High levels of carbon monoxide sent 6 people to the hospital Wednesday night in Watertown, Massachusetts.
The incident happened at an apartment complex on Riverside Road just after 8 p.m.
Fire officials said residents evacuated the building after they began getting headaches and feeling light headed.
Six people were taken by ambulance to Massachusetts General Hospital to be checked out but all were expected to be OK.
Authorities believe a faulty heating system was to blame for the high levels of CO.
Residents have since been allowed to go back into the building.