Sheridan County, Montana air quality history

Daily Air Quality Index summaries from EPA monitors in Sheridan, Montana, 1999–2025.

90%

Good days in 2021

Median daily AQI, 2021

Days above 100 in 2021

120

Days with an AQI in 2021 (of 365)

In 2021, EPA reported a daily Air Quality Index (AQI) for Sheridan, Montana on 120 days. 108 of those days (90%) were in the Good range, and the median daily AQI was 17. That median is lower than the typical monitored county in Montana (median 39.5 across 16 counties), and lower than the typical monitored U.S. county (41). The main pollutant was fine particles (PM2.5) on 120 days.

EPA did not report an AQI for every day of 2021 here (120 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.

Years with fewer than 120 monitored days (2019, 2020, 2022, 2023, 2024, 2025…) are shown in the table but left out of comparisons and trend statements.

How the days broke down each year

Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.

Good
Moderate
Unhealthy for Sensitive Groups
Unhealthy
Very Unhealthy
Hazardous

Typical and high-end AQI over time

The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.

Median daily AQI
90th-percentile daily AQI

Main pollutant by day, 2021

On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.

Pollutant	Days as main pollutant	Share of monitored days
Fine particles (PM2.5)	120	100%

Year-by-year data

Annual AQI summary by year
Year	Days with AQI	Good	Moderate	Unhealthy for Sensitive Groups	Unhealthy	Median AQI	90th pct.	Max AQI	Main pollutant
2025	91	79	11	0	1	18	54	197	PM2.5
2024	119	104	13	2	0	17	53	126	PM2.5
2023	112	96	12	3	1	18	56	160	PM2.5
2022	89	85	4	0	0	16	36	66	PM2.5
2021	120	108	12	0	0	17	52	98	PM2.5
2020	107	103	4	0	0	13	27	58	PM2.5
2019	104	101	3	0	0	16	36	57	PM2.5
2018	112	99	12	1	0	20	52	118	PM2.5
2017	114	109	5	0	0	17	37	75	PM2.5
2016	117	113	4	0	0	15	31	81	PM2.5
2015	115	109	3	2	1	16	38	155	PM2.5
2014	114	109	5	0	0	17	42	74	PM2.5
2013	107	102	5	0	0	19	41	70	PM2.5
2012	107	100	7	0	0	18	44	96	PM2.5
2011	115	111	4	0	0	17	36	56	PM2.5
2010	113	107	5	1	0	17	41	102	PM2.5
2009	120	120	0	0	0	18	34	45	PM2.5
2008	119	114	5	0	0	21	39	82	PM2.5
2007	118	111	7	0	0	18	42	71	PM2.5
2006	118	111	7	0	0	17	38	81	PM2.5
2005	110	107	3	0	0	16	33	54	PM2.5
2004	119	112	7	0	0	16	38	75	PM2.5
2003	107	107	0	0	0	19	34	47	PM2.5
2002	118	117	1	0	0	18	38	54	PM2.5
2001	113	104	9	0	0	21	46	58	PM2.5
2000	92	87	5	0	0	19	38	66	PM2.5
1999	4	4	0	0	0	9	16	16	PM2.5

What this data can and can’t tell you

It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.

It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.

Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.

All monitored counties in Montana

Source: U.S. EPA AirData, “Annual AQI by County” files for 1999–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem