The substance was hand-delivered to the center in an envelope Wednesday morning, the Milwaukee Journal Sentinel reported.
Three people who came in contact with the powder experienced burning in their eyes and noses, Beloit Fire Chief Brad Liggett said, adding that the substance was determined not to be a biological or chemical agent.
Nonetheless, 15 people were transported to hospitals for decontamination as a precaution, and another 60 people were quarantined at the health center before being sent home, police Capt. Vince Sciame said.
"There are no serious complications at this point," Sciame said.