Environments and Contaminants - Criteria Air Pollutants
The six most common air pollutants are called “criteria” air pollutants and include carbon monoxide (CO), lead (Pb), ground-level ozone (O3), particulate matter (PM), nitrogen dioxide (NO2), and sulfur dioxide (SO2).
Indicators
E1: Children in counties where air pollutants exceed current standards
Web update: 2025
Key Information
- In 2023, about 72% of children lived in counties where criteria air pollutants exceeded current standards.
- The standards for ozone and particulate matter are the most commonly exceeded. In 2023, almost 60% of children lived in counties where ozone levels exceeded the short-term (8-hour) standard at least one day during the year and about half of children lived in counties where particulate matter exceeded the short-term (24-hour) standard at least one day during the year.
- In 2023, less than 1% of children lived in counties that exceeded the standards for carbon monoxide, lead, or nitrogen dioxide, and less than 3% of children lived in counties that exceeded the standards for sulfur dioxide.
- Over 85% of Asian non-Hispanic children, about 80% of Hispanic and Native Hawaiian and Pacific Islander children, 77% of Black non-Hispanic children, 75% of children of multiple races, 64% of White non-Hispanic children, and 52% of American Indian and Alaska Native children, lived in counties in which air quality standards for any pollutant were exceeded in 2023.
Data Characterization
Data for this indicator are obtained from EPA’s Air Quality System (AQS) database.
Ambient air monitors are placed in locations throughout the United States, with an emphasis on areas expected to have higher pollutant concentrations or that have larger populations. Not all counties in the United States have air pollution monitors.
To support long-term trends and other data uses, monitors generally tend to stay in the same location over many years, but there may be some limited changes in the number or location of monitors providing data from year to year. Note that the PM 2.5 annual standard is assessed by the 2024 NAAQS level of 9 ug/m3 for all years presented in E1. See National Ambient Air Quality Standards (NAAQS) for PM for more information.
E2: Children living in counties with ozone and PM2.5 concentrations above short-term standards
Web update: 2025
Key Information
- In 2023, most children, 63% lived in counties where the 24-hour PM 2.5 standard was exceeded for 7 days or fewer. Additionally, only 5% of children lived in counties where the standard was exceeded for more than 7 days during the year.
- In 2023, 13% of children lived in counties with no days with ozone concentrations above the 8-hour ozone standard and about 40% of children lived in counties with concentrations above the standard on 1 to 10 days of the year. Less than 20% of children lived in counties with concentrations above the standard on more than 10 days of the year.
- In 2023, 32% of children lived in counties with no monitoring data for PM2.5 and 28% of children lived in counties with no monitoring data for ozone.
Data Characterization
Data for this indicator are obtained from EPA’s Air Quality System (AQS) database.
Ambient air monitors are placed in locations throughout the United States, with an emphasis on areas expected to have higher pollutant concentrations or that have larger populations. Not all counties in the United States have air pollution monitors.
E3: Days with good, moderate, or unhealthy air quality for children
Web update: 2025
Key Information
- The percentage of children's days that were designated as having “unhealthy” air quality decreased from 10% in 1999 to 4% in 2023.
- The percentage of children’s days with “good” air quality increased from 34% in 1999 to 40% in 2023.
Data Characterization
Data for this indicator are obtained from EPA's Air Quality System (AQS) database.
Ambient air monitors are placed in locations throughout the country, with an emphasis on areas expected to have higher pollutant concentrations or that have larger populations. Not all counties in the United States have air pollution monitors.
AQI values are based on daily monitoring data for up to five criteria air pollutants. Some counties do not have monitors, and some monitors do not operate every day, so some days do not have AQI values for every pollutant every day.
For this indicator, the available monitoring data are used to assign a value of “good,” “moderate,” “unhealthy,” or “no monitoring data” for each day in each U.S. county. The “unhealthy” category used in Indicator E3 represents all AQI categories above 100, including Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous.
About the Criteria Air Pollutants Indicators
Air pollution contributes to a wide variety of adverse health effects. The six most common air pollutants are called “criteria” air pollutants and include carbon monoxide (CO), lead (Pb), ground-level ozone (O3), particulate matter (PM), nitrogen dioxide (NO2), and sulfur dioxide (SO2). Exposure to these pollutants has been associated with effects on children's health including coughing and wheezing, aggravation of respiratory illnesses such as asthma, and neurodevelopmental effects. Children may be particularly susceptible to adverse effects because their lungs and other organ systems are still developing and because they may experience higher exposure due to their activities such as outdoor play.
EPA establishes national ambient air quality standards (NAAQS) for the criteria pollutants. The standards are set at a level to protect public health, including the health of at-risk populations such as children. Some of the standards are designed to protect against adverse health effects that can happen after exposure for a short time (e.g., ozone concentrations averaged over 8 hours) and others are designed to protect against effects from more long term exposure (e.g., rolling three month lead concentrations).
For historical and contextual information about criteria air pollutants and Indicators E1, E2, and E3, see the Criteria Air Pollutants section of America's Children and the Environment, Third Edition (pdf) .
Data Sources and Methods - Criteria Air Pollutants
EPA, state, local, and tribal air pollution control agencies collect and report data from thousands of monitors to the Air Quality System (AQS) database. For Indicators E1 and E2, these criteria pollutant measurements are compared to the levels of the National Ambient Air Quality Standards (NAAQS), established by EPA under the Clean Air Act. Indicator E3 uses EPA’s Air Quality Index (AQI), which is calculated from data in the AQS database. The AQI is based on daily measurements of all the criteria pollutants except for lead. Indicator E3 uses the AQI to present the percentage of days in each year considered to be in the categories of “good,” “moderate,” or “unhealthy” air quality for children ages 0 to 17 years.
- Detailed Methods for Indicators E1, E2, and E3 (pdf)
- Air Quality System (AQS) Metadata (pdf)
- Metadata for Census: Decennial Data (pdf)
- Metadata for Census: Intercensal and Postcensal Data