National rankings · 2025 EPA AQS

Kenai Peninsula County, Alaska leads; Maricopa County trails U.S. air quality

The cleanest air in the country sits in Kenai Peninsula County, Alaska (100.0% Good days); the most polluted in Maricopa County, Arizona (8.8%). Across 951 monitored counties the spread, not the average, is the real story.

100.0%
#1 cleanest · Kenai Peninsula Co.
8.8%
#1 worst · Maricopa Co.
951
Counties in release
74.8%
National Good-day share

The national picture

The cleanest air in the country sits in Kenai Peninsula County, Alaska (100.0% Good days); the most polluted in Maricopa County, Arizona (8.8%). Across 951 monitored counties the spread, not the average, is the real story.

100.0%
cleanest · Kenai Peninsula Co.
8.8%
most polluted · Maricopa Co.
74.8%
of monitored days rated Good
Hawaii
cleanest state · 93.8%

Widest state gap: Hawaii (93.8% of days Good) vs Arizona (60.7%).

Two rankings, two leaders

According to EPA AQS annual summaries on PlainAirData, Hawaii is #1 of 52 by day-weighted good-day share (93.8% of monitored days) at mean county median-AQI #4 (27.3) ≠ Alaska #1 lowest mean county median AQI (20.3) at good-day share #8 (84.4%). According to the 2025 EPA AQS county release on PlainAirData, Ponce County, Puerto Rico is #1 of 951 by median AQI (3, 91.4% Good days) at good-day-share #159 ≠ largest of 7 at 100.0% Good days #1 by good-day share (100.0%, median AQI 9) at median-AQI #7.

Day-weighted good-day share and mean county median AQI use different denominators; county median AQI and good-day share can crown different places.

What it takes to make the Top 25 cleanest counties list

#25 cutoff: Fauquier County, Virginia at 97.5% Good days among counties with at least 100 monitored days

97.5% Top 3% higher than 97% of 951 U.S. counties

0.0%–5.0%: 0 U.S. counties (0%). Below this entry. 5.0%–10.0%: 1 U.S. counties (0%). Below this entry. 10.0%–15.0%: 4 U.S. counties (0%). Below this entry. 15.0%–20.0%: 4 U.S. counties (0%). Below this entry. 20.0%–25.0%: 1 U.S. counties (0%). Below this entry. 25.0%–30.0%: 5 U.S. counties (1%). Below this entry. 30.0%–35.0%: 9 U.S. counties (1%). Below this entry. 35.0%–40.0%: 4 U.S. counties (0%). Below this entry. 40.0%–45.0%: 16 U.S. counties (2%). Below this entry. 45.0%–50.0%: 16 U.S. counties (2%). Below this entry. 50.0%–55.0%: 32 U.S. counties (3%). Below this entry. 55.0%–60.0%: 50 U.S. counties (5%). Below this entry. 60.0%–65.0%: 77 U.S. counties (8%). Below this entry. 65.0%–70.0%: 93 U.S. counties (10%). Below this entry. 70.0%–75.0%: 109 U.S. counties (11%). Below this entry. 75.0%–80.0%: 114 U.S. counties (12%). Below this entry. 80.0%–85.0%: 123 U.S. counties (13%). Below this entry. 85.0%–90.0%: 99 U.S. counties (10%). Below this entry. 90.0%–95.0%: 117 U.S. counties (12%). Below this entry. 95.0%–100.0%: 70 U.S. counties (7%). This entry sits in this band. 100.0%–105.0%: 7 U.S. counties (1%). Above this entry. #25: Fauquier 0.0% 100% every floor-qualified U.S. county, bucketed by value

Each bar is a band; taller bars hold more U.S. counties. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.

Source EPA Air Quality System (AQS) · 2025

The 10 most polluted U.S. counties

Ranked by median Air Quality Index, the typical day's AQI across the year (higher is worse)

  1. 1 Riverside, California 80
  2. 2 San Bernardino, California 77
  3. 3 Los Angeles, California 74
  4. 4 Maricopa, Arizona 73
  5. 5 Kern, California 70
  6. 6 El Paso, Texas 70
  7. 7 Pinal, Arizona 67
  8. 8 San Diego, California 67
  9. 9 Tulare, California 64
  10. 10 Imperial, California 64

Median AQI · EPA Air Quality System (AQS), 2025

Cleanest countiesTop 50 by good-day share. Most polluted countiesTop 50 by lowest Good-day share. Cleanest metro areasMetros by good-day share. Most improved countiesBiggest good-day-share gain. Biggest county declinesLargest good-day-share drop.

Top 25 Cleanest Counties

# County State Good % Median AQI
1 Kenai Peninsula Alaska 100.0% 9
2 Sonoma California 100.0% 9
3 Martin North Carolina 100.0% 31
4 Karnes Texas 100.0% 6
5 Wilson Texas 100.0% 8
6 Carbon Wyoming 100.0% 8
7 Uinta Wyoming 100.0% 9
8 Pacific Washington 99.7% 16
9 Denali Alaska 99.7% 33
10 Navajo Arizona 99.5% 13
11 Lake California 99.2% 31
12 Solano California 99.2% 33
13 Lincoln Wyoming 98.9% 10
14 Taos New Mexico 98.9% 16
15 Liberty Florida 98.5% 33
16 Flagler Florida 98.1% 34
17 Gallatin Montana 98.1% 11
18 Maui Hawaii 98.1% 21
19 Park Wyoming 98.0% 13
20 Elmore Alabama 97.9% 34
21 Prince Edward Virginia 97.8% 33
22 Mayagnez Puerto Rico 97.8% 9
23 Knox Maine 97.8% 33
24 Lenoir North Carolina 97.5% 35
25 Fauquier Virginia 97.5% 34

Top 25 Most Polluted Counties

# County State Good % Max AQI
1 Maricopa Arizona 8.8% 366
2 Los Angeles California 10.7% 201
3 Riverside California 10.7% 365
4 Harris Texas 11.0% 172
5 San Bernardino California 12.6% 207
6 Pinal Arizona 17.0% 1215
7 Kern California 17.5% 200
8 San Diego California 17.5% 161
9 El Paso Texas 18.1% 2122
10 Tulare California 24.1% 147
11 Bernalillo New Mexico 26.6% 204
12 Dona Ana New Mexico 27.4% 528
13 Imperial California 27.7% 426
14 Fresno California 28.8% 164
15 Cameron Texas 29.9% 139
16 Clark Nevada 31.0% 138
17 Pulaski Arkansas 31.2% 138
18 Tarrant Texas 31.8% 162
19 Butler Ohio 33.1% 151
20 Salt Lake Utah 33.1% 175
21 Pima Arizona 33.7% 357
22 St. Louis City Missouri 34.3% 738
23 Hamilton Ohio 34.3% 154
24 Kings California 34.5% 133
25 Wayne Michigan 36.2% 156

Top 25 Cleanest Metro Areas

# Metro Area Good % Median AQI
1 Evanston, WY 100.0% 9
2 Houghton, MI 100.0% 8
3 Los Alamos, NM 100.0% 13
4 Salem, OH 100.0% 12
5 Santa Rosa, CA 100.0% 9
6 Show Low, AZ 99.5% 13
7 Clearlake, CA 99.2% 31
8 Vallejo-Fairfield, CA 99.2% 33
9 Taos, NM 98.9% 16
10 Bozeman, MT 98.1% 11
11 Kahului-Wailuku-Lahaina, HI 98.1% 21
12 Guayama, PR 98.0% 19
13 Mayaguez, PR 97.8% 9
14 Rockland, ME 97.8% 33
15 Kinston, NC 97.5% 35
16 Mount Vernon-Anacortes, WA 97.5% 25
17 Aberdeen, WA 97.3% 22
18 Urban Honolulu, HI 97.3% 29
19 Concord, NH 96.7% 37
20 Arkadelphia, AR 96.7% 34
21 Sevierville, TN 96.7% 40
22 Hailey, ID 96.6% 17
23 Morehead City, NC 96.2% 35
24 Vineyard Haven, MA 95.9% 35
25 Palatka, FL 95.6% 21

All States Ranked by Air Quality

# State Avg Good % Avg AQI
1 Hawaii 93.8% 27
2 Maine 87.0% 35
3 Virginia 86.5% 34
4 Montana 85.9% 25
5 Wyoming 85.1% 31
6 Nebraska 85.0% 32
7 New Hampshire 84.9% 37
8 Alaska 84.4% 20
9 Puerto Rico 84.0% 22
10 Vermont 83.8% 32
11 Massachusetts 83.1% 38
12 Rhode Island 83.0% 36
13 Washington 82.2% 30
14 Maryland 81.8% 40
15 New York 81.3% 37
16 North Carolina 81.0% 39
17 South Dakota 80.8% 35
18 Kentucky 80.6% 39
19 Tennessee 80.3% 38
20 North Dakota 79.4% 38
21 Idaho 78.2% 33
22 West Virginia 78.2% 39
23 Florida 77.4% 41
24 Oregon 76.9% 32
25 Minnesota 76.8% 35
26 Connecticut 76.7% 40
27 Nevada 76.5% 41
28 Wisconsin 76.5% 38
29 Missouri 75.9% 40
30 New Jersey 75.5% 40
31 Kansas 74.7% 39
32 Michigan 74.7% 38
33 Louisiana 74.6% 41
34 Delaware 74.1% 41
35 Alabama 73.9% 40
36 Pennsylvania 73.8% 40
37 Mississippi 72.9% 42
38 Indiana 71.8% 41
39 Colorado 71.7% 40
40 South Carolina 71.3% 43
41 Arkansas 71.2% 45
42 Illinois 70.0% 42
43 New Mexico 69.1% 39
44 Oklahoma 68.1% 43
45 Ohio 67.0% 43
46 Iowa 66.8% 42
47 District of Columbia 66.0% 45
48 Texas 65.6% 42
49 Utah 65.4% 42
50 Georgia 65.0% 43
51 California 61.2% 46
52 Arizona 60.7% 45

Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), 2025

How to read these rankings

A ranking tells you where to start looking, not whether a place is safe to live.

  • Rankings here are sorted by Good-day share (the % of monitored days with AQI 0–50), so the top of a list is genuinely the cleanest air, not just the most data. Methodology
  • Monitor coverage varies: a county with few stations is described by a thinner sample, so compare like with like before concluding. Understand AQI
  • Looking for one place? Jump to its county or state profile for the full distribution, primary pollutant, and five-year trend. All counties

All rankings recompute directly from EPA AQS annual summaries (2020–2025) - no editorial curation, no advertiser influence. Found an error? The contact page has our correction process.

How These Air-Quality Rankings Are Compiled

Every ranking on this page is computed directly from the EPA Air Quality System (AQS) annual summary files, not editorially curated, and not influenced by advertisers. Counties and metros are ordered by the share of monitored days that fell in the "Good" band (AQI 0–50); states are ordered by the average of their counties' Good-day shares. The underlying numbers, Good %, median AQI, max AQI, monitored days, are visible on every county, metro, and state page so you can verify them. We recompute the rankings whenever EPA publishes a new annual release, and we document that cadence on the methodology page.

What These Rankings Mean (and What They Do Not)

Good-day share is a useful lens for comparing places, but it has limits. It reflects only the days a county's monitors actually reported, so a lightly monitored county is described by a thinner sample than one blanketed with stations. It also says nothing about which pollutant drove the bad days, ozone in summer behaves very differently from wildfire PM2.5, so a county near the bottom of one list may be perfectly fine for most of the year outside a short smoke season. Where a single year's wildfire activity skews a county, the five-year trend on its profile page tells the fuller story.

Why We Publish These Rankings

Air-quality data is published by the EPA as enormous annual CSV files that few people can read directly. Most visitors arrive with a concrete question - "where is the cleanest air?", "how does my county compare?", "which cities have the worst smog?" - and benefit from seeing a ranked, linked list backed by the official numbers. Publishing ranked views of public environmental data is long-established civic-data practice; we surface the raw figures, link to the EPA source, and avoid editorial spin. If a ranking ever reads as a value judgment the data does not support, email us via the contact page and we will review the wording.

Methodology, Sources, and Corrections

Every ranking is derived from the EPA AQS annual summaries linked on the methodology page. We do not blend proprietary signals and we do not substitute editor opinion for the data. If you believe a figure is miscomputed or a county is misclassified, contact us with the specific county or metro and the expected correction, and we will investigate against the source files. Corrections that affect a published ranking are rolled forward immediately; minor formatting fixes go out with the next scheduled refresh.

Download the EPA AQS annual county AQI extract cited on this page: annual-aqi-by-county.csv or the JSON distribution (public domain / CC0). Method notes live on /methodology and /data.

Every figure on PlainAirData is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.