The Des Moines Fire Department, Des Moines Hazmat and postal inspectors were called to the main post office late last night.
The fire department was called to 1165 2nd Avenue shortly after 11 p.m. after a postal employee became ill.
It was determined the man had been exposed to an unknown powdery substance.
Hazmat workers were called to test the powder and determined it was not harmful.
Postal inspectors were at the post office until early this morning trying to find the letter the powder came from.