A Garden City family was exposed to dangerously high levels of carbon monoxide in their homes Tuesday morning, emergency crews say.

Martin Arelleno, the owner of a home on Franklin Court East, says his cousin was staying with his wife and two kids in the basement. Arelleno says his cousin's two daughters fainted after being exposed to the gas.

A carbon monoxide detector was inside the home, but no alert was set off.

Garden City police say six out of the eight people inside the home were taken to the hospital for carbon monoxide poisoning. Arelleno says his family will be OK.

The source of what caused the high levels of carbon monoxide is still under investigation.

Garden City police say neighboring homes were also checked for carbon monoxide, but were found to be normal.

Nassau County Assistant Chief Fire Marshal John Priest says homeowners should get their heating systems checked by a professional.

"Carbon monoxide is a deadly killer. It is invisible, and it is odorless," says Priest. "Fire services across the United States will go to 60,000 carbon monoxide calls during 2016. We strongly suggest carbon monoxide and smoke detectors in the home on any floor."