What Counts as a Bad Air Day? AQI 100 Explained
The line is AQI 100
EPA's Air Quality Index runs from 0 to 500. At 100 it equals the national health standard for that pollutant. Above 100, EPA's advice is that children, older adults and people with asthma, heart or lung disease cut back on long or hard exertion outdoors. That is the line BadAirDays counts.
| AQI | EPA category | Who should cut back |
|---|---|---|
| 0-50 | Good | Nobody |
| 51-100 | Moderate | People unusually sensitive to the pollutant |
| 101-150 | Unhealthy for Sensitive Groups | Children, older adults, people with asthma, heart or lung disease |
| 151-200 | Unhealthy | Everyone; sensitive groups should avoid long time outdoors |
| 201-300 | Very Unhealthy | Everyone should avoid long or hard exertion outside |
| 301+ | Hazardous | Everyone should stay indoors |
How common is it?
Among 996 counties with a reliable monitor record for 2016-2025, half see 2.1 or fewer bad days a year, a quarter see 4.5 or more, and the worst tenth 8.8 or more. 173 counties average under half a day a year. 196 average at least one day over 150, the level where EPA's advice extends to everyone. The most bad days a year recently: San Bernardino County, California (138), Riverside County, California (132), Kern County, California (109).
A count, not a severity
A day at AQI 105 and a day at AQI 400 during a wildfire both count once. That is why each county page also lists its worst days on record and how many days went over 150: two counties with the same count can have lived very different years.
Moderate days are not nothing
Between 51 and 100 the air meets the standard but EPA still flags it for people unusually sensitive to the pollutant. Across monitored counties, 72% of recent monitored days rated Good. Each county page shows its own Good and Moderate shares and its median day, which says more about everyday air than the bad-day count does.