BRIDGEVIEW, Ill. (AP) _ An explosion at a chemical plant early today rocked this Chicago suburb, injuring four employees and forcing at least 300 people from their homes for almost two hours, officials said.

Police evacuated the residents living near Unocal Corp.'s chemical division polymer manufacturing plant as a precaution when irritating fumes drifted through the area following the 1 a.m. explosion, said dispatcher Mark McFall.

About 300 people, many in pajamas and carrying children, crowded into the John A. Oremus Community Center, said Sophie Robish, who helps out at the center.

''We heard the explosion and jumped out of bed like a spring,'' said Mrs. Robish, 50, who lives two blocks from the plant.

The residents were allowed back in their homes nearly two hours later, after the fumes cleared the area, McFall said.

Investigators from the U.S. Environmental Protection Agency and the Illinois EPA were at the scene, about 13 miles southwest of Chicago, trying to determine the origin and cause of the blaze, said Deputy Chief Chuck Corkery of the Cook County Emergency Services and Disaster Agency.

The plant, formerly known as the Union Chemical plant, makes latex, an adhesive used to bind materials in paint. The blast apparently occurred in ''an explosion-proof area'' equipped with special electrical wiring and reinforced walls, said John Maher, manager of field services at Unocal's chemical division in suburban Schaumburg.

Maher said damage to the plant was ''extensive.''

Nursing supervisor Sharon Hann of Christ Hospital and Medical Center said three victims were brought to the emergency room, one of whom was badly burned and transferred to Loyola University Medical Center. A fourth victim, also badly burned, was taken directly to Loyola.

Mrs. Robish said volunteers at the community center served coffee to the evacuees.

''A lot of people are in their night gowns, with their pillows and puppies and children,'' she said.

Among the evacuees was Katie Dyrda, 22.

''I just got done reading and was getting ready for bed when I heard this very loud boom,'' she said.

Mrs. Dyrda said she grabbed her 3-year-old son and headed for the center.