Royal Oak, MI – Ron Vample, Associated Press – More than 50 people were taken to a hospital Saturday after high levels of carbon monoxide were detected at a high school pool during swimming lessons, police said.

The children and adults fell ill Saturday morning inside Dondero High School in suburban Detroit. Doctors checked 35 people who complained of headaches, nausea and vomiting. Others left before being examined.

Five people were placed under increased pressure in the hospital’s hyperbaric chamber to help them better absorb oxygen into their blood, said Dr. Frank McGeorge, director of emergency medicine at William Beaumont Hospital. They were released later in the day.

Royal Oak police Sgt. Barry Gale said tests had found high levels of carbon monoxide, which were believed to be caused by a malfunctioning furnace. The incident was still under investigation.

Sandra Standel, the school district’s assistant superintendent, said the entire school was evacuated as a precaution.