It's commonly known as the silent killer.
"There is no smell. With carbon monoxide poisoning, it's odorless and colorless. So there was no odor," says Cathy Gerac, Commander, Austin-Travis County EMS.
It was Saturday morning when two ambulances were dispatched to a home on Wagon Crossing in southeast Austin.
EMS says a family of seven were all experiencing symptoms of carbon monoxide: headache, dizziness, nausea.
"It's pretty frightening because we could have had seven family members deceased in that home this morning. Fortunately, we didn't because somebody called for help," says Gerac.
Two adults were taken to St. David's Hospital and two children were taken to Dell Children's Medical Center.
Three other children had already left the home for their daily activities.
Two of them were taken by a private vehicle to Dell Children's and the third was evaluated by EMS and released.
EMS says they see a rise in these type of events during the winter, and have this warning:
"You want to make sure if you're burning fire in a fireplace, you have an open flue away to ventilate that fire. We highly recommend that you do not use the stove for heating in your home. It's very dangerous because of carbon monoxide. Also, have your gas heaters checked regularly," says Gerac.
The U.S. Environmental Protection Agency says at low concentrations, symptoms are fatigue in healthy people and chest pain in people with heart disease.
At higher concentrations, symptoms include impaired vision and coordination, headaches, dizziness, confusion and nausea.
Austin-Travis County EMS says work recently done on the homes' roof could have played a factor in the exposure but they are still trying to figure out the exact cause.