PUEBLO, Colo. (CBS4) – A family of 12 survived carbon monoxide poisoning in their Pueblo home on Wednesday.

The family’s father said he heard his 6-year-old screaming in pain while the rest of the family was sleeping in their home in west Pueblo.

READ MORE: Testing For COVID Ramping Up Again In Colorado

He found the rest of the children also sick. All 12 were transported to the hospital for treatment. The Pueblo Chieftain reported that the 10 children range in age from 2 to 20 years old.

Carbon monoxide levels were so high, firefighter wore oxygen masks. Chief Brad Davidson of Pueblo West Fire said a vent on a water heater wasn’t functioning properly and instead of pulling the deadly gas from the home it was pushing it back in.

The gas is colorless and odorless, making it difficult to notice without detectors until it starts causing physical problems.

The father, Tim English, told KKTV he heard his son about 1 a.m.

READ MORE: Vaccination Rate Keeps Colorado Hospitals Out of Jeopardy

“He kind of fell into my arms in the bathroom and almost fainted,” English said.

Investigators said they didn’t think the family had working carbon monoxide detectors. The family told KKTV they had one on the other side of the house from the water heater.

“I’m just thankful we did call 911, and we had a great group of people to come help us,” Sheri English said.

Lt. Mike Pylars of the Denver Fire Department said Thursday that firefighters respond to several carbon monoxide calls this time of year — mostly because of how some people try to heat their homes.

“People are using heaters designed for the outdoors. That’s going to cause carbon monoxide,” he said.

MORE NEWS: Gov. Jared Polis To Request Federal Disaster Declaration For I-70 Mudslide Damage Through Glenwood Canyon

Firefighters recommend a carbon monoxide detector on every level of homes and inside every bedroom. Any gas appliances should be inspected annually for problems.