The north side of the Police Department, located at 300 W. Winton Ave., was evacuated around 10:45 a.m. after a clerk opened an envelope and noticed white powder on her hand, Hayward fire Capt. LaShon Earnest said.
Fire crews and the Alameda County hazardous materials team responded.
Crews were able to flush the substance off the clerk's hand and she was not injured. No other employees came in contact with the powder, Earnest said.
The hazmat team was able to identify the powder as boric acid, a substance often used for pest control and not considered hazardous, Earnest said.
The evacuation order was lifted around 1 p.m., he said.
Police are investigating where the letter came from, he said.