Firefighters continued to battle the four-alarm blaze in an industrial section of the city at 11 p.m., more than four hours after the fire was first reported. A hazardous materials team and Westchester County Health Department officials were at the scene.
A preliminary report indicated that hydrochloric acid, nitric acid, sulfuric acid, alcohol and acetone may have been among the chemicals in the plant, which is the headquarters of KEM Chemical Corp.
Residents in Pelham, to the east of Mount Vernon, were advised to stay indoors and keep their windows shut because smoke from the fire was blowing in their direction.
The company's main building at the plant had collapsed, the Fire Department said. An area of several square blocks around the fire had been cordoned off, with no one allowed inside.
Angela Cox, a spokeswoman for Pelham's schools, said police interrupted a school play to tell everyone to leave because of the fire.
"They said it was a precaution. Everybody got out safely," Cox said.
It wasn't immediately clear what risks nearby residents faced by breathing fumes from the fire.
KEM Chemical specializes in supplying acids and other chemicals for microelectronics manufacturers, according to its Web site. The 47-year-old business is a relatively small, often producing chemicals in volumes smaller than the manufacturers want to make for themselves, the Web site says.
A call to the company yielded a busy signal Friday night, and an e-mail message to company representatives was not immediately answered.