What Is the Air Quality in Custer, Idaho?
Custer, Idaho has an Air Quality Grade of A (excellent) with a 5-year median AQI of 8. The dominant pollutant is Fine Particulate Matter (PM2.5), and air quality has been stable over the past decade.
Custer, Idaho Air Quality Snapshot
| Air Quality Grade | A80/100 |
| 5-Year Median AQI | 8 (Good) |
| Most Recent Median AQI (2023) | 8 (Good) |
| Dominant Pollutant | Fine Particulate Matter (PM2.5) |
| 10-Year Trend | Stable (-0.18 AQI/yr) |
| Unhealthy Days (last 5 yr) | 3 |
| Idaho Rank | #1 of 20 |
What Does the A Grade Mean?
Custer, Idaho earns an A — it is among the cleanest U.S. cities tracked by EPA monitoring, with median AQI averaging just 8 over the past five years. Days in the "Good" category dominate the calendar; air-quality alerts are rare.
Custer, Idaho's 5-year median AQI of 8 is 33 points below the national average of 41 — meaningfully cleaner than the typical U.S. metro tracked here. Within Idaho, Custer, Idaho runs cleaner than the state average of 33 — a positive signal that local conditions (terrain, wind patterns, emission sources) are working in residents' favor.
For context within Idaho: Jerome, Idaho currently holds the state's cleanest grade (B, AQI 17), while Valley, Idaho sits at the bottom (D, AQI 37).
What's in Custer, Idaho's Air?
The dominant pollutant in Custer, Idaho is Fine Particulate Matter (PM2.5). Fine particulate matter — particles less than 2.5 micrometers across — comes mostly from combustion: vehicle exhaust, wildfire smoke, residential wood burning, and industrial emissions. Because these particles are small enough to enter the bloodstream, PM2.5 is the pollutant most strongly linked to cardiovascular disease, respiratory illness, and premature death.
Days by Dominant Pollutant (2023)
| Pollutant | Days as Dominant | Share of Year |
|---|---|---|
| Fine Particulate Matter (PM2.5) | 104 | 100% |
Is the Air Getting Better or Worse?
Air quality in Custer, Idaho has held roughly steady over the past decade, with year-to-year shifts in median AQI of less than half a point. That stability makes the city's long-run grade a reliable signal of what residents can expect day-to-day.
In 2014, Custer, Idaho posted a median AQI of 9. By 2023 that figure was 8 — a drop of 1 AQI points cleaner across 10 years of EPA records.
Year-by-Year AQI in Custer, Idaho
| Year | Median AQI | Good Days | Unhealthy Days | Dominant Pollutant |
|---|---|---|---|---|
| 2014 | 9 | 107 | 0 | PM2.5 |
| 2015 | 8 | 113 | 0 | PM2.5 |
| 2016 | 11 | 103 | 4 | PM2.5 |
| 2017 | 7 | 100 | 3 | PM2.5 |
| 2018 | 7 | 85 | 2 | PM2.5 |
| 2019 | 8 | 95 | 0 | PM2.5 |
| 2020 | 7 | 105 | 3 | PM2.5 |
| 2021 | 8 | 93 | 0 | PM2.5 |
| 2022 | 7 | 98 | 0 | PM2.5 |
| 2023 | 8 | 103 | 0 | PM2.5 |
Health Context for Custer, Idaho
Across the past five years, this area has logged just 3 days where AQI rose into the "Unhealthy for Sensitive Groups" range or worse — about 1 days per year, or roughly one every other month. That is a low count by national standards.
For most healthy adults, current air quality in this area does not require any change in behavior. People with severe asthma, COPD, or recent cardiac events should still keep an eye on daily AQI alerts, especially during wildfire season. Because PM2.5 penetrates deep into the lungs and bloodstream, an N95 or KN95 mask provides meaningful protection on smoky or high-particulate days — surgical masks do not.
How This Grade Is Calculated
The AirHistory Air Quality Grade combines four signals: the 5-year median AQI (40% of the score), the 10-year trend direction (30%), the count of unhealthy days per year (20%), and the dominant pollutant type (10%). All four come directly from the EPA Air Quality System (AQS), which aggregates readings from federally certified monitors. Read the full methodology.
Custer, Idaho has an Air Quality Grade of A (excellent) with a 5-year median AQI of 8. The dominant pollutant is Fine Particulate Matter (PM2.5), and air quality has been stable over the past decade.
This answer pulls from the EPA Air Quality System (AQS), the authoritative federal source for U.S. air quality and pollution monitoring. The headline number above is the direct answer; what follows is the additional context most readers need to use the answer for a real decision rather than just a fact lookup.
For readers turning this answer into action: cross-reference against the underlying the EPA Air Quality System (AQS) record before acting on time-sensitive decisions. The site renders the data as it was published; subsequent revisions can shift the picture, and the live federal data is always the authoritative current reference.
Source: EPA Outdoor Air Quality Data, 2026.