Population 100K+ · Ranked by Median AQI

Best Air Quality (Large Counties)

Among counties with populations over 100,000, these have the cleanest air based on EPA monitoring data. Grades based on 3-year average median Air Quality Index (2021–2023). Lower AQI is better.

Best Air Quality (Large Counties)

Coastal & Mountain Air Reigns Supreme

A clear pattern emerges: counties with top-tier air quality often hug coastlines or nestle near mountain ranges. Washington and Hawaii, with five and three counties respectively, exemplify this, benefiting from prevailing winds that disperse pollutants and less concentrated industrial activity. New York also makes a strong showing with seven counties, many of which are upstate, away from major urban centers, suggesting that proximity to natural ventilation plays a significant role in maintaining clean air.

Ohio's Unexpected Breath of Fresh Air

Columbiana County, Ohio, claiming the #1 spot, might surprise many, given its location in a state often associated with heavy industry. However, its position along the Ohio River and its relatively rural character, even for a large county, likely contribute to its exceptional air quality. This highlights that even in industrial regions, specific geographic and economic factors can create pockets of remarkably clean air, defying broader state-level perceptions.

The Desert Paradox: Clean Air Amidst Growth

Mohave County, Arizona, at #2, presents an interesting dynamic. Despite experiencing robust population and GDP growth, it maintains excellent air quality. This suggests that its vast desert landscape, low population density relative to its size, and limited heavy industry allow for efficient dispersion of any localized emissions, proving that growth doesn't always have to come at the expense of environmental health in certain geographic contexts.

# County BTI Score Air Grade Median AQI Good Days Unhealthy Days Population
01 Columbiana County OH 78 A 13 99% 0 101,723
02 Mohave County AZ 60 A 17 97% 2 214,229
03 Maui County HI 1 A 21 99% 1 164,765
04 Cowlitz County WA 71 A 21 91% 2 110,621
05 Monroe County MI 77 A 23 89% 2 154,823
06 Thurston County WA 40 A 25 91% 3 294,272
07 Kitsap County WA 35 B+ 25 91% 5 275,411
08 Deschutes County OR 17 B+ 25 78% 52 199,352
09 Lubbock County TX 44 B+ 27 85% 3 311,509
10 Oneida County NY 84 B+ 28 85% 7 231,055
11 Bonneville County ID 65 B+ 28 85% 10 124,490
12 Hawaii County HI 22 B+ 28 91% 0 202,163
13 Humboldt County CA 1 B+ 28 87% 11 136,132
14 Pueblo County CO 29 B+ 29 86% 5 168,135
15 Potter County TX 83 B+ 29 86% 3 117,905
16 Honolulu County HI 24 B+ 29 97% 1 1,010,100
17 Skagit County WA 55 B+ 30 91% 6 129,480
18 Whatcom County WA 49 B+ 30 91% 11 226,523
19 Kootenai County ID 67 B+ 31 75% 19 173,396
20 Nassau County NY 51 B+ 31 80% 5 1,389,160
21 Saratoga County NY 64 B+ 31 97% 1 236,328
22 Jefferson County NY 76 B+ 32 96% 1 117,445
23 Ascension Parish LA 50 B+ 32 91% 5 126,973
24 Clackamas County OR 39 B+ 33 84% 10 420,925
25 Oswego County NY 94 B+ 33 96% 1 118,037
26 Gallatin County MT 9 B+ 33 79% 19 119,685
27 Rockwall County TX 3 B+ 33 94% 0 110,631
28 Kennebec County ME 49 B+ 33 89% 1 124,003
29 Manatee County FL 7 B+ 33 96% 0 405,069
30 Linn County OR 69 B+ 34 73% 7 128,598
31 Penobscot County ME 69 B+ 34 87% 0 152,640
32 Highlands County FL 59 B+ 34 96% 2 102,339
33 St. Lucie County FL 49 B+ 34 98% 0 334,682
34 Yellowstone County MT 41 B+ 34 78% 16 165,524
35 Bay County MI 100 B+ 34 75% 6 103,752
36 Niagara County NY 52 B+ 34 94% 2 212,230
37 Dutchess County NY 45 B+ 34 95% 2 296,467
38 Flagler County FL 43 B+ 34 97% 0 117,515
39 Gregg County TX 59 B+ 34 88% 3 124,245
40 Androscoggin County ME 42 B+ 34 86% 0 111,532
41 Columbia County GA 11 B+ 34 97% 0 156,921
42 Indian River County FL 10 B+ 34 97% 0 160,986
43 Osceola County FL 7 B+ 34 95% 0 393,745
44 San Mateo County CA 2 B+ 34 83% 0 754,250
45 Hendricks County IN 52 B+ 35 94% 4 175,639
46 Maury County TN 17 B+ 35 77% 1 102,002
47 Ector County TX 89 B+ 35 77% 0 162,300
48 Kent County RI 38 B+ 35 87% 7 170,168
49 Clark County WA 36 B+ 35 78% 7 504,091
50 Brazoria County TX 16 B+ 35 86% 37 374,600