The Air Quality Advisory issued by the Department of Environmental Protection is not the result of wildfire smoke, as it was last week, but instead ozone. Ground-level ozone is the main ingredient in smog, and is most likely to hit unhealthy levels on hot days in urban areas, according to the Environmental Protection Agency.

An air quality alert is in effect for all of Massachusetts as smoke from Canadian wildfires descends into the Boston area.

The Bay Area Air District has issued an air quality advisory in the region for Tuesday because of smoke from a wildfire in Central California.