A woman who works for the county's planning and building department at 455 County Center opened a letter Wednesday afternoon and found a white powder inside, San Mateo County sheriff's Deputy Rebecca Rosenblatt said.
Some of the substance got on her and she felt a burning sensation on her lips and fingers, but continued working and left the letter on her boss' desk to alert the boss to it later, Rosenblatt said.
The employee felt OK this morning, but decided to notify her office about the letter before she returned to the building. In turn, department officials called the sheriff's office.
Sheriff's personnel responded around 8:30 a.m. Thursday with a hazardous materials team.
The second floor of the county building was evacuated, and as of 10:30 a.m., emergency crews were still working to secure the building and examine the suspicious powder, Rosenblatt said.
The hazardous materials team removed the envelope from the building and brought it to a remote lab in a van parked outside, Rosenblatt said.
The substance was tested and determined not to be hazardous, and the evacuation order was lifted at about 11:30 a.m.
Other parts of the building were not affected by the incident, she said.