EDGEWATER PARK, N.J. (AP) _ An explosion and smoky fire at a plastics plant Sunday injured four men and forced the evacuation of about 500 people within a one-mile radius, authorities said.

The fire at the C-Lec Plastics Co. plant was brought under control 31/2 hours after it was reported by more than 100 firefighters, said Monica Gavio, supervisor at the Burlington County communications center.

She said the blaze's thick, black smoke prompted authorities to evacuate the surrounding residential area as a precaution. They were later allowed to return home, said Nancy Paolini, another county communications supervisor.

''Any time you burn plastics, you're going to have dangerous fumes,'' said James Staples, spokesman for the state Department of Environmental Protection.

But he added that samples taken found ''totally negligible'' concentrations of airborne toxins near the fire.

The tests recorded harmless levels of two hazardous chemicals, half a part per million of hydrogen cyanide and one part per million of styrene, said Staples.

Ms. Gavio said a former plant employee told her the company makes ''large plastic rings that are used in nuclear reactors,'' but added that she did not know if any toxic substances were stored in the 100-by-50-foot building.

Plant owner Dan Walsh, 31, of Riverton, was treated for burns on his hands and ear at Rancocas Valley Hospital in nearby Willingboro and released, a nursing supervisor said.

Three firefighters were treated for smoke inhalation at Zurbrugg Memorial Hospital in Riverside and released, a nursing supervisor said.

Town supervisor Robert Heal said the cause of the fire was under investigation.