Air Quality Rankings for North Dakota (2026)
North Dakota has 10 cities tracked by EPA air-quality monitors, with a state-wide 5-year median AQI of 37 — 4 points cleaner than the national average of AQI 41. Williams, North Dakota ranks #1 with the cleanest air (AQI 34, Grade B), while Cass, North Dakota sits at the bottom (AQI 41, Grade C).
How North Dakota Compares
North Dakota has 10 cities tracked by EPA air-quality monitors, with a state-wide 5-year median AQI of 37 — 4 points cleaner than the national average of AQI 41. Williams, North Dakota ranks #1 with the cleanest air (AQI 34, Grade B), while Cass, North Dakota sits at the bottom (AQI 41, Grade C). The rankings below are computed from the EPA Air Quality System (AQS), which aggregates daily AQI readings from federally certified monitors into annual averages. Cities are sorted by 5-year median AQI (lowest = cleanest = #1). The 5-year window smooths out year-to-year volatility from weather and wildfire events.
North Dakota is bucking the national trend of broad improvement: 9 of 10 monitored cities show measurably worse air over the past decade, more than the 1 that are improving. Across western states this usually traces back to expanding wildfire smoke exposure; elsewhere it can reflect rising local emissions from population or freight growth.
The dominant pollutant across 9 of 10 North Dakota cities is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days and is the leading air quality concern across much of the Sun Belt and California. Other North Dakota cities report Fine Particulate Matter (PM2.5) (1) as their dominant concern.
The fastest-improving city in North Dakota is Williams, North Dakota, with median AQI falling by 0.1 points per year. Steady improvement at that pace usually reflects fleet turnover (older diesels retiring), upwind power-plant retirements, or tighter regional emissions controls.
The city with the steepest decline is Ward, North Dakota, where median AQI is rising by 1.3 points per year. Rapid deterioration in a single city usually points to either wildfire-smoke exposure (in the West) or a new local emissions source — a power plant, port, or freight corridor coming online.
Full North Dakota Ranking
| # | City | 5yr Avg AQI | Current AQI | Worst Pollutant | Trend | Grade |
|---|---|---|---|---|---|---|
| 1 | Williams, North Dakota | 34 | 34 | Ozone | Stable | B |
| 2 | McKenzie, North Dakota | 36 | 39 | Ozone | Stable | C |
| 3 | Ward, North Dakota | 36 | 42 | Ozone | Worsening | C |
| 4 | Billings, North Dakota | 37 | 39 | Ozone | Stable | C |
| 5 | Dunn, North Dakota | 37 | 41 | Ozone | Worsening | C |
| 6 | Mercer, North Dakota | 38 | 41 | Ozone | Worsening | C |
| 7 | Burke, North Dakota | 38 | 44 | Ozone | Worsening | C |
| 8 | Burleigh, North Dakota | 38 | 42 | PM2.5 | Worsening | C |
| 9 | Oliver, North Dakota | 39 | 42 | Ozone | Worsening | C |
| 10 | Cass, North Dakota | 41 | 50 | Ozone | Worsening | C |
Air quality data for North Dakota is sourced from the EPA Air Quality System (AQS), which monitors outdoor air quality at thousands of stations nationwide.
Frequently Asked Questions
Williams, North Dakota has the best air quality in North Dakota with a 5-year average AQI of 34 and a Grade B (70/100). Its dominant pollutant is Ground-Level Ozone and the long-run trend is stable.
Cass, North Dakota has the worst air quality in North Dakota with a 5-year average AQI of 41 and a Grade C (59/100). Its dominant pollutant is Ground-Level Ozone.
North Dakota has 10 cities with EPA air quality monitoring data, covering 2014-2023 of daily AQI measurements aggregated into annual averages.
North Dakota's state-wide 5-year median AQI is 37, 4 points cleaner than the national average of AQI 41. North Dakota is bucking the national trend of broad improvement: 9 of 10 monitored cities show measurably worse air over the past decade, more than the 1 that are improving. Across western states this usually traces back to expanding wildfire smoke exposure; elsewhere it can reflect rising local emissions from population or freight growth.
Ground-Level Ozone is the dominant pollutant in 9 of 10 monitored North Dakota cities. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days and is the leading air quality concern across much of the Sun Belt and California.
North Dakota cities log an average of 5 days per year at "Unhealthy for Sensitive Groups" or worse, based on EPA monitor data over the last five years. Across all 10 North Dakota cities tracked, that totals 259 unhealthy days over the period.
Cities ranked by 5-year average AQI (lower is better). Grades factor in average AQI, trend direction, unhealthy days, and dominant pollutant.
The this entity category groups every U.S. air quality and pollution monitoring entity sharing this attribute. The list above is the data; the paragraphs below explain what the grouping means against the broader the EPA Air Quality System (AQS) distribution and how to read the relative rankings within the category.
For readers using this category as a starting point, the per-entity detail pages linked from the table above carry the underlying the EPA Air Quality System (AQS) data in full. The category-level view is the filter; the per-entity pages are the actual answer.
Source: EPA Outdoor Air Quality Data, 2026.