CHICAGO — At least four people were hospitalized after elevated carbon monoxide levels at a North Lawndale home.

According to Chicago fire personnel, crews registered dangerous carbon monoxide levels, nearing 500 parts per million (PPM) on Thursday in the 1800 block of S. St. Louis Avenue.

Three people were taken to nearby hospitals in serious-to-critical condition, including an 80-year-old woman who was found unconscious. The fourth victim’s condition is unknown.

Fire officials added that none of the smoke detectors were working in the home.

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The furnace may have been the cause of the leak, prompting firefighters to remind locals to check heating and have functioning carbon monoxide detectors.