality Standards for two sizes of particulate matter: PM2.5 and PM10. The numbers refer to the size of the particles in micrometers. PM2.5 is considered "fine" particulate matter, while PM10 is considered "inhalable coarse" particulate matter. These particles can enter the body through the nose or mouth during breathing and may cause serious health issues.
The EPA has been working to strengthen air quality standards for fine particle pollution (PM2.5). In 2024, the EPA is set to complete a review of secondary standards for ecological effects of oxides of nitrogen, oxides of sulfur, and particulate matter. State, local, and tribal agencies can use this information to develop emission reduction strategies, plans, and programs to ensure they achieve and maintain the NAAQS.
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Ground-level ozone
The formation of ground-level ozone is influenced by various factors. Firstly, it is more prevalent during the summer months due to increased heat and sunlight, which promote the chemical reactions that produce ozone. Secondly, urban environments, where major sources of ozone precursors are concentrated, tend to experience higher levels of ground-level ozone. However, wind can carry NOx over long distances, leading to ozone formation in rural regions as well.
To monitor ground-level ozone levels, various techniques are employed, including remote sensing technology such as LIDAR and in-situ monitoring technology like ozonesondes attached to meteorological balloons. These measurements help track ozone concentrations at different altitudes and contribute to air quality assessments.
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Carbon monoxide
The EPA works with state, tribal, and local air agencies to attain and maintain NAAQS for carbon monoxide and other criteria pollutants. These agencies develop emission reduction strategies, plans, and programs to meet the standards. The Menu of Control Measures (MCM) provides agencies with existing emission reduction measures and relevant information on their efficiency and cost-effectiveness.
To ensure compliance with NAAQS, the EPA has established several processes. These include reviewing the NAAQS, determining whether areas meet the standards, and working with areas to attain and maintain the required levels. The EPA also provides a range of resources, such as the Air Quality Management Process, AirNow (an air quality forecast tool), and the State Implementation Plan Status Tools, to support its collaborative efforts in managing carbon monoxide and other criteria air pollutants.
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Sulfur dioxide
The largest sources of SO2 emissions are from fossil fuel combustion at power plants and other industrial facilities. SO2 emissions that lead to high concentrations of SO2 in the air can also lead to the formation of other sulfur oxides (SOx). These compounds can react with other atmospheric compounds to form small particles that contribute to particulate matter (PM) pollution. These small particles may penetrate deeply into the lungs and, in sufficient quantities, can cause health issues. Additionally, at high concentrations, SOx can harm trees and plants by damaging foliage and impeding growth.
SO2 and other sulfur oxides can contribute to acid rain, which can harm sensitive ecosystems. They can also react with other atmospheric compounds to form fine particles that reduce visibility (haze) in parts of the United States, including national parks and wilderness areas. The deposition of these particles can stain and damage stone and other materials, including culturally significant objects.
To address SO2 pollution, the EPA identifies areas where air quality does not meet the established standards. State, local, and tribal governments then develop plans to reduce SO2 levels in those areas. The Menu of Control Measures (MCM) provides these governments with existing emission reduction measures and relevant information on their efficiency and cost-effectiveness.
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Nitrogen dioxide
The EPA has established NAAQS for NO2 to protect public health and welfare. The primary standards aim to protect the health of sensitive populations, including asthmatics, children, and the elderly, by limiting their exposure to harmful levels of NO2. The secondary standards focus on public welfare, aiming to prevent reduced visibility and damage to materials, crops, vegetation, and animals caused by NO2 pollution.
To attain and maintain the NAAQS for NO2, the EPA collaborates with state, tribal, and local air agencies. These agencies develop emission reduction strategies, plans, and programs to reduce NO2 levels and ensure compliance with the standards. The EPA's Menu of Control Measures (MCM) provides agencies with information on existing emission reduction measures, along with data on their efficiency and cost-effectiveness.
While the EPA has set standards and regulations to control NO2 pollution, there have been instances where NO2 levels have exceeded the EPA standards. In such cases, the EPA works with relevant agencies to implement corrective actions and ensure that NO2 emissions are reduced to meet the NAAQS.
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Frequently asked questions
The Clean Air Act requires the EPA to set National Ambient Air Quality Standards (NAAQS) for six common air pollutants known as "criteria pollutants". These pollutants are found all over the United States and can harm human health, the environment, and cause property damage.
The six criteria pollutants are particulate matter (often referred to as particle pollution), ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, and lead. Of these, particle pollution and ground-level ozone are the most widespread health threats. In 2021, 75% of Asian non-Hispanic children lived in counties where air quality standards for any pollutant were exceeded, with similar figures for other ethnicities.
Exposure to criteria pollutants has been associated with a range of adverse health effects, including coughing and wheezing, aggravation of respiratory illnesses such as asthma, and neurodevelopmental effects (in the case of lead).
