MILLBRAE, Calif. -- A hazardous materials crew didn't find any harmful substances at Mills High School in Millbrae Tuesday afternoon when they were called to investigate a mysterious vapor coming from the school's boiler room.

About two dozen firefighters and members of the county's hazardous materials team responded to the school, located at 400 Murchison Drive, after a fire alarm alerted authorities of the vapor just before noon, San Mateo County Office of Emergency Services district coordinator Steve Mahaley said.

School is out for the holidays, but about 30 students and staff on the campus were told to leave while emergency personnel investigated the vapor, he said.

"We went in and were able to determine there was nothing hazardous," Mahaley said.

He said everyone had cleared the scene just after 3 p.m.