Around 10 a.m. Saturday, at a Quality Inn and Suites in Niles, several victims began to feel nauseous, according to Niles City Dispatch.
The hotel was evacuated as high carbon monoxide levels were detected on all floors, dispatch said.
Niles City Fire Capt. Don Wise said 13 total patients were hospitalized, including children, staff members and officers. Six children were found by hotel staff unconscious in the pool area. No one was found in the pool.
The children affected range in age from 10 to 14. It is unclear how long they were unconscious.
Some victims were taken to Memorial Hospital in South Bend and another group was taken to Lakeland Michigan Hospital. Memorial Hospital confirmed that the patients there are in critical condition.
At Lakeland, one child was dead on arrival. Some hotel employees and first responders were being treated and were expected to make a full recovery, according to a hospital spokeswoman.
The highest carbon monoxide reading, officials said, was 800 parts per million in the pool area, which is an extremely dangerous level. Wise said a pool heater could have caused the problem, but that is still under investigation.
An off-duty firefighter on the scene helped provide medical help and pull the patients into a safer area.
He did not know the statuses of the children's parents.
The hotel had never had issues like this before, Niles said. It is unclear if there was a carbon monoxide detector in the pool area.