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.

The heat wave began Monday with most towns across central and eastern Massachusetts topping out between 90 to 92 degrees. This makes 17 days of 90+ in Boston this year, already three more than the average and the most we have had in three years.

A heat advisory continues in Massachusetts, are temperatures are expected to be the highest of the week on Tuesday.