Deadly levels of carbon monoxide (CO) sent a family of four to the hospital. It happened on Milwaukee's northwest side.
"They were having symptoms of headaches and a bit disoriented," said Battalion Chief James Hardy, with the Milwaukee Fire Department.
A carbon monoxide leak brought firefighters out on a cold day to a home just north of 79th and Hampton.
"Once we went inside we were able to ventilate and also find out the cause of the problem. From our investigation, it was the furnace," said Chief Hardy. Battalion Chief James Hardy said levels were deadly.
"Anything in that 100-150 range can be real dangerous and ultimately can be fatal," Hardy said.