The odor was described as that of "fuel" or "gas," dispatchers said.
The county's emergency management team set up a command post on Highland Avenue in Chester Township.
"Starting at 10 a.m. we've had numerous reports all across the county of odor of gasoline or fuel affecting businesses, schools, community," said Tim Boyce of Delaware County Emergency Management Services.
The smell was so strong it was reported 13 miles away in Broomall.
- Fmr. firefighter, deputy rescue woman from submerged car
Boyce said his team and partner agencies are checking the sky, rails and the refineries for the source of the smell.
"We're looking at every fixed site. Our pipeline, our industrial facilities, airport. We've checked the airport to make sure a plane didn't lose fuel. We excluded it," Boyce said.
Air quality tests found nothing out of the ordinary, leading Boyce to believe the odor wasn't toxic.
By mid- afternoon the smell had largely dissipated before officials could find the source.