Four people were in critical condition Wednesdsay night after they were exposed to what officials say was a "lethal level" of carbon monoxide.
Fire officials responded to the scene in the 300 block of West 107th Street on Chicago’s South Side where they say carbon monoxide levels were at readings of 1,000 parts per million.
"The furnace was malfunctioning and they had all burner of the stove on for heat," said Battalion Chief Tom Sutkus.
Fire Media Affairs said the home was a single family home.
Officials said three people were visiting the elderly homeowner who had just returned from the hospital.
Sutkus said the victims were in "various stages of consciousness."
The victims were taken to an area hospital in critical condition.
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"[The homeowner] just came home today and then the next thing you know they was laying on the ground and they was all passed out," said neighbor Greg Collins.
Carbon monoxide poisoning is known as a silent killer: an odorless, colorless gas that can be generated by any appliance that burns fossil fuel, including any appliance in your house that burns coal, wood, charcoal, gasoline, or propane, according to the U.S. Consumer Product Safety Commission.
Symptoms of carbon monoxide exposure include headache, fatigue, shortness of breath, nausea, and dizziness.
At sustained CO concentrations above 150 to 200 parts per million, disorientation, unconsciousness, and death are possible.