US Counties With the Lowest Median Air Quality Index

803 US counties ranked by median Air Quality Index, from lowest to highest. First are Rosebud County, MT (3), Uinta County, WY (6) and Karnes County, TX (7). The middle one of the 803 has 43.

What this measures

The Air Quality Index on the middle day of the year: half the days had cleaner air and half had dirtier air. The index runs from 0 to 500 and is set by the worst pollutant that day. Under 50 is good. Over 100 is unhealthy for sensitive groups, such as children and people with asthma. It comes from air monitors, so only about 800 counties have it. A county with no value has no monitor, not clean air. A year counts only if the monitors gave a reading on at least 274 days. 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 lowest counties

More on median Air Quality Index

• US Counties With the Highest Median Air Quality Index
• Median Air Quality Index 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.