Two firefighters stopped at a Subway for lunch, but instead of leaving with sandwiches, they randomly discovered potentially deadly carbon monoxide conditions that shut down a Lanham shopping center Friday morning.

Firefighters Melvin Batts and Sam Acala had popped into a Subway restaurant near Greenbelt and Good Luck roads around noon when the carbon monoxide detectors strapped to their walkie-talkies went off.

When other crews arrived, they discovered serious carbon monoxide levels in the air, said Battalion Chief Mike Linynsky. Safe readings of carbon monoxide should fall in the range of 0 to 5 parts per million, he said, with people typically experiencing headaches at 20 parts per million.

“The readings we found were as high as 600 ppm,” Linynsky said. Enough exposure to that and “you’d be dead.”

The shopping center was immediately evacuated, and firefighters suited up to find the leak. Emergency crews found that the exhaust vent from an air conditioning unit was broken and funneling carbon monoxide into stores instead of outside.

Crews shut down the unit, reduced the carbon monoxide levels and had natural gas to the building turned off before people were allowed back in about 2 p.m., according to a statement from the department and messages from its Twitter account. One person was transported to a hospital.

Often referred to as the “silent killer,” carbon monoxide is colorless, odorless and can’t be seen.

Ambulance crews have been carrying small detectors that constantly monitor the air for carbon monoxide levels since 2015. The devices, which are a little bit bigger than a lighter, are designed to alert firefighters to potentially hazardous conditions as they’re answering 911 calls.

“We normally get a call for a headache and it goes off when we get into someone’s house,” Linynsky said. “This is the first time it’s gone off randomly like this. The guys are just getting something to eat and it goes off.”