A Sioux Falls family is lucky to be alive tonight after deadly levels of carbon monoxide were found in there home. The levels were more than 10 times what's considered safe.

It's not uncommon for fire fighters to respond to carbon monoxide calls this time of year. But the levels inside this house were higher than they'd ever seen.

For firefighter Doug Stephenson, it seemed like a routine call. Around 11 o'clock at night a family of five, mom, dad and three kids all reported being sick. As he and other firefighters rode to the house, Doug grabbed the gas detector. They got inside the house, "and then all of a sudden the alarm went off."

The first reading was of carbon monoxide levels at 600 parts per million. The highest reading Doug had ever seen before was about 125. As they checked out the rest of the house, the detector registered as high as 800 ppm. Seventy parts per million is considered dangerous. Doug tells KSFY, "the feeling was that if this family had gone to bed, that it would have been a tragedy that would have been discovered the next day or whenever."

Firefighters discovered there was a problem with the home's furnace.

Katie Nelson lives just down the street. She was terrifying to learn what almost happened. "It kind of gets you thinking about the precautions you should follow though to keep your house safe and keep yourself safe."

Firefighters tell KSFY they understand the family has now recovered and is doing fine. We tried to talk to them, but there was no one home.