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.