CHICAGO (CBS) — Six people went to the hospital Thursday morning, after firefighters discovered a carbon monoxide leak at a home in the Marquette Park neighborhood.

Paramedics responded to a call for a sick person at a home near 64th and Homan around 1:15 a.m. When they arrived, sensors they carry with them alerted the paramedics to high levels of carbon monoxide in the home.

“Paramedics wear meters. So if they’re in a toxic environment, such as carbon monoxide, that meter will alert them. We had an incident like this a few months back, where the paramedics were instrumental in saving those lives. It happened again,” Chicago Fire Department Deputy District Chief Walter Schroeder said.

Two people were taken to hospitals in serious to critical condition, and four were taken to hospitals in fair to serious condition.

“We could have had six fatalities very easily,” Schroeder said.

The cause of the carbon monoxide leak was under investigation.

Meantime, firefighters also received a call for elevated carbon monoxide levels at a three-story apartment building near 87th and Paxton in the Calumet Heights neighborhood around 3 a.m.

No injuries were reported in that incident.

Schroeder said, since Wednesday, Fire Department crews have responded to at least four carbon monoxide calls.

“Especially with this cold weather, we do experience our run volumes to go up. Obviously people are cold, just as we are,” he said.

Schroeder said prevention is key. He urged Chicagoans to make sure their homes have working smoke and carbon monoxide detectors. Not having one could be a matter of life and death.

“It’s very deadly, and you don’t want to jeopardize yourself, or your family,” Schroeder said.

Illinois state law requires homes to have smoke and carbon monoxide detectors within 15 feet of any rooms used for sleeping and on every level of the home.

Fire Department officials said you should test smoke and carbon monoxide detectors every month and change the batteries every six months.

“You should also have your furnace, obviously, maintained if that needs to be checked out, as well as taking a good look at your hot water tank because that does give carbon monoxide also,” Schroeder said.