Baltimore, MD – WJZ -Fire officials say carbon monoxide was to blame for sickening residents of an apartment building in Baltimore.

It happened Monday night in the 700 block of North Carey Street. In all, 21 people were taken to two area hospitals.

The carbon monoxide originated in three furnaces. The furnaces have been turned off until they can be repaired by the building owner.

The Maryland poison control center takes up to 150 calls each year about carbon monoxide poisoning. Monday’s incident they say is a reminder to have heating systems inspected and carbon monoxide detectors installed. “Why that’s so critical is that carbon monoxide is colorless, it is odorless, you can’t detect it in your environment, and the initial symptoms of carbon monoxide poisoning are extraordinarily suable and look like the other diseases.” said Dr. Bruce Anderson.