Bad Air Days in Williams County, North Dakota: under 1 a Year

under 1 bad air day a year

Rare: among the cleanest-air counties EPA monitors. Ranked 817 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 Williams

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

What actually helps here

Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
On the odd bad day, check AirNow and keep windows shut.

The record for Williams

The worst recent year was 2017, with 2 bad days on 362 monitored days.

Date	AQI	Main pollutant
2017-09-12	131	PM2.5
2017-09-13	123	PM2.5
2018-06-09	100	Ozone
2018-08-09	99	PM2.5
2018-08-10	99	PM2.5

Most days, not the worst ones

Across 1,448 monitored days from 2016 to 2019, the air in Williams County rated Good on 87% of them, more often than the 72% typical of monitored counties, and Moderate on 12%. The median day sat at AQI 35; one day in ten went above 52.

The pollutant setting the AQI on an ordinary day here:

ozone71%1,023 days
fine particles (PM2.5)28%404 days
coarse dust (PM10)1%21 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2019	362	324	38	0	0	34	77
2018	360	291	69	0	0	38	100
2017	362	302	58	2	0	37	131
2016	364	349	15	0	0	32	94
2015	364	307	49	8	3	36	182
2014	365	295	69	1	0	37	114
2013	363	195	167	1	0	49	102

North Dakota 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.