Population 100K+ · Ranked by Median AQI · Data updated April 2026

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 42 A 13 99% 0 100,704
02 Mohave County AZ 92 A 17 97% 2 220,517
03 Maui County HI 62 A 21 99% 1 164,522
04 Cowlitz County WA 82 A 21 91% 2 112,360
05 Monroe County MI 36 A 23 89% 2 155,505
06 Thurston County WA 66 A 25 91% 3 299,067
07 Kitsap County WA 75 B+ 25 91% 5 277,881
08 Deschutes County OR 37 B+ 25 78% 52 206,334
09 Lubbock County TX 30 B+ 27 85% 3 318,884
10 Hawaii County HI 91 B+ 28 91% 0 205,769
11 Oneida County NY 80 B+ 28 85% 7 229,124
12 Bonneville County ID 61 B+ 28 85% 10 129,523
13 Humboldt County CA 1 B+ 28 87% 11 134,541
14 Pueblo County CO 14 B+ 29 86% 5 169,356
15 Grant County WA 6 B+ 29 81% 15 101,799
16 Potter County TX 89 B+ 29 86% 3 115,975
17 Honolulu County HI 73 B+ 29 97% 1 1,001,146
18 Skagit County WA 87 B+ 30 91% 6 131,328
19 Whatcom County WA 23 B+ 30 91% 11 230,503
20 Kootenai County ID 91 B+ 31 75% 19 181,996
21 Nassau County NY 33 B+ 31 80% 5 1,389,591
22 Saratoga County NY 61 B+ 31 97% 1 238,284
23 Jefferson County NY 85 B+ 32 96% 1 115,040
24 Ascension Parish LA 19 B+ 32 91% 5 130,314
25 Clackamas County OR 76 B+ 33 84% 10 423,975
26 Oswego County NY 98 B+ 33 96% 1 117,953
27 Gallatin County MT 32 B+ 33 79% 19 124,074
28 Rockwall County TX 4 B+ 33 94% 0 123,617
29 Manatee County FL 58 B+ 33 96% 0 429,792
30 Kennebec County ME 16 B+ 33 89% 1 126,808
31 St. Lucie County FL 97 B+ 34 98% 0 360,500
32 Penobscot County ME 71 B+ 34 87% 0 154,710
33 Highlands County FL 69 B+ 34 96% 2 105,702
34 Linn County OR 41 B+ 34 73% 7 130,706
35 Yellowstone County MT 21 B+ 34 78% 16 168,957
36 Niagara County NY 87 B+ 34 94% 2 210,721
37 Flagler County FL 73 B+ 34 97% 0 126,528
38 Dutchess County NY 57 B+ 34 95% 2 298,220
39 Bay County MI 50 B+ 34 75% 6 103,008
40 Indian River County FL 89 B+ 34 97% 0 166,936
41 Osceola County FL 82 B+ 34 95% 0 427,415
42 Androscoggin County ME 53 B+ 34 86% 0 113,423
43 Gregg County TX 45 B+ 34 88% 3 125,480
44 San Mateo County CA 9 B+ 34 83% 0 742,340
45 Columbia County GA 7 B+ 34 97% 0 162,434
46 Hendricks County IN 32 B+ 35 94% 4 183,344
47 Maury County TN 3 B+ 35 77% 1 107,791
48 Kent County RI 54 B+ 35 87% 7 171,456
49 Clark County WA 49 B+ 35 78% 7 516,959
50 Brazoria County TX 23 B+ 35 86% 37 391,255