Four people were overcome by carbon monoxide fumes when an unspecified leak caused fumes to spread throughout the home, fire officials said.
Two of the four people were taken to Cornell Hospital suffering from exposure to carbon monoxide, officials say, while two other people refused medical attention.
The incident occurred at 9:32 a.m. when firefighters responded to 905 Herkimer Street in Weeksville, Brooklyn, where they found at least one person unconscious inside the three-story home. Carbon monoxide alarms were ringing when firefighters arrived, officials said.
Firefighters evacuated 905, 907 and 903 Herkimer as a precaution after they found high levels of carbon monoxide in all three buildings.
EMS declared a mass casualty incident and brought in numerous ambulances to treat those exposed. The two who were hospitalized were taken to Cornell because EMS decided a hyperbaric chamber might help their recovery.
National Grid responded to the scene to shut off service and seek out where the leak may have occurred.
None of the injured have been identified and the incident is under investigation.
