A small explosion at the Samuel Adams brewery in Lehigh County injured two employees and forced the evacuation of about 100 others, officials said.

The explosion happened shortly before noon Thursday inside the boiler room of the brewery, which is located adjacent to Interstate 78, near Route 100, in Upper Macungie Township.

Workers were starting up one of the boilers when the explosion happened, said Michelle Sullivan, a spokeswoman for the Boston Beer Company, who added that the boiler room is a free-standing structure at the brewery.

Employees immediately called 911, shut down all operations and evacuated the brewery's buildings, as per the company's protocol, Sullivan said.

Two workers were treated at the scene for minor injuries. One complained of eye irritation; the other suffered a bump on the head, Sullivan said.

Samuel Adams brewery, Upper Macungie Twp., Lehigh Co.

Hazardous materials crews responded to the scene because asbestos had been used to wrap pipe in the boiler room, said Sullivan, who noted that an environmental monitoring expert was also brought in by the brewery to monitor the air quality.

The company was given the all-clear by the fire marshal and the fire department, who determined the air quality to be good and the building structurally safe, Sullivan said.

Brewing has not yet resumed, but Sullivan said that packaging and shipping operations have returned to normal.