New York, NY — Residents were evacuated from their Bronx apartments Monday after some people said they suddenly felt ill.

Investigators said the problem was carbon monoxide, CBS2’s Jessica Layton reported.

Parents hugged their kids in the cold, knowing that despite being stuck outside, they were lucky.

“Yes, because it’s like a silent killer. I wasn’t aware of anything,” mother Grismaldy Lopez said.

Lopez and her little boy didn’t know there was a problem until the FDNY knocked on their door and told them to get out. As neighbors evacuated, firefighters tried to find the source of the poison that sickened 11 people — four EMS workers and seven residents — including a mother and her two kids on the first floor.

“We have the detector, but it didn’t make any sound,” Lopez said. “Even right now I’m scared to go in, because I don’t know if anything can come back and something could happen during the night.”

EMS rushed to the corner of East 195th Street And Decatur Avenue in the Fordham Manor section of the borough during dinner time after residents called 911 to report they didn’t feel right.

“When they went into the building, their carbon monoxide readers on the coats went off with 200 parts per million,” FDNY Chief Michael Gunning said.

Gunning said anything over 100 parts per million is high. The FDNY quickly realized the leak was coming from the building’s basement furnace.

“Once we shut the furnace down and vented all the windows, the CO readings went down,” Gunning said.

Everyone is expected to be okay, including that mom and two kids. Layton has learned they plan to stay overnight in the hospital just to be safe.