UPDATE, 9:15 p.m.: Boston Fire Department reports: "All meter readings negative. Building will be turned over to BC. City Public Heath will check building again in the morning."

Boston firefighters are at the Merkert Chemistry Center at Boston College this evening, investigating the source of a vapor reported by students entering a third-floor lab. The department declared a Level 3 hazmat situation, which means that firefighters going into the building have to wear full hazmat suits because of the possible risk.

It's the second level-3 response to the Merkert building in a little over a month and at least the third large BFD response to the building in a little over a year.