Twelve people were taken to hospital on Thursday afternoon after an accidental chemical mix at a swimming pool in St. Catharines, Ont., exposed them to a dangerous gas.

Ten of the people who were at Lion Dunc Schooley Pool have since been released, but two people remain at St. Catharines General Hospital.

St. Catharines deputy mayor Pete Secord and fire chief Mark Mehlenbacher speak at a news conference about the accident. (Samantha Craggs/CBC)

Six pool patrons, five lifeguards and the contractor who caused the accident were sent to hospitals in St. Catharines and Niagara Falls. Victims ranged in age from seven to 50.

"All the patients we took to the hospital were experiencing some forms of exposure to chlorine gas," said Sylvan Lepine, the EMS incident commander. "They had scratchy throats or chest tightness. Shortness of breath. Some patients were wheezing. It was respiratory we were dealing with out in the field."

The accident occurred when a pool worker accidentally poured about 90 litres of chlorine into a tank that contained about 900 litres of muriatic acid, which is a common pool-cleaning chemical.

The worker who mixed the chemicals is one of the two people still in hospital. Lepine said he will be monitored for pulmonary edema — excess fluid in the lungs.

Hazmat workers, fire crews, the Ministry of Labour and the Ministry of the Environment all attended the scene. About 60 homes nearby were asked to keep their windows and doors closed and turn off their air conditioning. They were also asked to seek medical attention if they experienced coughing, a sore throat or difficulty breathing.

Close call for summer camp

It was a close call for a children's summer camp, which was leading a group over to the pool just minutes before the evacuation was called due to the leak.

"If we would have got there 15 minutes earlier, things could have been a lot worse. Luckily that didn't happen," said Tylar Schram, a camp counsellor.

St. Catharines Fire Chief Mark Mehlenbacher said the Ministry of Labour would be visiting the pool Friday morning "because essentially, we had staff that were injured."

A key question that remains to be answered is how the muriatic acid and chlorine came to be mixed in the first place, and how the dangerous mix-up could have been prevented.

Among the lifeguards sent to the hospital for treatment was Nigel Durrant's daughter.

"Very proud as a dad, and happy that she managed to do her job well," Durrant said of his daughter. "It's just unfortunate that because of the fact that she was the last person making sure that everybody was out of the area, she was exposed more than some of the other kids."

Crews used a vacuum truck to rid the pool tank of any remaining noxious chemicals.

The all-clear came at about 9 p.m. and the street was reopened.