Once the AQI reaches 101, air pollution is at a level that is unhealthy for sensitive groups, including the very old, the very young and anyone with a respiratory or immune condition.

Canadian wildfire smoke lingers in Michigan. Officials issued an air quality advisory in southeast Michigan for July 15.

California and Oregon are among the U.S. hotspots for dangerous submicron (PM1) air pollution—those with particles less than a micron, or a millionth of a meter wide—according to new research.