Bad Air Days in Cass County, North Dakota: 5 a Year

5 bad air days a year

More bad air days than three quarters of monitored counties. Ranked 241 of 996 monitored counties, where 1 is worst.

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

Which months to watch

Most bad days here fall in July and August and June. 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 1 bad days a year; in 2016 to 2025, 5. It has got worse. The worst recent year was 2021, with 17 bad days on 363 monitored days.

Date	AQI	Main pollutant
2021-07-30	211	PM2.5
2025-06-01	192	PM2.5
2025-07-12	178	PM2.5
2023-05-18	175	PM2.5
2021-07-15	173	PM2.5

An ordinary day in Cass

Across 3,627 monitored days from 2016 to 2025, the air in Cass County rated Good on 72% of them, close to the 72% typical of monitored counties, and Moderate on 26%. The median day sat at AQI 40; one day in ten went above 62.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)53%1,931 days
ozone45%1,649 days
coarse dust (PM10)1%28 days
nitrogen dioxide1%19 days

Cass County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	361	275	75	11	7	37	192
2024	366	288	76	2	1	38	158
2023	358	180	166	12	2	50	175
2022	365	285	79	1	0	40	111
2021	363	227	119	17	9	42	211
2020	363	303	60	0	0	37	80
2019	363	301	60	2	1	36	157
2018	360	271	88	1	0	39	121
2017	362	232	130	0	0	44	79
2016	366	263	103	0	0	37	87
2015	365	235	127	3	0	46	140
2014	365	285	79	1	0	37	113
2013	355	272	83	0	0	42	89
2012	366	241	125	0	0	43	92
2011	350	253	97	0	0	41	78
2010	365	225	139	1	0	44	124

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

Nearby counties on file

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.