Air Quality Rankings for Idaho (2026)
Idaho has 20 cities tracked by EPA air-quality monitors, with a state-wide 5-year median AQI of 33 — 8 points cleaner than the national average of AQI 41. Custer, Idaho ranks #1 with the cleanest air (AQI 8, Grade A), while Benewah, Idaho sits at the bottom (AQI 48, Grade C).
How Idaho Compares
Idaho has 20 cities tracked by EPA air-quality monitors, with a state-wide 5-year median AQI of 33 — 8 points cleaner than the national average of AQI 41. Custer, Idaho ranks #1 with the cleanest air (AQI 8, Grade A), while Benewah, Idaho sits at the bottom (AQI 48, Grade C). The rankings below are computed from the EPA Air Quality System (AQS), which aggregates daily AQI readings from federally certified monitors into annual averages. Cities are sorted by 5-year median AQI (lowest = cleanest = #1). The 5-year window smooths out year-to-year volatility from weather and wildfire events.
Idaho is bucking the national trend of broad improvement: 11 of 20 monitored cities show measurably worse air over the past decade, more than the 9 that are improving. Across western states this usually traces back to expanding wildfire smoke exposure; elsewhere it can reflect rising local emissions from population or freight growth.
The dominant pollutant across 18 of 20 Idaho cities is Fine Particulate Matter (PM2.5). PM2.5 (fine particulate matter) is most often driven by combustion sources — vehicle exhaust, industrial emissions, residential wood burning, and increasingly wildfire smoke. It penetrates deep into lung tissue and the bloodstream and is the air pollutant most strongly linked to long-term health impacts. Other Idaho cities report Ground-Level Ozone (2) as their dominant concern.
The fastest-improving city in Idaho is Lemhi, Idaho, with median AQI falling by 1.4 points per year. Steady improvement at that pace usually reflects fleet turnover (older diesels retiring), upwind power-plant retirements, or tighter regional emissions controls.
The city with the steepest decline is Valley, Idaho, where median AQI is rising by 2.8 points per year. Rapid deterioration in a single city usually points to either wildfire-smoke exposure (in the West) or a new local emissions source — a power plant, port, or freight corridor coming online.
Full Idaho Ranking
| # | City | 5yr Avg AQI | Current AQI | Worst Pollutant | Trend | Grade |
|---|---|---|---|---|---|---|
| 1 | Custer, Idaho | 8 | 8 | PM2.5 | Stable | A |
| 2 | Jerome, Idaho | 17 | 13 | PM2.5 | Improving | B |
| 3 | Latah, Idaho | 18 | 22 | PM2.5 | Worsening | B |
| 4 | Blaine, Idaho | 21 | 19 | PM2.5 | Worsening | C |
| 5 | Franklin, Idaho | 25 | 24 | PM2.5 | Improving | B |
| 6 | Twin Falls, Idaho | 27 | 25 | PM2.5 | Improving | B |
| 7 | Bonneville, Idaho | 30 | 25 | PM2.5 | Improving | B |
| 8 | Kootenai, Idaho | 30 | 32 | PM2.5 | Improving | B |
| 9 | Nez Perce, Idaho | 33 | 36 | PM2.5 | Worsening | C |
| 10 | Bonner, Idaho | 33 | 34 | PM2.5 | Worsening | C |
| 11 | Valley, Idaho | 37 | 36 | PM2.5 | Worsening | D |
| 12 | Bannock, Idaho | 37 | 41 | PM2.5 | Worsening | C |
| 13 | Lemhi, Idaho | 40 | 32 | PM2.5 | Improving | B |
| 14 | Canyon, Idaho | 41 | 34 | PM2.5 | Worsening | C |
| 15 | Butte, Idaho | 42 | 44 | Ozone | Worsening | C |
| 16 | Idaho, Idaho | 42 | 44 | PM2.5 | Worsening | C |
| 17 | Boise, Idaho | 43 | 39 | PM2.5 | Stable | C |
| 18 | Shoshone, Idaho | 43 | 47 | PM2.5 | Improving | B |
| 19 | Ada, Idaho | 45 | 44 | Ozone | Worsening | C |
| 20 | Benewah, Idaho | 48 | 48 | PM2.5 | Stable | C |
Air quality data for Idaho is sourced from the EPA Air Quality System (AQS), which monitors outdoor air quality at thousands of stations nationwide.
Frequently Asked Questions
Custer, Idaho has the best air quality in Idaho with a 5-year average AQI of 8 and a Grade A (80/100). Its dominant pollutant is Fine Particulate Matter (PM2.5) and the long-run trend is stable.
Benewah, Idaho has the worst air quality in Idaho with a 5-year average AQI of 48 and a Grade C (58/100). Its dominant pollutant is Fine Particulate Matter (PM2.5).
Idaho has 20 cities with EPA air quality monitoring data, covering 2014-2023 of daily AQI measurements aggregated into annual averages.
Idaho's state-wide 5-year median AQI is 33, 8 points cleaner than the national average of AQI 41. Idaho is bucking the national trend of broad improvement: 11 of 20 monitored cities show measurably worse air over the past decade, more than the 9 that are improving. Across western states this usually traces back to expanding wildfire smoke exposure; elsewhere it can reflect rising local emissions from population or freight growth.
Fine Particulate Matter (PM2.5) is the dominant pollutant in 18 of 20 monitored Idaho cities. PM2.5 (fine particulate matter) is most often driven by combustion sources — vehicle exhaust, industrial emissions, residential wood burning, and increasingly wildfire smoke. It penetrates deep into lung tissue and the bloodstream and is the air pollutant most strongly linked to long-term health impacts.
Idaho cities log an average of 6 days per year at "Unhealthy for Sensitive Groups" or worse, based on EPA monitor data over the last five years. Across all 20 Idaho cities tracked, that totals 573 unhealthy days over the period.
Cities ranked by 5-year average AQI (lower is better). Grades factor in average AQI, trend direction, unhealthy days, and dominant pollutant.
The this entity category groups every U.S. air quality and pollution monitoring entity sharing this attribute. The list above is the data; the paragraphs below explain what the grouping means against the broader the EPA Air Quality System (AQS) distribution and how to read the relative rankings within the category.
For readers using this category as a starting point, the per-entity detail pages linked from the table above carry the underlying the EPA Air Quality System (AQS) data in full. The category-level view is the filter; the per-entity pages are the actual answer.
Source: EPA Outdoor Air Quality Data, 2026.