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U.S. Air Quality Rankings

Best and worst counties for air quality based on EPA Air Quality System (AQS) annual data, drawn from regulatory monitors reporting across more than 3,000 US counties. See our methodology for how the rankings are computed. Rankings use the Air Quality Index (AQI) - tracking good days, unhealthy days, and peak readings.

National Summary (2024): EPA monitors air quality in 1,017 U.S. counties. The national average median AQI is 38.8 and 75.5% of monitored days are rated "Good" (AQI 0–50). The county with the most unhealthy air days is San Bernardino, CA (88 days above AQI 100).

Counties Monitored
1,017
Avg Median AQI (2024)
38.8
Avg Good Days %
75.5%
Data Year
2024

Browse Air Quality by State

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AQI Scale Reference

0–50
Good
51–100
Moderate
101–150
Unhealthy for Sensitive Groups
151–200
Unhealthy
201–300
Very Unhealthy
301+
Hazardous

Worst Air Quality Counties

Counties ranked by most days with AQI above 100 (Unhealthy or worse). Data: EPA AQS 2024.

# County Unhealthy Days Median AQI
1 San Bernardino 88 87
2 Riverside 67 87
3 Los Angeles 55 80
4 Boise 32 48
5 Tulare 28 71
6 Kern 21 72
7 Harney 20 46
8 Fresno 14 67
9 Grant 14 49
10 Harris 14 61
11 Inyo 13 61
12 Dona Ana 13 62
13 Cameron 13 61
14 El Paso 12 67
15 Catano Municipio 11 51
16 Fairbanks North Star 10 42
17 Plumas 9 48
18 Maricopa 8 77
19 Imperial 8 63
20 Jefferson 7 50
21 Ada 7 44
22 Bernalillo 7 61
23 Deschutes 7 39
24 Klamath 7 33
25 Sheridan 7 44
26 Powder River 6 44
27 Canyon 6 36
28 Ravalli 6 24
29 Lane 6 44
30 Tarrant 6 54
31 Pinal 5 77
32 Hidalgo 5 54
33 Salt Lake 5 54
34 San Diego 4 71
35 Douglas 4 48
36 Lemhi 4 37
37 Essex 4 37
38 Asotin 4 42
39 Orange 3 54
40 Denver 3 55
41 Fairfield 3 42
42 Miami-dade 3 48
43 Idaho 3 42
44 Twin Falls 3 23
45 Eddy 3 49
46 Dunn 3 37
47 Crook 3 24
48 Shelby 3 55
49 Collin 3 40
50 Dallas 3 55

Best Air Quality Counties

Counties with the most good-air days (AQI 0–50) and lowest median AQI. Data: EPA AQS 2024.

# County Good Days Median AQI
1 Routt 364 12
2 Pittsylvania 363 3
3 Karnes 363 7
4 Maui 363 21
5 Lake 361 32
6 Honolulu 360 31
7 Navajo 359 13
8 Prince Edward 358 33
9 Skagit 357 26
10 Taylor 357 34
11 Denali Borough 357 35
12 Carteret 357 35
13 Carbon 356 8
14 Hawaii 355 21
15 Fremont 354 13
16 Taos 354 16
17 Columbia 354 26
18 Rosebud 353 4
19 Grays Harbor 353 20
20 Oswego 353 32
21 St Lucie 353 35
22 Stafford 353 35
23 Jefferson 352 32
24 Washington 351 34
25 Apache 350 14
26 Monroe 350 36
27 Cowlitz 349 22
28 Archuleta 349 32
29 Sonoma 348 19
30 Macon 348 34
31 Mohave 347 19
32 York 346 34
33 Flagler 344 35
34 Indian River 344 35
35 Caguas 343 3
36 Knox 343 33
37 Elk 343 34
38 Solano 343 36
39 Tucker 343 37
40 Macoupin 342 35
41 Mayaguez Municipio 341 11
42 Luna 341 17
43 Mono 341 19
44 Kitsap 341 19
45 Mason 341 21
46 Wexford 341 34
47 Okaloosa 341 36
48 Hughes 340 8
49 Lincoln 340 31
50 Hamilton 340 33

Frequently Asked Questions

What is AQI and how is it measured?

The Air Quality Index (AQI) is a 0–500 scale used by the EPA to communicate daily air quality. AQI 0–50 is "Good," 51–100 is "Moderate," 101–150 is "Unhealthy for Sensitive Groups," 151–200 is "Unhealthy," and above 201 is "Very Unhealthy" or "Hazardous." It factors in ozone, particulate matter, carbon monoxide, sulfur dioxide, and nitrogen dioxide.

How does air quality vary across U.S. states?

Air quality varies significantly by region. Western states including California and Oregon often see elevated AQI from wildfire smoke in summer. Industrial corridors in the Midwest and Southeast face higher ozone and particulate levels. Mountain and rural areas generally have the cleanest air. The national average median AQI is 38.8 for 2024.

About This Data

Air quality data is from the EPA Air Quality System (AQS), Annual AQI by County dataset. Only counties with at least 50 monitored days are included to ensure statistical reliability. "Unhealthy days" counts days with AQI above 100 (Unhealthy for Sensitive Groups or worse). Data covers 2024 readings from EPA monitoring stations.

Source: U.S. Environmental Protection Agency, Air Quality System (AQS) U.S. Environmental Protection Agency, Air Quality System (AQS)