Big Horn County, Montana air quality history

Daily Air Quality Index summaries from EPA monitors in Big Horn, Montana, 1991–2008.

90%

Good days in 1997

Median daily AQI, 1997

Days above 100 in 1997

120

Days with an AQI in 1997 (of 365)

In 1997, EPA reported a daily Air Quality Index (AQI) for Big Horn, Montana on 120 days. 108 of those days (90%) were in the Good range, and the median daily AQI was 21. That median is about the same as the typical monitored county in Montana (median 20 across 13 counties), and lower than the typical monitored U.S. county (39). The main pollutant was coarse particles (PM10) on 120 days.

EPA did not report an AQI for every day of 1997 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.

The most recent year with data for this county is 2008; EPA's files for 2009–2025 contain no row for it, which usually means its monitors stopped reporting.

Years with fewer than 120 monitored days (2003, 2004, 2005, 2006, 2007, 2008…) 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, 1997

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
Coarse particles (PM10)	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
2008	62	54	8	0	0	25	54	83	PM10
2007	60	50	9	1	0	27	56	101	PM10
2006	61	45	15	0	1	34	62	151	PM10
2005	62	54	8	0	0	24	53	73	PM10
2004	62	54	8	0	0	25	52	67	PM10
2003	54	54	0	0	0	7	18	42	PM10
2002	86	82	4	0	0	13	35	64	PM10
1997	120	108	12	0	0	21	51	71	PM10
1996	121	98	23	0	0	21	60	77	PM10
1995	120	112	8	0	0	18	44	68	PM10
1994	64	56	8	0	0	23	54	66	PM10
1993	61	55	6	0	0	18	46	71	PM10
1992	61	53	8	0	0	16	53	90	PM10
1991	15	15	0	0	0	6	12	13	PM10

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.

Counties in Montana with a similar median AQI in 1997

Broadwater

All monitored counties in Montana

Source: U.S. EPA AirData, “Annual AQI by County” files for 1991–2008 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem