Montana Air Quality

EPA monitor data through 2025 · Updated August 13, 2026 · Methods

In 2025, Montana recorded 12 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 365 days with monitoring data. Most of those bad days were driven by fine particles. Under the AQI rules that applied before 2013, the same air would have scored 9 days. Since 2000, unhealthy days are 53% lower on a consistent definition.

Unhealthy days, 2025

AQI above 100

Same air, pre-2013 rules

for comparison with older reports

Good-air days

108

AQI 50 or below

Days monitored

365

full year

How to read a statewide figure. The Air Quality Index describes one place at one time. A statewide number is the worst monitor anywhere in the state on each day, so it answers “was the air unhealthy somewhere in Montana?” — not “what did a typical resident breathe?” In a large state with many monitors it will read high even if most of the state was clean. EPA publishes the AQI for metropolitan areas rather than states for exactly this reason. For what people actually breathed, use the metro pages below.

Unhealthy days by year, scored two ways

EPA changed what an AQI of 100 means in 2013 and again in 2015, so a count published before those changes is not comparable to one published after. Each panel below applies a single fixed rulebook to every year, so each is internally consistent. Why this matters.

Scored by today's rules

Scored by the pre-2013 rules

What 2025 looked like

Air quality category	Days	Share of monitored days
Good	108	30%
Moderate	245	67%
Unhealthy for sensitive groups	8	2%
Unhealthy or worse	4	1%

Median AQI 56 · 90th-percentile AQI 80 · worst day 197. Scored under current rules.

What drove the unhealthy days

Of the 12 days above 100 in 2025, 0 were driven by ozone and 12 by fine particles. The two behave differently. Ozone peaks on hot sunny afternoons; fine particles now spike mainly with wildfire smoke.

The unhealthy days in 2025

Every day the index passed 100, what it reached, and which pollutant drove it. 3 of these 12 would not have counted as unhealthy under the pre-2013 rules.

Date	Peak AQI	Driven by	Pre-2013 score
Jun 10	124	Fine particles	110
Jun 11	168	Fine particles	160
Jun 12	131	Fine particles	115
Jul 11	151	Fine particles	131
Jul 12	197	Fine particles	183
Jul 15	111	Fine particles	98 (not unhealthy)
Jul 22	127	Fine particles	112
Aug 9	146	Fine particles	127
Aug 10	165	Fine particles	156
Sep 2	102	Fine particles	91 (not unhealthy)
Sep 6	123	Fine particles	108
Sep 7	103	Fine particles	92 (not unhealthy)

Metro areas in Montana

Ranked by unhealthy days in their latest year. These are the geographies EPA actually reports the AQI for.

Metro area	Unhealthy days	Main driver
Kalispell, MT	1	fine particles
Billings, MT	0	ozone and fine particles equally
Bozeman, MT	0	ozone and fine particles equally
Butte-Silver Bow, MT	0	ozone and fine particles equally
Great Falls, MT	0	ozone and fine particles equally
Helena, MT	0	ozone and fine particles equally
Missoula, MT	0	ozone and fine particles equally

The full series

Year	Unhealthy days, current rules	Pre-2013 rules	Days monitored
2000	25	19	366
2001	21	14	365
2002	17	11	365
2003	27	20	365
2004	12	9	366
2005	26	18	365
2006	31	19	365
2007	38	32	365
2008	11	6	366
2009	26	14	365
2010	57	39	365
2011	69	50	365
2012	79	69	366
2013	35	27	365
2014	25	12	365
2015	55	41	365
2016	47	31	366
2017	88	77	365
2018	38	25	365
2019	10	8	365
2020	31	22	366
2021	59	47	365
2022	24	17	365
2023	29	25	365
2024	27	22	366
2025	12	9	365

Sources

EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
Download the full dataset for every state and metro.