Two Great Bend residents were treated for carbon monoxide exposure Wednesday evening, Fire Chief Mike Napolitano said. Great Bend police officers entered the apartment building at 3900 Forest Ave. around 6 p.m., and they found Brenda Pardee, 60, and Charles Morgan, 55, on the floor.

Great Bend Fire Department Ambulance transported both people to Great Bend Regional Hospital in serious condition, and they were later transferred to Wesley Medical Center. A spokesman for Wesley said they were both in critical condition Thursday afternoon.

"We’re not sure how long they were in the house like that," Napolitano said. The police department was asked through 911 to check the welfare of Pardee and Morgan, who had missed work for the past couple of days. Their supervisor said he had been unable to make contact with them and was concerned. A maintenance employee for the apartments let them in the building.

The eight-unit apartment complex was evacuated and all of the residents and apartments were checked for carbon monoxide. A Kansas Gas Service employee and City Building Inspector Lee Schneider assisted.

The source of the carbon monoxide was traced to the furnace in the apartment. The other apartments had separate gas meters and furnaces. Everyone else in the building was fine and they were all able to return to their apartments Wednesday night.

Carbon monoxide, often referred to as CO, is an odorless, colorless gas, found in combustion fumes, such as those produced by heating systems. According to information from the Centers for Disease Control, CO can build up in enclosed or semi-enclosed spaces. People and animals in these spaces can be poisoned by breathing it.

Symptoms of CO poisoning include headache, dizziness, weakness, nausea, vomiting, chest pain and confusion. People experiencing these symptoms sometimes call the fire department for a "CO check," and if firefighters find carbon monoxide levels are above 10 parts per million, they will call the gas company, Napolitano said. The culprit is often the furnace, but firefighters have also found that homes with attached garages can draw in exhaust fumes if a vehicle is started with the door open.

The Great Bend Fire Department recommends installing a battery-operated CO detector in the home. High levels of CO ingestion can cause loss of consciousness and death.