UPDATE 2:30: The crews, according to a nearby commenter, appear to have dispersed now. Not sure what the "mysterious white powder" was, but we'll update as soon as we know more.

2:10: Both St. Francis Memorial Hospital and the post office at 900 Hyde have now been shut down. The San Francisco post office was evacuated this morning at around 11:35 a.m. when employees found a "mysterious white powder on their clothing," prompting them to walk to the nearby hospital's emergency room.

KRON has more:

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.

12:48: We're getting word that the emergency room at St. Francis Memorial Hospital (1400 Pine) has shut down today due to a hazmat situation.

According to KRON 4, "Five people at the hospital...have similar symptoms caused by an unknown substance."

The San Francisco Department of Public Health is currently investigating the incident. Fire spokeswoman Mindy Talmadge tells BCN/SFAppeal that fire crews are “testing a substance on somebody's clothing” at the hospital.

We'll update as soon as we know more.