Chicago, IL — Four people were rushed to the hospital Thursday night after dangerously high carbon monoxide levels were found in a North Lawndale home, Chicago fire officials said.

The carbon monoxide scare serves as a warning to everyone.

Three of the four individuals transported to hospitals were in serious to critical condition.

CFD spokesman Larry Langford said CFD responded around 6 p.m. to the home in the 1800-block of South St. Louis for a person feeling sick on the first floor. As EMTs treated that person, they learned there was an 80-year-old woman on the second floor who had been unresponsive for “the whole day.”

They called in more help and when paramedics got to the second floor they found the elderly woman unresponsive and the crews’ carbon monoxide alarms started to alarm, indicated high levels in the home.

CFD tweeted their readings showed carbon monoxide near 500 parts per million. Over 101 parts per million is considered to be a dangerous level of carbon monoxide exposure. Levels this high are considered deadly, Langford said.

CFD said the home had no working carbon monoxide detectors and wants to warn the public about the dangers of this odorless and colorless gas as temperatures plunge.

Officials said the detectors should be installed on every floor of a home near sleeping areas so they can be heard.

CFD is working with Peoples Gas to determine the exact cause. At this point believe it may be linked to the furnace or heating systems. Langford said this is a good reminder for people to check their heating systems and to make sure they have working CO monitors.

“Get one as soon as you possibly can. At least one for every floor where you have sleeping areas. And install them according to instructions. It’s very cheap insurance to keep your family safe and keep you from making trips to the hospital,” he said.