Description of County Grading System

Ozone and short-term particle pollution (24-hour PM_2.5)
The grades for ozone and short-term particle pollution (24-hour PM_2.5) were based on a weighted average for each county calculated using the Air Quality Index as noted above. The number of orange days experienced by each county was assigned a factor of 1; red days were assigned a factor of 1.5 and purple days were assigned a factor of 2. By multiplying the total number of days within each category by their assigned factor, a total was determined. Because the monitoring data was collected over a three-year period, the total was divided by three to determine the weighted average. Each county’s grade was determined using the weighted average. Counties were ranked by weighted average. Metropolitan areas were ranked by the highest weighted average among the counties in the Census Bureau-defined Metropolitan Statistical Area. In 2003, the U.S. Census Bureau published revised definitions for the nation’s Metropolitan Statistical Areas. Therefore, comparisons between MSAs of the State of the Air reports from 2000 to 2003 and the State of the Air reports in 2004 and later should be made with caution.

All counties with a weighted average of zero (corresponding to no exceedances of the 8-hour standard over the three year period) were given a grade of “A.” Counties with a weighted average of 0.3 to 0.9 (corresponding to 1 to 2 orange days) received a “B.” Counties receiving a “C” had only 3 to 6 days over the standard, including at most one red day, and scored a weighted average of 1.0 to 2.0. Counties received a “D” if they had a weighted average of 2.1 to 3.2, which meant they had 7 to 9 days over the standard. Counties with weighted averages of 3.3 or higher (corresponding to approximately the 8-hour standard) received an “F.” These counties generally had at least 10 orange days or 9 days over the standard with at least one or more days in the red or purple category. The number of days for an “F” grade was set to roughly correlate with the number of days that would place a county in nonattainment for the ozone standard. For short-term particle pollution, the number of days required for an F would roughly approximate a 99th percentile form of the PM_2.5, a form the American Lung Association supports.

Grading System
Grade	Weighted Average	Approximate Number of Allowable Orange/Red/Purple days
A	0.0	None
B	0.3 to 0.9	1 to 2 orange days with no red
C	1.0 to 2.0	3 to 6 days over the standard: 3 to 5 orange with no more than 1 red OR 6 orange with no red
D	2.1 to 3.2	7 to 9 days over the standard: 7 total (including up to 2 red) to 9 orange with no red
F	3.3 or higher	9 days or more over the standard: 10 orange days or 9 total including at least 1 or more red or purple

Weighted averages allow comparisons to be drawn based on severity of air pollution. For example, if one county had 9 orange days and 0 red days, it would earn a weighted average of 3.0 and a D grade. However, another county which had only 8 orange days, but it also had 2 red days, which signify days with more serious air pollution, would receive a 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 standard and the 24-hour PM_2.5. EPA determines whether a county violates the standard based on the 4th 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, while still meeting EPA’s 1997 ozone standard or the 1-hour ozone standard set in 1979.

Year-round particle pollution (Annual PM_2.5)
Since no comparable Air Quality Index exists for year-round particle pollution (annual PM_2.5), the grading was based on EPA’s determination of violations of the national ambient air quality standard for annual PM_2.5 of 15 µg/m³, as reported October 12, 2005 in personal correspondence from Mark Schmidt, EPA. Counties that EPA listed as being in attainment of the standard were given grades of “Pass.” Counties EPA listed as being in nonattainment were given grades of “Fail.” Where insufficient data existed for EPA to determine attainment or nonattainment, those counties received a grade of “Incomplete.” Counties were ranked by design value. Metropolitan areas were ranked by the design value among the counties in the Census Bureau-defined Metropolitan Statistical Area as of 2003. The 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 or not the air quality in a county meets the standard.

With the assistance of the State and Territorial Air Pollution Program Administrators and the Association of Local Air Pollution Control Officials, all state and local agencies were provided the opportunity to review and comment on the data in draft tabular form. The Lung Association reviewed all discrepancies with the agencies and, if needed, with A.S.L. & Associates. Questions about the annual PM design values were referred to Mr. Schmidt of EPA, who reviewed and had final decision on those determinations. 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.

Methodology continued...