Seven people, including two children, taken to hospital after carbon monoxide exposure in Mattapan

One of seven people exposed to carbon monoxide gas at a triple-decker home in Mattapan remained hospitalized Wednesday night, according to a Boston fire official.

Five residents, including two children, of the Woolson Street building were released from various Boston hospitals by Wednesday night, along with a worker who was also exposed to the fumes, said Steve MacDonald, Boston Fire Department spokesman. That worker had been repairing the heating unit in the building, which likely prompted the release of the gas, Boston EMS spokeswoman Jennifer Mehigan.

All heating systems in the three apartments were shut down and the windows opened to release any gas, and the city’s environmental health inspector was summoned to the scene.

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After residents were evacuated, the company initially working on the heating unit tightened a loose duct that may have been leaking gas, MacDonald said. After closing windows and doors and running the heat for 45 minutes, no carbon monoxide was present in the department’s readings, MacDonald said.

By 9 p.m., residents were allowed to reenter the building. Only one person, an adult female resident, remained in the hospital, though she may be receiving treatment unrelated to carbon monoxide poisoning, MacDonald said.

“At this time of year, we recommend getting your heating unit or furnace checked out by a licensed professional, especially in older homes,” Mehigan said. “Nights are just starting to get colder, so many might not have thought about their heating units.”

People exposed to carbon monoxide often feel dizzy or nauseous. When people in a building suddenly share those symptoms, it can be caused by the presence of the odorless and colorless gas, Mehigan said.

Too much exposure to carbon monoxide can be deadly.

MacDonald said a couple of carbon monoxide detectors were missing from the building. Another lacked batteries. The Fire Department will follow up Thursday to make sure the detectors are satisfactory, MacDonald said.

“As the heating season is now upon us, people should make sure their systems are working, and, if necessary, get them cleaned or serviced,” MacDonald said.