Four people were hospitalized as a precaution after an envelope with white powder was found in the mail room of the Coffee Creek prison in Wilsonville.
Tualatin Valley Fire and Rescue responded with its Hazardous Materials team.
The mail room was secured and the ventilation system was turned off.
20 to 30 people in the area were evacuated.
The four people who were exposed didn't show major symptoms, but two had minor symptoms.
The powder is being tested and police are investigating.