EnviroNearby

Williams County, North Dakota air quality history

Daily Air Quality Index summaries from EPA monitors in Williams, North Dakota, 1985–2019.

90%
Good days in 2019
34
Median daily AQI, 2019
0
Days above 100 in 2019
362
Days with an AQI in 2019 (of 365)

In 2019, EPA reported a daily Air Quality Index (AQI) for Williams, North Dakota on 362 days. 324 of those days (90%) were in the Good range, and the median daily AQI was 34. That median is about the same as the typical monitored county in North Dakota (median 35 across 10 counties), and lower than the typical monitored U.S. county (41). The main pollutant was ozone on 263 days. Across the years with fuller monitoring, the share of Good days rose: an average of 73% in 2013–2015 versus 85% in 2017–2019.

The most recent year with data for this county is 2019; EPA's files for 2020–2025 contain no row for it, which usually means its monitors stopped reporting.

Years with fewer than 120 monitored days (1993, 1994, 1995, 1996, 1997, 1998…) are shown in the table but left out of comparisons and trend statements.

How the days broke down each year

Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.

Share of monitored days in each AQI category, by year0%25%50%75%100%1985: Good 92 of 95 days1985: Moderate 3 of 95 days19851986: Good 114 of 116 days1986: Moderate 2 of 116 days1987: Good 41 of 41 days19871988: Good 43 of 45 days1988: Moderate 2 of 45 days1989: Good 42 of 43 days1989: Moderate 1 of 43 days19891990: Good 52 of 54 days1990: Moderate 1 of 54 days1990: Unhealthy for Sensitive Groups 1 of 54 days1991: Good 58 of 58 days19911992: Good 61 of 61 days1993: Good 55 of 55 days19931994: Good 54 of 56 days1994: Moderate 2 of 56 days1995: Good 53 of 53 days19951996: Good 56 of 56 days1997: Good 54 of 54 days19971998: Good 27 of 27 days2013: Good 195 of 363 days2013: Moderate 167 of 363 days2013: Unhealthy for Sensitive Groups 1 of 363 days20132014: Good 295 of 365 days2014: Moderate 69 of 365 days2014: Unhealthy for Sensitive Groups 1 of 365 days2015: Good 307 of 364 days2015: Moderate 49 of 364 days2015: Unhealthy for Sensitive Groups 5 of 364 days2015: Unhealthy 3 of 364 days20152016: Good 349 of 364 days2016: Moderate 15 of 364 days2017: Good 302 of 362 days2017: Moderate 58 of 362 days2017: Unhealthy for Sensitive Groups 2 of 362 days20172018: Good 291 of 360 days2018: Moderate 69 of 360 days2019: Good 324 of 362 days2019: Moderate 38 of 362 days2019
  • Good
  • Moderate
  • Unhealthy for Sensitive Groups
  • Unhealthy
  • Very Unhealthy
  • Hazardous

Typical and high-end AQI over time

The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.

Median and 90th-percentile daily AQI, by year0501001985: 90th percentile AQI 351986: 90th percentile AQI 311987: 90th percentile AQI 291988: 90th percentile AQI 281989: 90th percentile AQI 241990: 90th percentile AQI 291991: 90th percentile AQI 201992: 90th percentile AQI 181993: 90th percentile AQI 181994: 90th percentile AQI 241995: 90th percentile AQI 211996: 90th percentile AQI 191997: 90th percentile AQI 241998: 90th percentile AQI 262013: 90th percentile AQI 632014: 90th percentile AQI 562015: 90th percentile AQI 562016: 90th percentile AQI 452017: 90th percentile AQI 542018: 90th percentile AQI 592019: 90th percentile AQI 511985: Median AQI 211986: Median AQI 141987: Median AQI 151988: Median AQI 141989: Median AQI 131990: Median AQI 141991: Median AQI 121992: Median AQI 101993: Median AQI 121994: Median AQI 101995: Median AQI 121996: Median AQI 81997: Median AQI 91998: Median AQI 112013: Median AQI 492014: Median AQI 372015: Median AQI 362016: Median AQI 322017: Median AQI 372018: Median AQI 382019: Median AQI 3419851987198919911993199519972013201520172019
  • Median daily AQI
  • 90th-percentile daily AQI

Main pollutant by day, 2019

On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.

PollutantDays as main pollutantShare of monitored days
Ozone26373%
Fine particles (PM2.5)9727%
Coarse particles (PM10)21%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2019362324380000345177Ozone
20183602916900003859100Ozone
20173623025820003754131Ozone
2016364349150000324594Ozone
20153643074953003656182Ozone
20143652956910003756114Ozone
201336319516710004963102PM2.5
1998272700000112644PM10
199754540000092432PM10
199656560000081920PM10
1995535300000122139PM10
1994565420000102456PM10
1993555500000121825PM10
1992616100000101824PM10
1991585800000122036PM10
19905452110001429143PM10
1989434210000132458PM10
1988454320000142852PM10
1987414100000152940PM10
198611611420000143191PM10
1985959230000213555PM10

What this data can and can’t tell you

It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.

It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.

Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.

All monitored counties in North Dakota

Source: U.S. EPA AirData, “Annual AQI by County” files for 1985–2019 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem