Firefighters and police were responding to a reported chemical leak in South Philadelphia late Tuesday afternoon.
Motorists were advised to avoid the area around 2801 S. Christopher Columbus Blvd. near Oregon Avenue.
The location is the site of a facility operated by Ashland, a global chemical business.
"Businesses in area of Columbus Blvd. & Oregon Ave. are asked to shelter in place with doors closed and air condition system off," police said on Twitter.
Firefighters are "investigating a substance leaking from a tank," police said.
Ashland spokesman Gary Rhodes said the plant "experienced a release of phthalic anhydride, a material used in the making of plastic that goes into fiberglass."
The material was "released into a containment dike on the plant property. Vapor from the molten material condensed in the air, causing flakes of the material to settle on or just outside the plant property," he said.
The cause of the accident was unknown, he added.
"Please avoid area & take a alt streets," the city Office of Emergency Management said on Twitter.