The Philly region is under an Excessive Heat Warning, with heat indexes up to 105 degrees possible

The Philadelphia region could see heat index values in the triple digits on Wednesday.

An Excessive Heat Warning is in effect through 8 p.m. Wednesday for Delaware, Eastern Montgomery, Lower Bucks and Philadelphia counties in Pennsylvania, and in Mercer, Camden, Gloucester and Northwestern Burlington counties in New Jersey.

Heat-related illnesses, like heatstroke and exhaustion, increase significantly during extreme heat and high humidity events, per the National Weather Service.

NWS Meteorologist Ray Martin said Wednesday’s heat spike feels hotter because it cooled down first.

Code Orange in effect in Philly, suburbs

Pennsylvania officials have declared a Code Orange Air Quality Action Day for Ozone for the Philly region on Wednesday. That includes Philadelphia, Bucks, Chester, Delaware and Montgomery counties.

Under a Code Orange, air pollution concentrations may be unhealthy for vulnerable groups such as children, people with asthma, people with heart or lung disease and older adults.

The state Department of Environmental Protection cited mostly sunny skies, light winds and high temperatures in the low- to mid-90s as likely contributors to 8-hour average concentrations of ground-level ozone in the Code Orange range.

Ozone is formed from burning fossil fuels. Cars, trucks, power plants and industrial facilities contribute nitrogen oxides and volatile organic compounds which, when combined with sunlight, create smog, also known as ground-level ozone.

Residents and businesses are also encouraged to reduce ozone air pollution by driving less, limiting engine idling and conserving electricity by setting air conditioning to a higher temperature and turning off lights not in use.

Residents are encouraged to visit AirNow.gov to check local conditions.

If you need a primer, here’s how to understand your air quality index.

How to help

During extreme heat, check in on elderly neighbors and relatives. If you see someone experiencing symptoms of heatstroke, like rapid heartbeat, difficulty breathing or chest pain, call 911.