The evacuation occurred at about 1:45 p.m., forcing everyone inside the building to go outside into the rain, Reiter said.
Authorities swept through City Hall to ensure that all four floors and the building's basement were cleared out and to determine whether there was any merit to the alarm. The cause was identified and people were allowed to return to the building by 2 p.m., he said.
Mayor Gavin Newsom and most members of the Board of Supervisors were out of their offices at the time, but their staff members were forced to leave during the evacuation, Reiter said.
Reiter said the alarm was activated by a staff member in the City Hall day care center who had been caught out in the rain and was using a hair dryer in a room with a low ceiling.
"Whenever you're drying your hair you're essentially burning it, and that smell was picked up by the detector," he said.
The staff member's attempt to dry off ended up getting everyone else wet, since it was pouring outside at the time of the evacuation.
"Nobody's ever happy about going out in the rain," Reiter said.