Air Quality

Desired Trend


Current Trend

Baseline (2009-2011): 30.3 days
Current (2020-2022): 12.3 days

Theme Green


Average number of days per year the EPA Air Quality Index exceeds 100 for ozone (unhealthy for sensitive groups)

Why is it Important?

The quality of air impacts the health of residents, the ability of the region’s economy to grow, and the environment. Ground-level ozone is a common air pollutant and the main component of smog. It is distinct from the ozone layer found in the upper atmosphere, which shields the earth from harmful ultraviolet radiation. Ground-level ozone is formed by the chemical reaction of precursors – oxides of nitrogen (NOx) and volatile organic compounds (VOCs) – on hot sunny days. These precursors are emitted by industrial activity, automobile exhaust, gasoline vapors, and commercial products like solvents and paints. The health effects of ground-level ozone include shortness of breath, coughing, throat irritation, aggravation of lung diseases such as asthma, and permanent lung damage and is most likely to affect children, older adults, people with lung disease, and people who exercise outdoors. Poor air quality may harm the economy by reducing tourism due to health concerns and may increase the cost of business by requiring more stringent pollution control equipment. Air pollution also harms the environment by damaging ozone-sensitive plants such as cottonwood and black cherry trees.1

The EPA monitors air quality daily and reports an index score that is categorized into one of six levels ranging from “Good” to “Hazardous”.2,3 Index values between 101 and 150, also called Orange Days, indicate air quality that is unhealthy for sensitive groups such as children, older adults, and people with lung disease. Index values above 150 are unhealthy for everybody (Red Day), very unhealthy (purple), or hazardous (maroon). This indicator measures the number of days classified as unhealthy for sensitive groups or worse. The number of poor air quality days are averaged over a three year period to reduce variations due to climate and make the long term trend more apparent.

How are we Doing?

Air quality in the St. Louis region improved from an average of 30.3 days of poor air quality per year in the baseline time period of 2009-2011 to an average of 12.3 days per year in 2020-2022. Since 2000, air quality improved overall with an annual average of 34 fewer days of poor air quality per year in 2020-2022 than in 2000-2002. The average number of days was slightly higher for the most recent three-year period than it was for the previous three with an average of 10 days per year for 2017-2019, 11.3 days for 2018-2020, 10.7 for 2019-2021, and 12.3 for 2020-2022.

Over the long term, air quality has improved in part due to mild summer weather as well as improvements in motor vehicle technology; cleaner burning gasoline; inspection/maintenance program; transportation projects to reduce congestion; ridesharing program and MetroLink; controls on industry and power plants; and individual behavior decisions.

Most of the poor air quality days are classified as Orange Days, or “Unhealthy for Sensitive Groups”. The St. Louis region had an annual average of 21.1 Orange Days and 6.8 Red days (“Unhealthy for Everyone”) between 2000 and 2022. In the last 10 years (2012-2022), the annual average for orange days (14.4 days) as well as red days (2.1 days) was fewer. Over the entire 23-year-period the region experienced 12 Purple Days, which are “Very Unhealthy for Everyone,” none of which were in the last 15years. The days with exceedances are based on the Eight-Hour Ozone Standard, which the U.S. EPA strengthened in 2015, setting it at 70 ppb (parts per billion). The EPA states the new level is “based on extensive scientific evidence about ozone’s effects on public health and welfare.”5

After the passage of the Clean Air Act Amendments of 1990, the St. Louis region was classified as a Moderate ozone non-attainment area. Non-attainment means that an area does not meet National Ambient Air Quality Standards (NAAQS).NAAQS, or concentration limits, are set by the U.S. Environmental Protection Agency (EPA) for pollutants with the most widespread threat to public health and the environment. Since that time, the region has made substantial progress in reducing ozone levels, and in 2018 part of the region was reclassified as a Marginal non-attainment area, which is the level closest to attainment of the 2015 ozone standard.The EPA found that all of Jefferson County, the majority of Franklin County (Boles Township being the exception), and Monroe County were in attainment of this ozone standard. However, in 2021, Jefferson and Monroe counties were reclassified by EPA to non-attainment. Effective November 7, 2022, EPA reclassified the St. Louis (MO-IL) ozone nonattainment area from Marginal to Moderate, along with 21 other Marginal nonattainment areas in the country, for not attaining the 2015 ozone standard by the August 3, 2021 attainment deadline for Marginal nonattainment areas (based on monitoring data).

Geographic Level

St. Louis eight-county bi-state region, including Franklin, Jefferson, St. Charles, and St. Louis counties and the city of St. Louis in Missouri and Madison, Monroe, and St. Clair counties in Illinois. View map.


1U.S. Environmental Protection Agency, Ecosystem Effects, 1 November 2012; accessed on 4 February 2014 at

2U.S. Environmental Protection Agency, Air Quality Guide for Ozone, March 2008; accessed on 4 February 2014 at

3The ozone air quality index is determined based on the highest 8-hour average concentration of ozone. The current health-based standard for ozone is .070 parts per million (ppm). 

4Missouri Department of Natural Resources, Possible Effects of COVID-19 Stay-at-Home Order on St. Louis Area Air Quality, 16 October 2020, accessed on 22 February 2021 at 

5 U.S. Environmental Protection Agency, Overview of EPA’s Updates to the Air Quality Standards for Ground-Level Ozone, 1 October 2015; accessed on 17 January 2017 at

6 Levels of non-attainment, from worst to best are: extreme, severe 17, severe 15, serious, moderate, and marginal. For more information, see EPA Ozone Designation and Classification Information at

Data Sources

United States Environmental Protection Agency, Air Quality Index; East-West Gateway Council of Governments