The Department of Environmental Protection has issued an ozone air quality alert for several counties in southeastern Pennsylvania.
The DEP said the code orange alert is in effect Tuesday for Bucks, Chester, Delaware, Montgomery and Philadelphia counties.
Under the system, green represents good air quality, yellow is moderate, orange represents unhealthy pollution levels for sensitive people, and red warns of unhealthy pollution levels for everyone, officials said.
Ground-level ozone, a key component of smog, forms during warm weather when pollution from vehicles, industry, households and power plants heats up, making it hard for some people to breathe.
The DEP said the elderly and those with respiratory problems such as asthma are especially vulnerable to air pollution and should limit outdoor activities.