Allston, MA – About 200 students were evacuated and four were taken to a hospital this morning after carbon monoxide leaked in an apartment building, according to broadcast reports.

Fire crews responded to the scene after a carbon-monoxide detector went off in another apartment. The four were taken to St. Elizabeth’s Hospital after telling firefighters they were suffering from nausea.

Firefighters were called to the complex at 48 Brighton Ave., about 5:30 Thursday after a resident in a basement apartment called to say a carbon monoxide detection alarm had gone off, according to WCVB-TV.

Fire investigators found that an oil burner in the heating system was not ventilating properly, according to acting district Fire Chief Ed Callaghan. He said the alarm saved the residents’ lives.

Fire officials told WBZ radio that they measured carbon monoxide in the building measured at several hundred parts per million. Under 50 ppm is considered safe.

Carbon monoxide is a colorless, odorless gas produced by the incomplete combustion of the fossil fuels. When a person breathes it, the gas displaces oxygen in the bloodstream and interferes with vital body functions.

Authorities were ventilating the building after moving the students elsewhere.