State air quality · 2025

Idaho Air Quality Ranking

Across 19 monitored counties, the typical county in Idaho posts a median AQI of 33 (Good) and 78.2% of monitored days were rated Good, from EPA AQS.

33
Avg median AQI
78.2%
Good-air days
19
Counties monitored
60%
Cleaner than US states

Idaho's typical county median AQI is 33 (Good) across 19 monitored counties

According to the U.S. EPA Air Quality System, Idaho monitors 19 counties with a day-weighted Good-day share of 78.2%. Cleanest: Blaine County (96.6% Good). Most polluted: Bonner County (56.2%). These are annual summaries, not live AirNow readings.

Data source EPA Air Quality System (AQS)

Cleaner than US states
Idaho's day-weighted Good-day share of 78.2% places it cleaner than 60% of US states.
In-state spread
Within Idaho, Custer County leads at 96.9% Good days while Bonner County sits at 56.2% across 19 monitored counties.
2024→2025 release
Idaho's Good-day share moved +6.0 points vs the 2024 EPA annual release.
Typical median AQI
Typical median AQI: 33

Verify with AirNow → · EPA AQS →

EPA state monitoring brief

AQS state key IDAHO

Idaho · 2025 EPA AQS · 19 counties · med AQI 33 · 78.2% Good · Blaine→Bonner spread

  • GOOD 33 Median AQI · Good
  • SHARE 78.2% Good-day share
  • COUNT 19 Counties monitored
  • UNH 8 Unhealthy+ days
  • RANK 60% Cleaner than states
  • PM25 84.9% Main pollutant share

Good-day share peers

North Dakota79.4%Idaho (this)78.2%West Virginia78.2%Florida77.4%
Good-day share neighbourhood · nearest ABS peers vs Idaho

Idaho's 78.2% Good-day share is cleaner than 60% of monitored US states; Blaine and Bonner County bracket the in-state range. Open Custer County →

According to EPA AQS annual summaries on PlainAirData, Idaho ranks #21 of 52 by day-weighted good-day share (78.2%) and #10 by mean county median AQI (32.8). 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%).

Data updated August 2026

What changed, what drives it, and how thick is the sample

Three derived views WPR-style word count does not substitute for: release-over-release movement from county annual rows, the EPA main-pollutant day mix, and how many counties sit below the 100-day ranking floor.

Release change

78.2%

Good-day share in 2025, up 6.0 percentage points vs 2024. County-mean median AQI 33, within half a point of the 2024 county-mean median.

Pollutant mix

PM2.5

Main driver on 84.9% of pollutant-assigned days.

  • PM2.5 84.9%
  • Ozone 13.5%
  • PM10 1.7%

Monitoring coverage

346

Average monitored days per county in the latest release. 1 of 19 counties sit below the 100-day floor used for cleanest/most-polluted claims.

Counties Monitored

19

Good Days %

78.2%

Avg Median AQI

33

Total Unhealthy Days

8

Best Air Quality

Blaine County

Worst Air Quality

Bonner County

Where Idaho sits nationally

COUNT-derived placement among 52 US states with at least 365 monitored days in the latest county release. Idaho's day-weighted Good-day share is 78.2% - cleaner than 60% of that comparison set.

Idaho Good-day share vs. every US state

Day-weighted share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release

78.2% Cleaner than 60% higher than 60% of 52 US states

0.0%–5.0%: 0 US states (0%). Below this entry. 5.0%–10.0%: 0 US states (0%). Below this entry. 10.0%–15.0%: 0 US states (0%). Below this entry. 15.0%–20.0%: 0 US states (0%). Below this entry. 20.0%–25.0%: 0 US states (0%). Below this entry. 25.0%–30.0%: 0 US states (0%). Below this entry. 30.0%–35.0%: 0 US states (0%). Below this entry. 35.0%–40.0%: 0 US states (0%). Below this entry. 40.0%–45.0%: 0 US states (0%). Below this entry. 45.0%–50.0%: 0 US states (0%). Below this entry. 50.0%–55.0%: 0 US states (0%). Below this entry. 55.0%–60.0%: 0 US states (0%). Below this entry. 60.0%–65.0%: 2 US states (4%). Below this entry. 65.0%–70.0%: 9 US states (17%). Below this entry. 70.0%–75.0%: 11 US states (21%). Below this entry. 75.0%–80.0%: 11 US states (21%). This entry sits in this band. 80.0%–85.0%: 13 US states (25%). Above this entry. 85.0%–90.0%: 5 US states (10%). Above this entry. 90.0%–95.0%: 1 US states (2%). Above this entry. 95.0%–100.0%: 0 US states (0%). Above this entry. 100.0%–105.0%: 0 US states (0%). Above this entry. This state 0.0% 100% every floor-qualified US state, bucketed by value

Each bar is a band; taller bars hold more US states. 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 U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025

Which counties in Idaho have the cleanest and most polluted air?

Cleanest counties in Idaho

Share of monitored days rated Good (AQI 0–50)

  1. 1 Custer County 96.9%
  2. 2 Blaine County 96.6%
  3. 3 Franklin County 96.3%
  4. 4 Bonneville County 93%
  5. 5 Twin Falls County 92.9%
  6. 6 Butte County 90.1%
  7. 7 Bannock County 88.8%
  8. 8 Latah County 83.4%
  9. 9 Kootenai County 77.8%
  10. 10 Canyon County 77.1%
  11. 11 Ada County 74%
  12. 12 Valley County 73.3%

Good-day share · EPA Air Quality System

County Good % Median AQI
Custer 96.9% 10
Blaine 96.6% 17
Franklin 96.3% 21
Bonneville 93.0% 22
Twin Falls 92.9% 26
Butte 90.1% 42
Bannock 88.8% 36
Latah 83.4% 31
Kootenai 77.8% 31
Canyon 77.1% 36
Ada 74.0% 42
Valley 73.3% 34
Nez Perce 72.5% 33
Idaho 71.8% 40
Lemhi 71.5% 29
Boise 70.4% 42
Benewah 63.4% 41
Shoshone 56.2% 44
Bonner 56.2% 47

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025. See our methodology for how AQI and median figures are computed.

Coverage: 19 counties in Idaho with active monitoring stations. Unmonitored counties are not represented.

Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files, aggregated to the state level · 2020–2025

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.

Informational only, not professional advice. Data from EPA AQS. See our disclaimer.