An air quality alert is in place for the Philadelphia region today.
The state environmental protection departments in both Pennsylvania and New Jersey say today is a "Code Orange" day, meaning the air is unhealthy for sensitive groups like young children, the elderly, and people with respiratory problems.
Those groups should limit their time outdoors today.
The all-afternoon sun, combined with temperatures reaching into the 90s, "will promote ozone formation," says AirNow.gov, the air-quality website for the Environmental Protection Agency.
Ground-level ozone forms when pollution -- such as from vehicles, houses, power plants and industry -- "bakes" in the sun during warm weather, according to environmental protection officials. That can lead to breathing problems for some people.