Bad Air Days in Flathead County, Montana: 6 a Year

6 bad air days a year

More bad air days than three quarters of monitored counties. Ranked 184 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (100% particle days).

When the bad days come in Flathead

Most bad days here fall in August and September. Averaged over 2016 to 2025.

What to buy, and what not to

Particles (wildfire smoke, dust) cause enough bad days here to matter. A HEPA purifier sized to your bedroom removes them; run it with windows shut on bad days.
Upgrade the furnace or AC filter to MERV 13 so the whole house is filtered, not just one room.

The worst days on file

In 2000 to 2009 this county averaged 2 bad days a year; in 2016 to 2025, 6. It has got worse. The worst recent year was 2018, with 16 bad days on 365 monitored days.

Date	AQI	Main pollutant
2017-09-07	275	PM2.5
2017-09-08	272	PM2.5
2017-09-06	238	PM2.5
2020-09-14	206	PM2.5
2018-08-19	193	PM2.5

The air on a normal day

Across 3,653 monitored days from 2016 to 2025, the air in Flathead County rated Good on 71% of them, close to the 72% typical of monitored counties, and Moderate on 27%. The median day sat at AQI 41; one day in ten went above 64.

The pollutant setting the AQI on an ordinary day here:

ozone47%1,715 days
fine particles (PM2.5)46%1,695 days
coarse dust (PM10)7%243 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	269	95	1	0	41	103
2024	366	253	113	0	0	42	95
2023	365	237	120	8	2	43	160
2022	365	249	113	3	1	41	152
2021	365	268	88	9	1	39	155
2020	366	286	72	8	5	37	206
2019	365	267	97	1	0	39	103
2018	365	222	127	16	5	45	193
2017	365	250	104	11	6	42	275
2016	366	298	68	0	0	37	80
2015	365	216	137	12	9	44	190
2014	365	254	109	2	0	41	141
2013	365	235	126	4	0	44	125
2012	366	258	108	0	0	41	95
2011	365	221	144	0	0	45	89
2010	365	188	177	0	0	49	96

*2025 may be a partial year in EPA's file.

Montana compared

How this is counted

A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day.