Two teenage sisters visiting from South Carolina spent Tuesday in the hospital after a carbon monoxide leak in the Garden City home where they were staying, the homeowner said.

Martin Arellano said around 7:30 a.m. the girls complained of dizziness. He said the two, his cousin’s daughters, ages 15 and 17, felt hot and then fainted. Police and fire rescuers arrived minutes after they were summoned by the family, Arellano said.

All eight of the home’s occupants, including two other children, ages 19 and 11, were taken to Nassau University Medical Center and seven were found to have elevated levels of carbon monoxide. The teens from South Carolina were admitted, said Arellano and hospital officials.

VideoCarbon monoxide sickens family in Garden City

Garden City police Insp. Michael J. Doyle said “a high level” of carbon monoxide was found inside the home on Franklin Court West. He said the source was not yet determined.

But Arellano said the girls were sleeping in the basement, near a newly installed heater. The homeowner said he learned afterward from National Grid, which responded and shut off the gas, that the device wasn’t properly installed in June. The exhaust was streaming straight into the basement where the girls slept, he said.

The family didn’t learn of the error until now because a mild winter had rendered the heater unnecessary — it didn’t kick on until 6 a.m., Arellano said.

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A nearby carbon monoxide detector did not go off, he said, adding that “there was some paper and boxes around it.”

Arellano said the girls were to stay in the hospital overnight for observation but that they were expected to be released Wednesday. And while their planned visit Tuesday to the American Museum of Natural History was postponed, they still will have time to enjoy the city before returning home.

“Everything turned out OK,” Arellano said. “This is just one of those cautionary tales. You have to . . . double check, don’t take things for granted and always be vigilant and make sure everything is working properly.”

Ann Marie Studdert, director of intergovernmental affairs at the hospital, confirmed the teens were admitted. She had no information on their condition.

Nassau County Assistant Chief Fire Marshal John Priest said that carbon monoxide is the leading cause of accidental death in homes across the United States and that there are, on average, about 60,000 emergency calls annually related to CO incidents nationwide.

“We strongly recommend CO detectors in any home, on any floor with a heating appliance and on any floor with a bedroom,” Priest said, noting that carbon monoxide is both colorless and odorless — and can only be detected with a CO alarm.

Additional information on CO poisoning and how to prevent it can be found at on the Nassau Fire Commission homepage at nassaucountyny.gov/firemarshal.

With John Valenti