An air quality alert is in effect for the Philadelphia region and other parts of Pennsylvania today.
The state Department of Environmental Protection says people who are sensitive to air pollution should limit outdoor activities.
The code orange alert is in effect in Philadelphia, Bucks, Chester, Delaware and Montgomery counties in southeastern Pennsylvania. Other parts of eastern and south-central Pennsylvania are also covered by the alert.
A code orange alert means pollution standards may become unhealthy for groups that include children, the elderly and those with asthma or lung and heart conditions.
The high particle conditions in the Philadelphia area today are "due primarily to stagnant conditions," according to AirNow, the Environmental Protection Agency's air-quality site.
Light to calm winds are expected for the day, AirNow says, "with limited vertical mixing and recirculating flow," which will keep particles in the "unhealthy for sensitive groups" range.
Relatively warm and humid weather is also keeping the particle concentrations high. Forecasters say to expect a high temperature of 52 degrees in Philadelphia today, 3 degrees above average for the date.
The code orange alert is also expected to be in effect Wednesday.
"Stagnation overnight and through the morning rush hour, combined with very limited vertical mixing, will allow particles to accumulate," AirNow says. "Surface winds will pick up in the afternoon, but they will most likely be insufficient to adequately ventilate the atmosphere."