What is the air quality in Sheridan County, Montana?

EPA monitors reported only 91 days for Sheridan County in 2025, below the 100-day floor used for annual conclusions. Within that limited sample, the median AQI was 18 and 79 days were rated Good; this does not describe the full year.

What is the main pollutant in Sheridan County?

The primary pollutant in Sheridan County is Fine Particles (PM2.5), which was the main AQI driver on 91 days (100% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.

How many unhealthy air days does Sheridan County have?

Among 91 reported days in 2025, 1 reached Unhealthy or worse. Because the record is below 100 monitored days, that count cannot establish how many unhealthy days occurred across the full year.

Is the air quality in Sheridan County getting better or worse?

Air quality in Sheridan County is worsening across floor-qualifying years (2020-2024); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2020 at 96.3% (103 of 107 days). Lowest qualifying Good-day share: 2023 at 85.7% (96 of 112 days).

What does AQI 18 mean for health?

The median of the 91 reported days was AQI 18, in the "Good" category. The limited sample is not enough to assign the county a full-year health grade. See the AQI category guide for day-level meaning and EPA AirNow for current conditions.

How does Sheridan County compare to other counties in Montana?

Sheridan County is not placed in county comparisons because its 91-day record is below the 100-day comparison floor.

What is the 90th percentile AQI in Sheridan County?

The 90th percentile AQI for Sheridan County is 54, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 197 (Unhealthy).

Montana · County air quality · 2025

Sheridan County, Montana

Limited EPA sample: 91 monitored days, below the 100-day floor for a full-year verdict. The figures below describe only those reported days.

Compiled by PlainAirData · August 23, 2026

18
Sample median AQI: 86.8%
Good in sample: 1
Unhealthy in sample: 91
100-day comparison floor

Sheridan County has only 91 EPA-monitored days in 2025, below the 100-day annual floor

According to the U.S. EPA Air Quality System, Sheridan County, Montana reported 91 monitored days in 2025. That sample is too thin for a full-year air-quality grade or peer ranking. Use the figures only as the reported-day record, and check AirNow for current conditions.

Data source EPA Air Quality System (AQS)

Monitored days: Sheridan County reported 91 monitored days in 2025, below the 100-day floor: sample-only, not a full-year peer rank.
Sample median AQI: Sample median AQI: 18
Annual floor: Annual floor: 100

Sheridan County's peak single-day AQI fell from 160 to 126 between the 2023 and 2024 EPA annual releases, a change of -34 points.
Sheridan County's Good-day share rose from 85.7% to 87.4% between the 2023 and 2024 EPA annual releases, a change of +1.7 percentage points.
Sheridan County's Unhealthy-category days fell from 1 to 0 between the 2023 and 2024 EPA annual releases, a change of -1 days.

Data updated August 2026

Good · 79 d · 86.8%
Moderate · 11 d · 12.1%
Unhealthy · 1 d · 1.1%

How did Sheridan County's reported sample break down in 2025?

Across 91 monitored days, 79 fell in the Good band (AQI 0–50) and 1 reached Unhealthy or worse. The worst single day peaked at AQI 197 (Unhealthy).

EPA AQS →

Pollutant Breakdown

Days where each pollutant was the primary driver of AQI readings in Sheridan County.

PM2.5 · 91 days · 100.0%

Sample Coverage and Health Context

Limited sample

Sheridan County has no annual health grade because only 91 days were reported, below the 100-day floor. The 86.8% Good-day share describes only that sample.

Reported days do not reveal what happened during unmonitored parts of the year, including seasonal smoke, ozone, or inversion periods. Use these values as a description of the available EPA sample only, and check AirNow.gov for current health guidance.

The dominant pollutant is PM2.5, driving AQI on 91 of 91 monitored days.

5-Year Air Quality Trend

Air quality is worsening in Sheridan County. Highest Good-day share: 2020 (96.3%, 103 of 107 monitored days). The 2025 row is shown for transparency but excluded from the direction because it has fewer than 100 monitored days.

Year	Good	Moderate	USG	Unhealthy	Median AQI	Max AQI
2020	103	4	0	0	13	58
2021	108	12	0	0	17	98
2022	85	4	0	0	16	66
2023	96	12	3	1	18	160
2024	104	13	2	0	17	126
2025	79	11	0	1	18	197

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.

Informational only, not professional advice. Data from EPA AQS. See our disclaimer. Spotted a figure that looks wrong? Report a correction.

Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Sheridan County, Montana · 2020–2025 Accessed 2026-08-23

Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.

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.