FDNY reports the cause of the elevated CO readings was due to a gas leak from a faulty furnace. The building is located on 232 East 165th Street in the Melrose section.
The gas was turned off as crews were in the process of ventilating the building.
A total of 13 people were sent to the hospital. One patient is critical, and the remaining 12 are stable.
The building is a five-story multiple dwelling with 14 residential units.