Air pollution is forecast Thursday to reach unhealthy levels for sensitive groups, including children, the elderly, and people with asthma, heart disease or lung disease.
The Maryland Department of the Environment has issued a "Code Orange" alert across the Baltimore and Washington, D.C., regions. Ozone, a pollutant created by reactions between chemicals emitted by cars and industrial facilities, is the chief concern.
Meanwhile, smoke from wildfires in Canada has been drifting across the region, adding to the pollution.
The heat and sunshine coupled with calm winds tend to make air pollution worse. Partly cloudy skies and highs in the lower 90s are forecast Thursday. Some afternoon storms are possible, and could improve air quality.
More poor air quality is possible Friday, according to MDE. Clean Air Partners, a nonprofit air quality advocacy group, is forecasting another "Code Orange" day Friday.
Ozone is a natural compound present in upper levels of the atmosphere and consisting of three oxygen atoms. But when it is created near the ground, by chemicals interacting in the presence of sunlight, it can contribute to smog and reduce lung function and irritate lung tissue, according to the Environmental Protection Agency.