The report, in its 26th edition, uses data collected by air quality monitors run by the Environmental Protection Agency.

As hazy air from fires out West wafts across the country, some people are at higher risk of health problems, including children, older people and those with lung conditions. Here's how to stay safe.

In Madison, Wis., the air quality was measured at 242 Thursday morning— a "very unhealthy" purple alert rating that many other communities shared, particularly in the Midwest.

Chicago saw its air quality plummet, while other cities including Detroit, Milwaukee and Pittsburgh were experiencing unhealthy air quality levels.

Everyone knows that red means danger, but how did purple become a cautionary color? At an EPA conference in the late 1990s, attendees nearly came to blows over color coding on the Air Quality Index.

Some of the worst air quality was recorded in the Great Lakes region and in Pennsylvania, New York, New Jersey and the Washington, D.C., metro area.

Traditionally, red is the color associated with alarm, but in California, Colorado and Ohio the worst COVID-19 tiers are purple. So how did purple become the cautionary color of the pandemic?