An ammonia leak at the Midcoast Frozen blueberry processing facility March 6 made for pockets of pungent air behind the Route 1 building but appeared to pose no immediate danger.

Firefighters responded to a 911 call from two workers in an attached office building that is home to several small businesses. One of the workers told The Republican Journal that the alarm with a flashing green light went off twice before they called 911. Initially, they thought it was a test, she said.

Firefighters walked around the 248 Northport Ave. building with handheld ammonia detectors that occasionally let out a chirp like a smoke detector that needs a battery. A representative of the building arrived soon after and found the source of the leak, which continued, occasionally concentrating in a gust of wind and causing the assembled workers to drift one way or another to get away from it.

Anhydrous ammonia is commonly used as a coolant in commercial refrigeration systems. If released into the air, the fumes can be toxic and, in high concentrations, even fatal. The system at Midcoast Frozen runs refrigerators and massive deep freezer added to the building in 2012 by Midcoast's predecessor Coastal Farms Food Processing.

Among the handful of large ammonia-based refrigeration systems in the city, one at Penobscot McCrum potato products factory on the Belfast waterfront, has received the most attention. A 2014 inspection and findings of violation by the U.S. Environmental Protection Agency kickstarted improvements at the plant, including a siren to warn the public of a major ammonia leak, and emergency planning between city and county emergency officials.

As part of a resolution, Penobscot McCrum bought equipment for local first responders.

The call on March 6 was the third ammonia leak since the fire department got the additional gear, Fire Chief Jim Richards said. One of the previous calls was to Midcoast Frozen, while the other was to Penobscot McCrum.

The firefighters stayed at the Midcoast Frozen facility Monday night until the leak was repaired.