US Counties With the Highest Share of Days with Unhealthy Air
803 US counties ranked by share of days with unhealthy air, from highest to lowest. First are San Bernardino County, CA (32.3), Riverside County, CA (31.2) and Los Angeles County, CA (23.8). The middle one of the 803 has 1.1.
What this measures
The percentage of days when the Air Quality Index was over 100, which is unhealthy for sensitive groups or worse. The index runs from 0 to 500 and is set by the worst pollutant that day. Smoke from wildfires and summer ozone cause most of these days. It counts only the days the monitors gave a reading, and a year counts only if there were at least 274 of them. Only about 800 counties have a monitor, mostly the larger ones. Source: EPA Air Quality System.
For most measures, a national ranking mostly sorts places by income and by how urban they are. Each county’s own page compares it with the 50 counties most like it instead, which is often the fairer view.
The 100 highest counties
More on share of days with unhealthy air
- US Counties With the Lowest Share of Days with Unhealthy Air
- Share of days with unhealthy air by US county: map by year and what it goes with
A ranking describes places, not the people in them, and a high or low place does not by itself say why. All US rankings.