GORHAM — About 200 students had to be evacuated from a dormitory on the Gorham campus of the University of Southern Maine late Tuesday after an estimated 20 gallons of silver nitrate spilled in a photography lab.

College spokesman Bob Caswell said three students and one USM security officer were taken to a hospital to be examined, as a precautionary measure.

Caswell said the students seemed fine and were not exhibiting any symptoms of being exposed to silver nitrate, a potentially poisonous compound that is more of an irritant if it touches skin.

The accident took place around 9:30 p.m. in a ground floor lab of Robie Andrews dormitory at 39 University Way.

More than 50 firefighters, emergency medical technicians and members of the Presumpscot Valley HazMat team responded.

Gray Fire Chief Ricky Plummer, who is a member of the HazMat team, said it would likely be several hours before the students would be allowed back into their rooms.

Plummer said the silver nitrate was being used to process photos. Investigators think students tipped over a tank containing the chemical.