TABLE 1. Air-quality measures in urban and rural counties — United States, 2008–2012.
| County urban-rural category* No. (%) | Air-quality measure† |
|||||
|---|---|---|---|---|---|---|
| Ozone days§ |
PM2.5 days¶ |
Average PM2.5 concentration (µg/m3)** |
||||
| Mean (SE)†† | Estimate§§ (95% CI) | Mean (SE)†† | Estimate§§ (95% CI) | Mean (SE)†† | Estimate§§ (95% CI) | |
| Large central metropolitan
68 (100) |
47.54 (1.12) |
Reference |
11.21 (0.25) |
Reference |
11.15 (0.03) |
Reference |
| Large fringe metropolitan
368 (100) |
20.67 (0.09) |
0.62 (0.59 to 0.65)
|
2.82 (0.01) |
0.54 (0.49 to 0.59)
|
10.51 (0.004) |
-1.18 (-1.4 to -0.97)
|
| Medium metropolitan
370 (99) |
13.89 (0.10) |
0.45 (0.43 to 0.47)
|
4.93 (0.05) |
0.76 (0.7 to 0.83)
|
10.17 (0.004) |
-1.33 (-1.55 to -1.12)
|
| Small metropolitan
355 (99) |
7.95 (0.04) |
0.32 (0.3 to 0.33)
|
3.55 (0.03) |
0.62 (0.56 to 0.68)
|
9.85 (0.004) |
-1.63 (-1.85 to -1.41)
|
| Micropolitan
637 (99) |
5.83 (0.01) |
0.23 (0.22 to 0.24)
|
1.78 (0.01) |
0.35 (0.32 to 0.39)
|
9.47 (0.003) |
-1.92 (-2.14 to -1.71)
|
| Noncore 1,311 (98) | 3.81 (0.005) | 0.17 (0.16 to 0.18) | 0.95 (0.002) | 0.23 (0.21 to 0.25) | 8.87 (0.001) | -2.29 (-2.49 to -2.08) |
Abbreviations: CI = confidence interval; EPA = U.S. Environmental Protection Agency; MSA = metropolitan statistical area; NAAQS = National Ambient Air Quality Standards; PM2.5 = particulate matter ≤2.5 microns in diameter (fine particulate matter); PPM = parts per million; SE = standard error.
* Number represents the number of counties in the data by urban-rural classification. Percent represents the percentage of U.S. counties in that urban-rural classification in the data. The metropolitan categories are large metropolitan (MSA population ≥1 million), medium metropolitan (MSA population 250,000–999,999) and small metropolitan (MSA population <250,000). Large metropolitan counties are subdivided into large central metropolitan counties and large fringe metropolitan counties based on the location and size of the MSA principal city. The nonmetropolitan categories are micropolitan (counties in a micropolitan statistical area defined as urban clusters with a population of 2,500–49,999) and noncore (counties not in a micropolitan statistical area) (Source: Ingram DD, Franco SJ. 2013 NCHS urban-rural classification scheme for counties. Vital Health Stat 2 2014;166:1–73).
† Data for all three measures were obtained from the National Environmental Public Health Tracking Network and include 2008–2012 combined.
§ Total number of days with maximum 8-hour average ozone concentrations greater than EPA’s NAAQS of 0.070 ppm.
¶ Total number days with 24-hour average PM2.5 levels greater than EPA’s NAAQS of 35 µg/m3.
** Annual average ambient PM2.5 concentration (µg/m3).
†† Mean and mean SE of the air-quality measure for all counties in that urban-rural classification.
§§ Effect estimates for the total number of PM2.5 days and the total number of ozone days are odds ratios calculated with Poisson regression. Effect estimates for average PM2.5 concentration were calculated with linear regression. Estimates in bold are significant (based on the 95% CIs).