Methodology

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 Allen S. Lefohn, Ph.D., A.S.L. & Associates, Helena, Montana, to characterize the hourly averaged ozone concentration information and the 24-hour averaged fine particulate matter (PM_2.5) concentration information for the three-year period for 2020-2022 for each monitoring site. 

Year-round particle pollution. Design values for the annual PM_2.5 concentrations by county for the period 2020-2022 were retrieved December 18, 2023 from data posted on May 23, 2023, at the U.S. Environmental Protection Agency’s website at https://www.epa.gov/air-trends/air-quality-design-values.

The Lung Association received important 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 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 2020, 2021 and 2022 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 26, 2023, 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 a situation in which anomalies of weather or other factors in any single year create air pollution levels that inaccurately reflect typical conditions. The highest 8-hour daily maximum concentration in each county was identified for each day of the EPA-defined ozone season for 2020, 2021 and 2022. 

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 during the ozone season when the ozone level was within the ranges identified by EPA based on the Air Quality Index: 

The approach 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 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. 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) or purple (Very Unhealthy). When insufficient data were available in any 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. 

A.S.L. & Associates identified the maximum daily 24-hour AQS PM_2.5 concentration for each county for each day in 2020, 2021 and 2022 with monitoring information. The 24-hour averaged PM_2.5 data were downloaded on August 2, 2023, following the close of the authorized period for quality review and assurance certification of data. The hourly averaged PM_2.5 concentration data in the EPA AQS database were characterized into 24-hour average PM_2.5 values by EPA and provided to A.S.L. & Associates. Using these results, 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 PM_2.5 concentration was within the ranges identified by EPA based on the Air Quality Index, as adopted by EPA on December 14, 2012¹: 

All previous data collected for 24-hour average PM_2.5 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 PM_2.5 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. Both 24-hour averaged PM data, as well as hourly averaged PM data averaged by EPA over 24 hours, were used. Included in the analysis are data collected using only FRM and FEM methods, which reported hourly and 24-hour averaged 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-hour PM_2.5concentration 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 PM_2.5 monitor experienced air quality designated as orange (Unhealthy for Sensitive Groups), red (Unhealthy), purple (Very Unhealthy) or maroon (Hazardous). 

The grades for ozone and short-term particle pollution (24-hour PM_2.5) 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.
2. 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] 
   
3. 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
4.  Weighted average was then used to determine each county’s grades for ozone and 24-hour PM_s.5 according to the following table:

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 2012 national air quality 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 could be as many as 21 unhealthy days in three years. The grading used in this report is roughly equivalent to allowing only about one percent of the days to be those on which PM2.5 concentrations were over 35 µg/m3 (called a “99th percentile form”). The American Lung Association supports using the 99th percentile form as a more appropriate form of the 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 had nine orange days and no red days, it would earn a weighted average of 3.0 and a D grade. However, another county that had 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. 

Note that this system differs significantly from the methodology EPA uses to determine violations of both the ozone and the 24-hour PM2.5 standards. 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. Multiple days of unhealthy air beyond the highest four in each year are not considered. By contrast, the system used in this report recognizes when a community’s air quality repeatedly results in unhealthy air throughout the three years. Consequently, some counties will receive grades of “F” in this report, showing repeated instances of unhealthy air, even while still meeting the EPA’s 2015 ozone standard. This is consistent with the American Lung Association’s position is that the evidence shows that the 2015 ozone standard fails to adequately protect public health. 

Counties were ranked by weighted average. Metropolitan areas were ranked by the highest weighted average among the counties within a given Metropolitan Statistical Area as of 2020 as defined by the White House Office of Management and Budget (OMB). 

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 PM_2.5), the grading was based on the 2024 National Ambient Air Quality Standard for annual PM_2.5 of 9 µg/m³. Counties that EPA listed as being at or below 9 µg/m³ were given grades of “Pass.” Counties that EPA listed as being above 9 µg/m³were given grades of “Fail.” Where data was collected but was insufficient for EPA to determine a design value, those counties received a grade of “Incomplete.” 

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 among the counties within a given Metropolitan Statistical Area as of 2020 as defined by the OMB. 

The Lung Association calculates the county population at risk from these pollutants based on the population from the entire county where the monitor is located. The Lung Association then calculates the metropolitan population at risk based upon the largest metropolitan area that contains that county. Not only do people from that county or metropolitan area circulate within the county and the metropolitan area, but the air pollution also circulates to that monitor from throughout the county and metropolitan area. 

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