It was unclear how long the deadly gas, which was detected at 5:30 p.m., had been freely flowing through 781 Drew St. in Cypress Hills.
The sickened residents were evacuated and hospitalized with dizziness and nausea, according to fire officials.
"I went upstairs to see my neighbor, and her son was passed out," said resident Melissa Perez, 13. The son is 2.
Eight people were taken to Jamaica Hospital in Queens, while another three went to Brookdale Hospital. Their conditions were unknown.
Some neighborhood residents complained of minor headaches and dizziness but were not hospitalized, according to witnesses.
A flu pipe that had been removed from the building's boiler, preventing proper ventilation, appeared to be the cause of the leak, said Ed Yutkowitz, a spokesman for Keyspan Energy.