The Massachusetts Department of Environmental Protection has issued an air quality alert for parts of the state, including Essex County.
With temperatures in the 90s and high humidity, DEP spokesman Ed Coletta said certain groups of people should avoid going outside if at all possible.
At-risk groups include "people with heart or lung disease such as asthma, older adults, children, teenagers, and people who are active outdoors," according to the DEP's MassAir website. People sensitive to the poor air quality should also avoid heavy exertion and take breaks throughout the day.
"It's a problem with the ozone spiking this afternoon into this evening," Coletta said.
The unhealthy conditions could extend into Wednesday morning, but air quality should improve for the rest of the week, he added.
The National Weather Service also issued a heat advisory for eastern Massachusetts until 8 p.m.
Temperatures and the dew point should slowly drop by Thursday, providing some relief before the muggy air returns this weekend, according to meteorologist Dave Epstein.