A woman, her two children and her boyfriend were sent to the hospital late Sunday after dangerous levels of carbon monoxide leaked into her West Side apartment.

The poisonous gas collected in the basement of the six-unit apartment building in the 1200 block of North Keeler Avenue because of six water heaters and six boilers packed into a small area, People’s Energy spokeswoman Elizabeth

Castro said.

The area was not properly vented, and the flue system was not adequate for 12 major appliances, she said.

The carbon monoxide seeped into a second-floor apartment, sickening the four about 11:30 p.m. They were taken to Norwegian-American Hospital. The adults were treated and released Monday, according to a nursing supervisor. The children were listed in stable condition, she said.