Methodology

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Since its inception in 2000, the American Lung Association "State of the Air" report has used a methodology that starts with reliable quality-assured data from EPA and applies an unbiased grading system to provide credible, easy-to-understand information to the public about the air they breathe.

Statistical Methodology: The Air Quality Data

Ozone and short-term particle pollution. The data on air quality throughout the United States were obtained from the U.S. Environmental Protection Agency’s Air Quality System (AQS). The American Lung Association contracted with Dr. Allen S. Lefohn, A.S.L. & Associates, Montana, to characterize the hourly averaged ozone concentration information and the 24-hour averaged PM2.5 concentration information for the three-year period for 2022-2024 for each monitoring site.

Year-round particle pollution. Design values for the annual PM2.5 concentrations by county (or county equivalent, hereinafter referred to for simplicity as "county") for the period 2022-2024 were retrieved July 24, 2025 from data posted on June 3, 2025 at the U.S. Environmental Protection Agency’s (EPA) website.

The Lung Association received critical assistance from members of the National Association of Clean Air Agencies and the Association of Air Pollution Control Agencies. With their assistance, all state and local agencies were provided with the opportunity to review and comment on the data in draft tabular form. The Lung Association reviewed any discrepancies with the agencies and, if needed, with Dr. Lefohn at A.S.L. & Associates. The American Lung Association wishes to express its continued appreciation to the state and local air directors for their willingness to assist in ensuring that the characterized data used in this report are correct.

The 2022, 2023 and 2024 AQS hourly ozone data were used to calculate the daily 8-hour maximum concentration for each ozone-monitoring site. The hourly averaged ozone data were downloaded on June 30, 2025, following the close of the authorized period for quality review and assurance certification of data. Only the hourly average ozone concentrations derived from FRM and FEM monitors were used in the analysis. The data were considered for a three-year period for the same reason that EPA uses three years of data to determine compliance with the ozone standard: to prevent anomalies in weather or other factors in any single year from creating air pollution levels that inaccurately reflect normal conditions. The highest 8-hour daily maximum concentration in each county for 2022, 2023 and 2024, based on the EPA-defined ozone season, was identified.

The current National Ambient Air Quality Standard for ozone is 70 parts per billion (ppb) measured over eight hours. EPA’s Air Quality Index reflects the 70 ppb standard. A.S.L. & Associates prepared a table by county that summarized, for each of the three years, the number of days the ozone level was within the ranges identified by EPA based on the following Air Quality Index:

0 – 54 ppbGood (Green)
55 – 70 ppbModerate (Yellow)
71 – 85 ppbUnhealthy for Sensitive Groups (Orange)
86 – 105 ppbUnhealthy (Red)
106 – 200 ppbVery Unhealthy (Purple)
>200 ppbHazardous (Maroon)


The goal of this report was to identify the number of days that 8-hour daily maximum concentrations in each county occurred within the defined ranges. This approach provided an indication of the level of pollution for all monitored days, not just those days that fell under the requirements for attaining the National Ambient Air Quality Standards. Therefore, no data capture criteria were applied to eliminate monitoring sites or to require a number of valid days for the ozone season.

The daily maximum 8-hour average concentration for a given day is derived from the highest of the 17 consecutive 8-hour averages beginning with the 8-hour period from 7:00 a.m. to 3:00 p.m. and ending with the 8-hour period from 11:00 p.m. to 7:00 a.m. the following day. This follows the process EPA uses for the current ozone standard adopted in 2015 but differs from the form used under the previous 0.075 ppm 8-hour average ozone standard that was established in 2008. All valid days of data within the ozone season were used in the analysis. However, for computing an 8-hour average, at least 75 percent of the hourly concentrations (i.e., 6-8 hours) had to be available for the 8-hour period. In addition, an 8-hour daily maximum average was identified if valid 8-hour averages were available for at least 75 percent of possible hours in the day (i.e., at least 13 of the possible 17 8-hour averages). Because EPA includes days with inadequate data (i.e., not 75 percent complete) if the standard value is exceeded, our data capture methodology also included the site’s 8-hour value if at least one valid 8-hour period were available, and it was 71 ppb or higher.

As instructed by the Lung Association, A.S.L. & Associates included the exceptional (e.g., wildfires) and natural events (e.g., stratospheric intrusions) that were identified in the database and identified for the Lung Association the dates and monitoring sites that experienced such events. Some data have been flagged by the state or local air pollution control agency to indicate that they had raised issues with EPA about those data. For each day across all sites within a specific county, the highest daily maximum 8-hour average ozone concentration was recorded and then the results were summarized by county for the number of days the ozone levels were within the ranges identified above.

Following receipt of the above information, the American Lung Association identified the number of days each county, with at least one ozone monitor, experienced air quality designated as orange (Unhealthy for Sensitive Groups), red (Unhealthy), purple (Very Unhealthy) or maroon (Hazardous). When sufficient data were available for at least one year in a county, but insufficient data were available in any other year, an “incomplete” was identified for the 3-year period. Insufficient data exist for various reasons. For example, when a specific monitor was used for a special study and the monitor was then discontinued in other years, an “incomplete” is assigned. When no data were collected in a county for all three years of the study period, a “did not collect” was assigned.

A.S.L. & Associates identified the maximum daily 24-hour AQS PM2.5 concentration for each county in 2022, 2023 and 2024 with monitoring information. The 24-hour averaged PM2.5 data were downloaded on August 21, 2025 from the EPA website following the correction of the hourly values by the EPA of the PM2.5 data associated with monitors using method codes 236 and 238. The current short-term National Ambient Air Quality Standard for PM2.5 is 35 micrograms per cubic meter (µg/m3) measured over twenty-four hours. Using the downloaded PM2.5 daily data from the EPA website, A.S.L. & Associates prepared a table by county that summarized, for each of the three years, the number of days the maximum of the daily PM2.5 concentration was within the ranges identified by EPA based on the Air Quality Index, as adopted by EPA on February 7, 2024 as follows:

0.0 µg/m3 to 12.0 µg/m3Good (Green)
12.1 µg/m3 to 35.4 µg/m3Moderate (Yellow)
35.5 µg/m3 to 55.4 µg/m3Unhealthy for Sensitive Groups (Orange)
55.5 µg/m3 to 125.4 µg/m3Unhealthy (Red)
125.5 µg/m3 to 225.4 µg/m3Very Unhealthy (Purple)
greater than or equal to 225.5 µg/m3Hazardous (Maroon)

All previous data collected for 24-hour average PM2.5 were characterized using the AQI thresholds listed above. 

The goal of this report was to identify the number of days that the maximum in each county of the daily PM2.5 concentration occurred within the defined ranges. This approach provided an indication of the level of pollution for all monitored days, not just those days that fell under the requirements for attaining the National Ambient Air Quality Standards. Therefore, no data capture criteria were used to eliminate monitoring sites. The 24-hour average PM data were used. Included in the analysis are data collected using only FRM and FEM methods, which reported 24-hour average data. As instructed by the Lung Association, A.S.L. & Associates included the exceptional and natural events that were identified in the database and identified for the Lung Association the dates and monitoring sites that experienced such events. Some data have been flagged by the state or local air pollution control agency to indicate that they had raised issues with EPA about those data. For each day across all sites within a specific county, the highest daily maximum 24-h PM2.5 concentration was recorded and then the results were summarized by county for the number of days the concentration levels were within the ranges identified above.

Following receipt of the above information, the American Lung Association identified the number of days each county, with at least one PM2.5 monitor, experienced air quality designated as orange (Unhealthy for Sensitive Groups), red (Unhealthy), purple (Very Unhealthy) or maroon (Hazardous). When sufficient data were available for at least one year in a county, but insufficient data were available in any other year, an “incomplete” was identified for the 3-year period. When no data were collected in a county for all three years of the study period, a “did not collect” was assigned.

Design values for the annual PM2.5 concentrations by county for the period 2022-2024 were retrieved from EPA’s website. The Lung Association did not conduct further analysis on these data, beyond using them to assign “passing” or “failing” grades. When EPA did not post a design value for a county, but the Lung Association assigned either an “incomplete” or a grade to that county for the daily measure of fine particle pollution, an “incomplete” was identified for that county for the annual measure of particle pollution for the 3-year period. Otherwise, when no data were collected in a county for all three years of the study period, a “did not collect” was assigned.

How “State of the Air’s” Approach to Air Quality Data Differs from EPA’s

EPA distinguishes between actual air quality (i.e., the air people breathe that may cause health effects) and regulatory air quality (i.e., the data used to determine if a region meets federal air pollution standards). EPA uses actual air quality concentration data to produce trends summaries. These summaries are intended to reflect changes in actual air quality that at times may be influenced by naturally-caused episodic events (e.g., wildfires, stratospheric ozone intrusions, volcanic activity and dust storms).

For regulatory purposes, air quality design values are used to designate and classify nonattainment areas (areas that do not meet National Ambient Air Quality Standards). The designated design values are intended to reflect air quality not impacted by EPA-defined “exceptional events.” EPA defines exceptional events as unusual or naturally-occurring events that affect air quality but are not reasonably controllable using techniques that tribal, state or local air agencies may implement. If EPA concurs with a state or tribal entity that a pollution spike is an "exceptional event," the recorded concentration is removed from the design value calculations. Therefore, for assessing attainment status, that spike is officially disregarded in the regulatory record by EPA. Thus, while an area may be in "attainment" of air pollution standards for regulatory purposes, residents may still be exposed to unhealthy levels of ozone and PM2.5. 

The grading system that the Lung Association uses in this report differs significantly from the methodology EPA uses to determine violations of both the ozone and 24-hour PM2.5 standards. For example, EPA determines whether a county violates the ozone standard based on the fourth maximum daily 8-hour ozone reading each year averaged over three years. Neither multiple days of unhealthy air beyond the highest four in each year, nor the full severity of the highest three in each year, are considered. In contrast, the system used in the Lung Association’s report recognizes every day that a community’s air quality results in unhealthy air throughout the three years, and the severity of such days’ air pollution. Consequently, certain counties will receive grades of “F” in this report, showing repeated instances of unhealthy air, while still meeting the corresponding national standard for regulatory purposes. The Lung Association’s position is that if an air pollution exceedance is ruled an “exceptional event” and is omitted for the purposes of regulatory compliance, it remains a threat to people’s health. Our report seeks to paint a full picture of the air pollution experienced by each given community, regardless of the source of that pollution. 

Description of County Grading System

The grades for ozone and short-term particle pollution (24-hour PM2.5) were based on a weighted average calculation. To determine weighted averages, the Lung Association followed these four steps separately for each pollutant in each county:

  1. Assigned weighting factors to each category of the Air Quality Index. Days of poor air quality were given the following weighting factors:

Orange days    1.0

Red days           1.5

Purple days      2.0

Maroon days   2.5

This ensured that days when the air pollution levels were worse received appropriately greater weight. 

  1. Multiplied the total number of days within each AQI category by their assigned factor, and added all the categories to calculate a total:

Total = [Orange days x 1]  +  [Red days x 1.5]  +  [Purple days x 2]  +  [Maroon days x 2.5]

  1. Divided the total by three to determine the weighted average, since the monitoring data were collected over a three-year period:

Weighted Average   =    Total  ÷  3

  1. Weighted average was then used to determine each county’s grades for ozone and 24-hour PM2 5 according to the following table:
Weighted AverageGrade
0.0A
0.3 - 0.9B
1.0 - 2.0C
2.1 - 3.2D
3.3 or higherF


All counties with a weighted average of zero (corresponding to no exceedances of the standard over the three-year period) were given a grade of “A.” 

For ozone, an “F” grade was set to generally correlate with the number of unhealthy air days that would place a county in nonattainment for the ozone standard.

For short-term particle pollution, fewer unhealthy air days are required for an F than for nonattainment under the PM2.5 standard. The 2006 24-hour PM2.5 standard is set to allow two percent of the days during the three years to exceed 35 µg/m3 (called a “98th percentile” form) before violating the standard. That would be roughly 21 unhealthy days in three years. The grading used in this report would allow only about one percent of the days to be over 35 µg/m3 (called a “99th percentile” form) of the PM2.5. The American Lung Association supports using the tighter limits in a 99th percentile form as a more appropriate standard that is intended to protect the public from short-term episodes or spikes in pollution.

Weighted averages allow comparisons to be drawn based on severity of air pollution. For example, if one county experienced nine orange days and no red days, it would earn a weighted average of 3.0 and a D grade. However, another county that experienced only eight orange days but also two red days, which signify days with more serious air pollution, would receive an F. That second county would have a weighted average of 3.7.

Counties were ranked by weighted average.  Metropolitan areas were ranked by the highest weighted average among the counties within the largest Metropolitan Statistical Area containing those counties as defined by the White House Office of Management and Budget (OMB) as of 2023.

Weighted average values from earlier years are from prior reports and updated when new standards are implemented.

Since no comparable Air Quality Index exists for year-round particle pollution (annual PM2.5), the grading was based on the 2024 National Ambient Air Quality Standard for annual PM2.5 of 9.0 µg/m3. Counties that EPA listed as being at or below 9.0 µg/m3 were given grades of “Pass.” Counties that EPA listed as being above 9.0 µg/m3were given grades of “Fail.” 

A design value is the calculated concentration of a pollutant based on the form of the National Ambient Air Quality Standard and is used by EPA to determine whether the air quality in a county meets the standard. Counties were ranked by design value. Metropolitan areas were ranked by the highest design value for year-round particle pollution among the counties within the largest Metropolitan Statistical Area containing those counties as defined by OMB as of 2023.

The Lung Association calculates the county population at risk from these pollutants using the total population of the county where the monitor is located. The Lung Association then calculates the metropolitan population at risk based on the largest metropolitan area that includes that county. Because both people and air pollution move freely throughout the county and surrounding metropolitan area, the monitor reflects exposure across that broader region. Metropolitan statistical areas are delineated by OMB based on published standards applied to Census Bureau data. Metropolitan statistical areas must have at least one urban area with a population of 50,000 or more and include neighboring counties if they meet specified standards for commuting to or from the central counties.

The U.S. Census Bureau provides 2024 population estimates at the county level overall, for those under 18 years of age or 65 years of age and older, and by race and ethnicity.

Population and poverty estimates for Puerto Rico come from the U.S. Census Bureau’s American Community Survey, 2019-2023.

Pregnancies by county are based on the number of births to women aged 18-49 by state in 2024 and the estimated population of women in the same age range by county. Puerto Rico births are for women of any age and from 2023.

Poverty estimates by county come from the U.S. Census Bureau’s Small Area Income and Poverty Estimates program. The estimates are derived from a model using estimates of income or poverty from the Annual Social and Economic Supplement and the Current Population Survey, 2023.

Health condition counts are determined by applying state-level disease rates to county population estimates. Pediatric asthma, adult asthma, chronic obstructive pulmonary disease (COPD), and cardiovascular (CV) disease rates come from the Centers for Disease Control and Prevention’s Behavioral Risk Factor Surveillance System. Lung cancer incidence rates come from StateCancerProfiles.gov.

Further details about how the populations-at-risk numbers are derived can be found in the Understanding Grades and Tables section.

Did You Know?

  1. 33.5 million children (46% of all kids) in the U.S. live in an area that received a failing grade for at least one measure of air pollution.
  2. More than 7 million children in the United States (10% of all kids) live in a community with failing grades for all three measures.
  3. More than four in 10 (44%) people of all ages in the U.S. live where the air they breathe earned an F in “State of the Air” 2025.
  4. Nearly 33 million people live in counties that got an F for all three air pollution measures in “State of the Air” 2025.
  5. Infants, children and teens as a group are more susceptible to the health impacts of air pollution. Their lungs are still developing, they breathe more air for their body size than adults, and they are frequently exposed to outdoor air.
  6. Breathing ozone irritates the lungs, resulting in inflammation—as if your lungs had a bad sunburn.
  7. Breathing in particle pollution can increase the risk of lung cancer.
  8. Particle pollution can cause early death and heart attacks, strokes and emergency room visits.
  9. Particles in air pollution can be smaller than 1/30th the diameter of a human hair. When you inhale them, they are small enough to get past the body's natural defenses.
  10. Ozone and particle pollution are both linked to increased risk of premature birth and lower birth weight in newborns.
  11. If you live or work near a busy highway, traffic pollution may put you at greater risk of health harm.
  12. People who work or exercise outside face increased risk from the effects of air pollution.
  13. Millions of people are especially vulnerable to the effects of air pollution, including children, older adults and people with lung diseases such as asthma and COPD.
  14. Research shows that people of color and people with lower incomes are disproportionately affected by air pollution that puts them at higher risk for illness.
  15. Air pollution is a serious health threat. It can trigger asthma attacks, harm lung development in children, and even be deadly.
  16. You can protect yourself by checking the air quality forecast in your community and avoiding exercising or working outdoors, if possible, when unhealthy air is expected.
  17. Climate change enhances conditions for ozone pollution to form and makes it harder to clean up communities where ozone levels are high.
  18. Climate change increases the risk of wildfires whose smoke spreads dangerous particle pollution.
  19. Policymakers at every level of government must take steps to clean the air their constituents breathe.
  20. The nation has the Clean Air Act to thank for decades of improvements in air quality. This landmark law has successfully driven pollution reduction for over 55 years.
  21. This U.S. Environmental Protection Agency (EPA) is rolling back clean air protections and has eliminated health costs from its economic analyses. Both actions threaten clean air progress.
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