An ammonia leak Tuesday injured a maintenance worker at a storage facility in St. Paul and forced the evacuation of 10 nearby buildings.

The worker, whose name was not released, was treated at the scene and taken to Regions Hospital. He was initially harmed by "significant exposure" but was released Tuesday afternoon and was doing well, said St. Paul Fire Marshal Steve Zaccard.

The leak was detected about 8:50 a.m. at Midway Cold Storage on Territorial Road by a passerby who smelled the ammonia and saw a vapor cloud. A worker told authorities the leak was under control, but authorities found increasing levels of ammonia.

Firefighters closed two valves on the building's roof about noon and allowed ammonia to dissipate from a third. The building remained vacant all day as firefighters ventilated it. Nearby offices reopened about 2 p.m.

The ammonia circulates through coils in the company's freezers and coolers.

Kevin Zwiefelhofer, a mechanical engineer at Harris Companies, which services Midway's refrigeration systems, said the ammonia leaked from an evaporator in one of the freezers.

Zwiefelhofer said it's unlikely that a coil burst or that someone tampered with the evaporator because it hangs from the ceiling. It could take a day or two to determine the cause.

The city recertified Midway Cold Storage this May. Inspection records from this year and 2008 show minor "deficiencies" unrelated to ammonia.

No reports concerning the business have been filed with the Occupational Safety and Health Administration.