State ranking · ozone · 2025

States with the Fewest Ozone Air Days (2025)

Where ozone is the primary air-quality driver on the fewest days, averaged across each state's monitored counties from EPA AQS.

28.4
Fewest: Puerto Rico
12
States ranked

Puerto Rico averages the fewest ozone-primary AQI days (28.4/yr) among 12 states with ≥3 monitored counties

This ranking uses the 2025 EPA AQS county release: for each state with at least three monitored counties, we average the count of days ozone was the primary AQI pollutant. It is annual monitoring data, not a same-day AirNow advisory.

Data source EPA Air Quality System (AQS)

Puerto Rico ranks #1 with 28.4 ozone-primary days per year averaged across 8 monitored counties.

Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions, so it tends to dominate the Air Quality Index least in cooler, less-developed, or higher-latitude states. The figures below count the days each year that ozone was the primary AQI pollutant, averaged across the monitored counties in each state (states with at least three monitored counties). See the methodology for details.

from the compiled EPA AQS release in this database.

Fewest ozone-driven days, by state

Average days per year ozone was the primary AQI pollutant

  1. 1 Puerto Rico 28.4
  2. 2 Washington 38
  3. 3 Idaho 46.5
  4. 4 Oregon 48
  5. 5 Alaska 55.5
  6. 6 Montana 62.5
  7. 7 Hawaii 75.3
  8. 8 Georgia 87.5
  9. 9 Nebraska 96.5
  10. 10 Iowa 104.2
  11. 11 Minnesota 118.5
  12. 12 Alabama 121.7

Avg ozone-primary days/yr · EPA Air Quality System

# State Avg ozone days/yr Monitored counties
1 Puerto Rico 28.4 8
2 Washington 38 31
3 Idaho 46.5 19
4 Oregon 48 21
5 Alaska 55.5 8
6 Montana 62.5 23
7 Hawaii 75.3 3
8 Georgia 87.5 26
9 Nebraska 96.5 11
10 Iowa 104.2 15
11 Minnesota 118.5 23
12 Alabama 121.7 15

How to read this ranking

A low count does not always mean cleaner air overall: in some states fine-particle pollution (PM2.5) is the dominant problem instead of ozone, so ozone simply drives the index on fewer days. Pair this view with the cleanest-air ranking and each state's full profile to see the whole picture. Counts are averaged across a state's monitored counties and reflect the latest EPA reporting year.

Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS). Ozone days are days with ozone as the primary AQI pollutant, averaged across monitored counties per state.

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.