Meghan Hodges
The Environmental Protection Agency (EPA) in the United States identifies six criteria air pollutants that are commonly found throughout the country. These pollutants are carbon monoxide (CO), lead (Pb), ground-level ozone (O3), particulate matter (PM), nitrogen dioxide (NO2), and sulfur dioxide (SO2). The Clean Air Act requires the EPA to set National Ambient Air Quality Standards (NAAQS) for these six pollutants, which can be harmful to public health, the environment, and regulated industries. Ambient air monitors are placed across the country, with a focus on areas expected to have higher pollutant concentrations or larger populations. The EPA's Air Quality Index (AQI) is based on daily measurements of up to five criteria air pollutants, and the data is used to evaluate and improve air quality.
| Characteristics | Values |
|---|---|
| Number of pollutants | 6 |
| Names of pollutants | Carbon monoxide (CO), lead (Pb), ground-level ozone (O3), particulate matter (PM), nitrogen dioxide (NO2), and sulfur dioxide (SO2) |
| Harmful effects | Health, environment, and property damage |
| Regulatory activity | Evolving, with areas designated as "attainment" or "nonattainment" for NAAQS |
| National Ambient Air Quality Standards (NAAQS) | Set by the EPA under the Clean Air Act, with primary and secondary standards for public health and welfare protection |
| Air Quality Index (AQI) | Based on daily measurements of up to five criteria air pollutants, with values ranging from "good" to "hazardous" |
| Data sources | EPA's Air Quality System (AQS) database, ambient air monitors |
| Monitoring coverage | Not all counties in the US have air pollution monitors |
| PM2.5 standards | 24-hour: 35 µg/m3; Annual: 12 µg/m3 |
| Ozone standards | 8-hour: 0.070 ppm |
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What You'll Learn
Carbon Monoxide (CO)
The EPA plays a crucial role in monitoring and regulating CO levels. They work with state, tribal, and local agencies to ensure that CO is maintained at safe levels through the establishment of National Ambient Air Quality Standards (NAAQS). These standards are set by the Clean Air Act, which categorises them into primary and secondary standards. Primary standards focus on protecting public health, especially sensitive groups such as children and the elderly. On the other hand, secondary standards aim to safeguard public welfare, including protection against damage to the environment, animals, crops, and buildings.
The EPA collects and analyses data from thousands of ambient air monitors placed across the United States, focusing on areas with expected higher pollutant concentrations or larger populations. This data is then compared to the NAAQS levels to assess the percentage of children living in counties with pollutant concentrations exceeding these standards. The EPA's Air Quality Index (AQI) is also utilised, providing daily measurements of criteria pollutants, including CO, to determine air quality levels in different counties.
While the EPA's review in 2011 found no evidence of ecological effects from CO at ambient levels, CO is still a significant concern due to its indirect contribution to climate change. CO participates in atmospheric chemical reactions that produce ozone, a known climate change gas. Additionally, indoor CO levels can be significantly higher than outdoors, with sources including gas stoves, malfunctioning gas appliances, and tobacco smoke. As a result, the EPA and air quality regulators are committed to reducing CO emissions to mitigate the potential health risks and environmental impacts associated with this harmful pollutant.
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Ground-level Ozone (O3)
Ozone is a colorless and highly irritating gas that can trigger a variety of health problems, particularly for children, the elderly, and people with lung diseases such as asthma. Exposure to ozone has been linked to premature mortality and a range of health issues requiring hospital admissions. It can also negatively impact vegetation, decreasing crop productivity, and damaging flowers, shrubs, and textiles.
Ozone levels are highest on hot, sunny days in urban environments, but wind can transport ozone over long distances, so even rural areas can experience high ozone levels. To address ground-level ozone pollution, the EPA works with states and tribes to monitor air quality and designate areas as attainment or nonattainment based on national ambient air quality standards. States with nonattainment areas must develop implementation plans to improve air quality and meet the EPA's standards.
To support the reduction of ground-level ozone and overall air quality, individuals can check ozone levels through online resources and take precautionary measures to protect their health.
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Particulate Matter (PM)
The health risks associated with PM depend on its size. Larger particles, such as sand and large dust, which are bigger than 10 micrometers in diameter, are not regulated by the EPA. On the other hand, particles smaller than 10 micrometers (known as PM10) can be inhaled into the lungs and cause adverse health effects, particularly for individuals with respiratory conditions. Fine particles, or PM2.5, with diameters of 2.5 micrometers or less, pose the greatest risk to health. These fine particles can penetrate deep into the lungs and may even enter the bloodstream. Long-term exposure to PM2.5 has been linked to premature death, especially in people with chronic heart or lung diseases, and impaired lung development in children.
The EPA has established regulations to address the health risks posed by PM. The Clean Air Act requires the EPA to set National Ambient Air Quality Standards (NAAQS) for criteria air pollutants, including PM. These standards aim to protect public health and the environment. The EPA also provides tools like AirNow, which offers daily Air Quality Index (AQI) updates, helping individuals understand the air quality in their area and take necessary precautions.
To reduce the formation of PM, the EPA has implemented national and regional rules to decrease emissions of pollutants that contribute to its creation. These regulations support state and local governments in meeting the Agency's national air quality standards. Additionally, the EPA reviews and strengthens these standards periodically, as seen in the 2024 update to the secondary standards for fine particle pollution (PM2.5). Overall, the EPA's efforts to monitor and regulate PM aim to safeguard public health and mitigate the harmful impacts of particulate matter pollution on both people and the environment.
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Nitrogen Dioxide (NO2)
NO2 is primarily introduced into the air through the burning of fossil fuels such as coal, oil, methane gas, and diesel at high temperatures. It is a byproduct of emissions from cars, trucks, buses, power plants, and off-road equipment. Indoors, exposure can arise from cigarette smoke, and butane and kerosene heaters and stoves. The levels of NO2 are, on average, at least three times higher in homes with gas stoves compared to electric stoves.
Breathing air with high concentrations of NO2 can irritate the airways in the human respiratory system. Short-term exposure can aggravate respiratory diseases, especially asthma, leading to coughing, wheezing, and difficulty breathing. Prolonged exposure to elevated levels of NO2 may contribute to the development of asthma and potentially increase susceptibility to respiratory infections. Scientific evidence also suggests that exposure to NO2 could cause asthma in children.
NO2 and other NOx interact with water, oxygen, and other chemicals in the atmosphere to form acid rain, which harms sensitive ecosystems such as lakes and forests. The EPA has established rules to reduce emissions of NO2 and NOx, helping state and local governments meet the National Ambient Air Quality Standard (NAAQS).
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Sulfur Dioxide (SO2)
As a pollutant, SO2 has harmful effects on human health, particularly the respiratory system. Exposure to SO2 can cause wheezing, shortness of breath, chest tightness, and other respiratory problems, especially during physical activity. Long-term exposure to high levels of SO2 can increase respiratory symptoms and reduce lung function. Even short exposures to peak levels of SO2 can cause breathing difficulties for people with asthma when they engage in outdoor activities.
The dominant use of sulfur dioxide is in the production of sulfuric acid, where it serves as an intermediate in the synthesis of sulfolane, an important solvent in the petrochemical industry. SO2 is also involved in the global atmospheric sulfur cycle, reacting with water to form clouds of sulfurous acid (H2SO3). This reaction plays a role in the warming of planets like early Mars and Venus, where it is a significant component of the atmosphere.
In the context of the Environmental Protection Agency (EPA), SO2 is one of the six "criteria" air pollutants identified by the Clean Air Act. The EPA is responsible for setting National Ambient Air Quality Standards (NAAQS) for these pollutants, which can harm public health and the environment. To monitor SO2 levels, ambient air monitors are placed across the United States, particularly in areas expected to have higher pollutant concentrations or larger populations. The EPA collects and reports data from these monitors to its Air Quality System (AQS) database, which helps assess and address SO2 pollution.
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