A late-night carbon monoxide incident sent emergency crews rushing to a residential building in Gravesend, leaving 12 people sick and eight taken to the hospital, officials said.

Firefighters were called to a building on West 6th Street, between Avenue U and Avenue T, around 11 p.m. Sunday after reports of an unconscious person. When crews arrived, they quickly realized the situation involved more than one person.

Officials say elevated levels of carbon monoxide were detected inside the building, and multiple residents were feeling ill. Firefighters and EMS treated people at the scene while checking the building for the dangerous gas.

In total, 12 people were affected. Eight were transported to Nassau University Medical Center. Four others refused medical attention. Officials did not release the ages of those involved or say what caused the carbon monoxide buildup.

The incident was closed just after 1:30 a.m.

Carbon monoxide is often called the “silent killer” because it is invisible and odorless. Symptoms can include headache, dizziness, nausea and fatigue — and can easily be mistaken for the flu.

City health officials remind residents to never use gas stoves, ovens, grills, or generators to heat their homes. They also stress the importance of having working carbon monoxide detectors, which are required in most New York City homes near sleeping areas.

If a carbon monoxide alarm goes off, officials say residents should open windows, leave the home immediately, and call 911.