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Appendix A: Description of Methodologycont'd
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 (MSA). Therefore, comparisons of MSAs from the American Lung Association State of the Air 2005 or any State of the Air reports prior to 2004 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.
|
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 standare: 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 used 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 the 8-hour ozone standard set in 1997 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 PM2.5 of 15 µg/m3, as reported in communication from EPA’s Office of Air Quality Planning and Standards in February, 2005 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.
Methodology continued... |
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