A cloud of noxious white smoke from the back of a trash truck forced the brief evacuation of about a dozen Bethesda residents yesterday, disrupted traffic around Wilson Lane and Bradley Boulevard and sent three rubbish haulers and a firefighter to the hospital.

The fire broke out about noon on Wilson Lane near Bradley Boulevard after the three trash collectors picked up a plastic bag that apparently contained a flammable chemical, fire officials said.

As smoke poured from the truck, police rerouted traffic and firefighters evacuated about a dozen people from houses in the area, keeping them away for about 90 minutes.

Capt. Raymond Mulhall, a spokesman for Montgomery County's Department of Fire and Rescue Service, said firefighters were unable to determine exactly what the bag contained or why it ignited. He said it had been left out for rubbish haulers by a resident of Wilson Lane, whom he would not identify.

The three rubbish haulers, complaining of throat and eye irritation, were treated at Suburban Hospital and released. Capt. Richard Hobbs of the Bethesda Fire Department identified them as Aron Artis, 45, of Washington; Jerry Davis, 47, of Riverdale; and George Dorsey, 48, of Mount Airy in Carroll County.

Firefighter Douglas Bennett of the Chevy Chase Fire Department also was treated at the hospital for throat and eye irritation.

Mullhall said firefighters allowed the fire to burn because hosing it down might have caused the chemical to spill onto the street.

Hobbs said members of the department's Hazardous Materials Response Team examined the charred trash and determined that it could be safely disposed of at the county's landfill in Laytonsville.