A male employee began work at 6 a.m., restocking shelves in the cooler at the Speedway at 66 Prospect Street in Waltham when he began to feel lightheaded and called 911, according to the Waltham Fire Department.
Firefighters arrived at the scene around 7 a.m., at which point the gas station's atmosphere had become oxygen deficient.
A tier-one hazmat response was declared, sending the state Hazmat team to the scene.
The problem, according to the Hazmat team's Facebook page, was a leak from a large carbon dioxide cylinder.
Hazmat responders put on protective clothing and used meters to determine there were no other problems.
The male worker was transported to a hospital, according to the Waltham Fire Department.
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