The incident was reported at 11:38 a.m. at the post office at 1400 Pine St. The five postal workers walked over to the hospital, located about a block away at 900 Hyde St., a fire official said.
San Francisco Department of Public Health spokeswoman Eileen Shields said the five employees were evaluated at the hospital and released around 1 p.m.
The hospital was not evacuated, hospital spokeswoman Dee Mostofi said.
The Office of Emergency Services is asking the public to avoid the area around Pine and Larkin streets. The post office remained shut down as of 1:15 p.m.