A Subway shop in a Lanham, Md. shopping center was reported as having dangerously high levels of carbon monoxide, according to the Prince George’s County Fire Department.

The leak was discovered when two firefighters stopped at the shop for lunch at the Aerospace Place shopping center located at 9900 Greenbelt Road on Oct. 14.

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The Prince George’s County Fire Department said, in a statement, that two firefighters (Melvin Batts and Sam Acala) discovered the leak when their portable carbon monoxide detectors started going off.

The firefighters called for additional resources and responders found that an exhaust vent from the boiler/HVAC unit had broken and was sending carbon monoxide directly into the store. The entire shopping complex was evacuated.

According to the Washington Post, the levels were around 600 parts per million (ppm). Safe levels range from 0-5 ppm.

Firefighters turned the unit off and used fans to bring down the carbon monoxide levels. No one was injured. The fire department says it appears the unit had just turned on for the day.