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 78 A 13 99% 0 100,704
02 Mohave County AZ 53 A 17 97% 2 220,517
03 Maui County HI 36 A 21 99% 1 164,522
04 Cowlitz County WA 57 A 21 91% 2 112,360
05 Monroe County MI 58 A 23 89% 2 155,505
06 Thurston County WA 20 A 25 91% 3 299,067
07 Kitsap County WA 82 B+ 25 91% 5 277,881
08 Deschutes County OR 51 B+ 25 78% 52 206,334
09 Lubbock County TX 26 B+ 27 85% 3 318,884
10 Oneida County NY 78 B+ 28 85% 7 229,124
11 Bonneville County ID 61 B+ 28 85% 10 129,523
12 Hawaii County HI 40 B+ 28 91% 0 205,769
13 Humboldt County CA 2 B+ 28 87% 11 134,541
14 Pueblo County CO 11 B+ 29 86% 5 169,356
15 Grant County WA 2 B+ 29 81% 15 101,799
16 Potter County TX 97 B+ 29 86% 3 115,975
17 Honolulu County HI 49 B+ 29 97% 1 1,001,146
18 Skagit County WA 47 B+ 30 91% 6 131,328
19 Whatcom County WA 11 B+ 30 91% 11 230,503
20 Nassau County NY 76 B+ 31 80% 5 1,389,591
21 Kootenai County ID 69 B+ 31 75% 19 181,996
22 Saratoga County NY 79 B+ 31 97% 1 238,284
23 Jefferson County NY 96 B+ 32 96% 1 115,040
24 Ascension Parish LA 42 B+ 32 91% 5 130,314
25 Clackamas County OR 72 B+ 33 84% 10 423,975
26 Oswego County NY 98 B+ 33 96% 1 117,953
27 Gallatin County MT 37 B+ 33 79% 19 124,074
28 Rockwall County TX 17 B+ 33 94% 0 123,617
29 Manatee County FL 21 B+ 33 96% 0 429,792
30 Kennebec County ME 13 B+ 33 89% 1 126,808
31 St. Lucie County FL 88 B+ 34 98% 0 360,500
32 Yellowstone County MT 72 B+ 34 78% 16 168,957
33 Penobscot County ME 68 B+ 34 87% 0 154,710
34 Highlands County FL 47 B+ 34 96% 2 105,702
35 Linn County OR 42 B+ 34 73% 7 130,706
36 Niagara County NY 87 B+ 34 94% 2 210,721
37 Dutchess County NY 85 B+ 34 95% 2 298,220
38 Flagler County FL 84 B+ 34 97% 0 126,528
39 Bay County MI 52 B+ 34 75% 6 103,008
40 Osceola County FL 51 B+ 34 95% 0 427,415
41 Indian River County FL 38 B+ 34 97% 0 166,936
42 Gregg County TX 28 B+ 34 88% 3 125,480
43 San Mateo County CA 15 B+ 34 83% 0 742,340
44 Androscoggin County ME 12 B+ 34 86% 0 113,423
45 Columbia County GA 9 B+ 34 97% 0 162,434
46 Hendricks County IN 38 B+ 35 94% 4 183,344
47 Maury County TN 22 B+ 35 77% 1 107,791
48 Kent County RI 69 B+ 35 87% 7 171,456
49 Clark County WA 62 B+ 35 78% 7 516,959
50 Montgomery County TN 35 B+ 35 78% 2 234,153
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Counties ranked
Cite this ranking

BoomTownIndex, “Best Air Quality (Large Counties).” Data: April 2026. https://boomtownindex.com/rankings/best-air-quality/

Free to reference with a link. See methodology →