Bad Air Days in Fergus County, Montana: 4 a Year

4 bad air days a year

About the middle of monitored US counties. Ranked 254 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (91% particle days).

The bad-air season here

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

Is a purifier worth it here?

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.

Better or worse than it was

The worst recent year was 2021, with 15 bad days on 365 monitored days.

Date	AQI	Main pollutant
2023-09-05	185	PM2.5
2023-09-04	162	PM2.5
2018-08-19	161	PM2.5
2020-09-15	159	PM2.5
2020-09-14	157	PM2.5

An ordinary day in Fergus

Across 3,651 monitored days from 2016 to 2025, the air in Fergus County rated Good on 88% of them, more often than the 72% typical of monitored counties, and Moderate on 11%. The median day sat at AQI 40; one day in ten went above 54.

The pollutant setting the AQI on an ordinary day here:

ozone83%3,025 days
fine particles (PM2.5)17%614 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	364	330	33	1	0	41	131
2024	366	312	50	4	1	40	155
2023	364	315	43	6	2	41	185
2022	365	341	22	2	0	40	122
2021	365	288	62	15	1	41	156
2020	366	330	33	3	2	39	159
2019	365	343	22	0	0	38	86
2018	365	313	46	6	1	40	161
2017	365	296	62	7	2	42	155
2016	366	349	17	0	0	36	88
2015	365	328	28	9	4	37	167
2014	365	337	27	1	0	37	135
2013	365	348	17	0	0	35	62
2012	217	206	11	0	0	27	71
2011	114	114	0	0	0	12	47
2010	121	118	3	0	0	14	58

*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.