# Counties in North Dakota by share of days unhealthy for sensitive groups or worse (US EPA AQI), 2001

All 12 values come from U.S. Environmental Protection Agency for 2001, so the places are comparable. 3 counties have no value for that period and are not ranked.

- Indicator id: `aqi_unhealthy_sensitive_or_worse_days_share`
- Unit: %
- Definition: Days with a daily Air Quality Index above 100 (unhealthy for sensitive groups, unhealthy, very unhealthy or hazardous) as a percentage of the days of the year with an AQI in a US county. Calculated by Invokely from the day counts in the EPA AirData annual summaries; the counts are stored with each value.
- Source: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts (https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual)
- Retrieved: 2026-10-12
- Citation: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: days with an AQI above 100 as a share of days with an AQI, calculated with stored counts. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
- Web page: https://invokely.ai/rankings/unhealthy-air-quality-days-share/north-dakota-counties

| Rank | Place | % |
|---:|---|---:|
| 1 | Cass County | 0.3 |
| 2 | Billings County | 0 |
| 2 | Burke County | 0 |
| 2 | Burleigh County | 0 |
| 2 | Dunn County | 0 |
| 2 | Grand Forks County | 0 |
| 2 | McKenzie County | 0 |
| 2 | McLean County | 0 |
| 2 | Mercer County | 0 |
| 2 | Oliver County | 0 |
| 2 | Stark County | 0 |
| 2 | Steele County | 0 |