Chicago, IL  — Four people were taken to the hospital Tuesday morning after a faulty furnace apparently caused elevated carbon monoxide levels in a high-rise apartment building in Uptown.

The Chicago Fire Department said crews responded to 4541 N. Sheridan Rd. and found carbon monoxide readings of 200 parts per million (ppm), which can be potentially deadly in cases of prolonged exposure.

According to the Consumer Products Safety Commission, extended exposure to carbon monoxide levels above 70 ppm can cause headaches, fatigue, and nausea. At sustained concentrations above 150 to 200 ppm, carbon monoxide exposure can cause disorientation, unconsciousness, and death.

A total of four people from the building were taken to hospitals in fair to serious condition. Residents were allowed back in the building after carbon monoxide levels returned to normal.

The Fire Department said it appeared a faulty furnace was the cause of the carbon monoxide leak.