A malfunctioning ice-making machine at the University of Southern Maine ice arena forced an evacuation of the university’s Gorham campus Friday night.

Even though the leak, caused by an overheated cooling unit, was small and contained, rain and humidity created an extremely strong ammonia odor in the area, Gorham Fire Chief Robert Lefebvre said in an interview Saturday.

About 50 people were evacuated from the campus, which was sealed off as a precaution until emergency crews understood the extent of the chemical leak, Lefebrve said.

“At the time we did the evacuation we didn’t know how severe of an incident we were dealing with,” Lefebvre said. “Ammonia can be very hazardous to the respiratory system. We erred on the side of caution.”

A USM employee who was exposed to ammonia fumes near the building was sent to the hospital for evaluation, as was a firefighter, who was evaluated for exertion, Lefevbre said. Both were later released.

Crews were called to the university about 11 p.m. Friday, according to Lefebvre.

The ammonia leak appears to have been caused by an overheated rooftop cooling unit for the arena’s ice machine. The malfunction triggered a release valve that spilled about 100 pounds of ammonia into the ice-making room, according to Lefebvre.

Ammonia is a widely used industrial refrigerant.

The ice-making system sensed the ammonia spill and started its emergency shutdown, closing valves, locking the room’s door, and starting fans to ventilate the chemical. When personnel from the Presumpscot Valley Hazardous Materials team got into the room, they found that the spill was fairly minor, Lefebvre said.

But outside, the ammonia had reacted to Friday night’s rain and humidity, causing a powerful ammonia odor that suggested the leak could be serious.

“Ammonia reacts with water; it even makes the smell worse than it actually is,” Lefebvre said. “The rain and humid atmosphere at the time, it just kept that smell hanging low. Had it been a clear night without the rain, it probably would have dissipated into the air.”

Crews cleared the area about 3 a.m. Saturday, Lefevbre said.

Most of the people who were evacuated were in the same dormitory near the ice arena, according to Lefebvre.

The Presumpscot Valley Hazardous Materials Team has members from the Gorham, Scarborough, Westbrook, Windham, Standish and Gray fire departments and is based in Gorham.