The Illinois Environmental Protection Agency has extended its Air Pollution Action Day due to an increase in ground-level ozone. That’s created when chemicals and exhaust from industrial smokestacks and car tailpipes “cook” in the heat.

The agency announced an air pollution warning for the Chicago area, for the fourth consecutive day, as high temperatures continue to hit triple digits. The agency says elevated ozone levels pose a potential health hazard to sensitive populations, especially individuals with respiratory or pulmonary disorders as well as active children and adults.

“We’re certainly concerned because we’ve had this continuing for the past three days,” said Kim Biggs, Illinois EPA special programs coordinator. “We do expect that the levels are going to drop off over the weekend. There’s a chance of rain coming into the area which will certainly help it.”

Regional air quality is expected to reach the “Unhealthy for Sensitive Groups” category on the national Air Quality Index through Saturday, the seventh Action Day issued for 2012, according to the Illinois EPA. 

The agency is also advising people to keep cool, limit driving and avoid physical activity outdoors when air pollution levels are high.