Police say the woman was found unconscious Tuesday night in the building on Leavitt Street in Flushing.
Other residents were temporarily evacuated and the building was vented.
The woman was rushed to Flushing Hospital but was pronounced dead.
The leak was traced to a faulty water heater in the woman's apartment, and is under investigation.
Fire officials say smoke and carbon monoxide alarms were present but not operational.
No other apartments or residents were affected, the utility said.
It's not yet known what caused the appliance to malfunction.
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