What Is the Air Quality in Williams, North Dakota?
Williams, North Dakota has an Air Quality Grade of B (good) with a 5-year median AQI of 34. The dominant pollutant is Ground-Level Ozone, and air quality has been stable over the past decade.
Williams, North Dakota Air Quality Snapshot
| Air Quality Grade | B70/100 |
| 5-Year Median AQI | 34 (Good) |
| Most Recent Median AQI (2019) | 34 (Good) |
| Dominant Pollutant | Ground-Level Ozone |
| 10-Year Trend | Stable (-0.11 AQI/yr) |
| Unhealthy Days (last 5 yr) | 0 |
| National Rank (cleanest = #1) | #208 of 1,020 (20th cleanest percentile) |
| North Dakota Rank | #1 of 10 |
What Does the B Grade Mean?
Williams, North Dakota earns a B — air quality is reliably in the safe range for most residents most of the time, with a 5-year median AQI of 34. Sensitive groups will see occasional caution days, but the typical resident will not need to change behavior based on air quality.
Williams, North Dakota's 5-year median AQI of 34 is 7 points below the national average of 41 — meaningfully cleaner than the typical U.S. metro tracked here. Within North Dakota, Williams, North Dakota runs cleaner than the state average of 37 — a positive signal that local conditions (terrain, wind patterns, emission sources) are working in residents' favor.
For context within North Dakota: Billings, North Dakota currently holds the state's cleanest grade (C, AQI 37), while Ward, North Dakota sits at the bottom (C, AQI 36).
What's in Williams, North Dakota's Air?
The dominant pollutant in Williams, North Dakota is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle exhaust and industrial emissions. It is worst on hot, sunny, stagnant summer days. Ozone irritates the lungs, triggers asthma attacks, and reduces lung function — even healthy adults can feel chest tightness and shortness of breath after exercising in elevated ozone.
Days by Dominant Pollutant (2019)
| Pollutant | Days as Dominant | Share of Year |
|---|---|---|
| Ground-Level Ozone | 263 | 73% |
| Fine Particulate Matter (PM2.5) | 97 | 27% |
| Coarse Particulate Matter (PM10) | 2 | 1% |
Is the Air Getting Better or Worse?
Air quality in Williams, North Dakota has held roughly steady over the past decade, with year-to-year shifts in median AQI of less than half a point. That stability makes the city's long-run grade a reliable signal of what residents can expect day-to-day.
In 2014, Williams, North Dakota posted a median AQI of 37. By 2019 that figure was 34 — a drop of 3 AQI points cleaner across 6 years of EPA records.
Year-by-Year AQI in Williams, North Dakota
| Year | Median AQI | Good Days | Unhealthy Days | Dominant Pollutant |
|---|---|---|---|---|
| 2014 | 37 | 295 | 1 | Ozone |
| 2015 | 36 | 307 | 8 | Ozone |
| 2016 | 32 | 349 | 0 | Ozone |
| 2017 | 37 | 302 | 2 | Ozone |
| 2018 | 38 | 291 | 0 | Ozone |
| 2019 | 34 | 324 | 0 | Ozone |
Health Context for Williams, North Dakota
Across the past five years, this area has logged just 0 days where AQI rose into the "Unhealthy for Sensitive Groups" range or worse — about 0 days per year, or roughly one every other month. That is a low count by national standards.
For most healthy adults, current air quality in this area does not require any change in behavior. People with severe asthma, COPD, or recent cardiac events should still keep an eye on daily AQI alerts, especially during wildfire season. Because ozone peaks in the afternoon on hot sunny days, plan outdoor exercise for early morning or after sunset on bad-air days.
How This Grade Is Calculated
The AirHistory Air Quality Grade combines four signals: the 5-year median AQI (40% of the score), the 10-year trend direction (30%), the count of unhealthy days per year (20%), and the dominant pollutant type (10%). All four come directly from the EPA Air Quality System (AQS), which aggregates readings from federally certified monitors. Read the full methodology.
Williams, North Dakota has an Air Quality Grade of B (good) with a 5-year median AQI of 34. The dominant pollutant is Ground-Level Ozone, and air quality has been stable over the past decade.
The data source behind this answer is the EPA Air Quality System (AQS). Every figure on the page traces back to that source; the methodology page describes the inputs and the refresh cadence in full detail.
For readers turning this answer into action: cross-reference against the underlying the EPA Air Quality System (AQS) record before acting on time-sensitive decisions. The site renders the data as it was published; subsequent revisions can shift the picture, and the live federal data is always the authoritative current reference.
Source: EPA Outdoor Air Quality Data, 2026.