SOUTH PORTLAND (AP) – The city’s emergency plan adopted after the terrorist attacks on Sept. 11, 2001, worked smoothly when a supermarket was evacuated because of an airborne irritant that caused burning throats, difficulty breathing and nausea. Within minutes, 21 ambulances were parked outside the Shaw’s Supermarket at Mill Creek treating customers and workers and carting them off to the hospital.

“That’s something we’ve drilled on since 9/11,” Fire Lt. Robb Couture said Tuesday. “We were able to triage 43 patients in under an hour, and get them to the hospital.”

All told, more than 50 complained of symptoms and went to one of three hospitals to be checked out Monday night. Most of the people who fell ill were feeling better within an hour, officials said.

Officials believe a problem with the heating system in the recently renovated store was to blame for the symptoms that led to the evacuation at about 6:45 p.m. Monday.

After the supermarket was cleared, workers turned on the heating system and it produced a metallic-type odor, Couture said. Workers noted that part of the ductwork had come loose, and the problem was fixed, he said.

The Northern New England Poison Control Center initially suspected Freon. But workers tested the refrigeration system and there were no leaks, Couture said.

“It could be Freon. It could be another nuisance substance. It could be something that we never know what it really was,” said Dr. Tamas Peredy, a medical toxicologist who was consulting with medical workers on the incident.

Fire officials were unable to detect any foreign substances using sensitive monitoring equipment that can detect everything from carbon monoxide to a chemical or biological attack, Couture said.

Shaw’s was working with local emergency workers to determine the source of the problem. Spokeswoman Judy Chong declined further comment, saying she would let the fire department answer questions about the incident.