The "Code Red" affects Berks, Bucks, Carbon, Chester, Delaware, Lehigh, Monroe, Montgomery, Northampton, and Philadelphia counties until 11:59 p.m. Thursday night according to the National Weather Service.

There is also a Code Red Air Quality Alert in the central and northern mountains of Pennsylvania, the Susquehanna Valley, and northeast Pennsylvania.

"Members of sensitive groups may experience more serious health effects. The effects of air pollution can be minimized by avoiding strenuous activity or exercise outdoors," said the National Weather Service.

State air quality monitors registered "hazardous" air over 300 AQI Thursday in the Lehigh Valley and Susquehanna Valley according to AirNow.gov. The Philadelphia area was registering "very unhealthy" air pollution levels of 270 at 11 a.m.

Related article: When Will The Smoke Clear In PA? Latest Forecast For Thursday

The highest concentrations of wildfire smoke should wane Thursday morning and into the afternoon across Pennsylvania, the National Weather Service said, though haze and smoke is expected to become more dense into Thursday evening.

Thursday's Air Quality Index (AQI) levels are expected to be slightly better, but still unhealthy for people with heart or lung disease, older adults, children, and teens, said the weather service, though Pennsylvania may not see the conditions clear completely until Friday or Saturday.

The frequency, extent and severity of wildfires mark important indicators of climate change, environmental officials say. The peak of the nation's wildfire season is also occurring earlier — peaking in August from 1984-2001 and then in July from 2002-20, according to research from the U.S. Environmental Protection Agency.