A chlorine gas exposure at a Reading, Pennsylvania, YMCA sickened dozens and sent 20 people to the hospital.

Reading First Deputy Fire Chief Tom Kemery said the gas was created by improper mixing of the chemicals used to treat the pool.

He said the chemicals created a violent reaction, releasing potentially deadly chlorine gas, and this gas spread throughout the basement area and up through the northern part of the building.

Approximately 60 people were affected and sent to the Penn State Health St. Joseph Community Campus building shelter. Five ambulances were called to the scene to transport 14 children and six adults to the hospital.

Lt. Trent Zulick of the city fire marshal’s office said that he’d never seen a pool chemical incident on the scale of what occurred at the YMCA, although toxic reactions like this have been known to occur in the area, and they usually involve residential pools.

According to Zulick, patients were treated for mild medical issues, including skin and eye irritation, burning in the throat, and shortness of breath.

Using fans, first response crews vented the high concentration of gas in the building. They covered the canisters where the gas was leaking to slow the flow and directed the chemicals into the basement pool, using the pool water to dilute them.

Officials said the pool water would dissolve the chemicals harmlessly and the pool’s chlorine levels should have normalized by the following day.

The incident occurred at 3 p.m. on August 5 and was declared under control by 4:30 p.m., with staff and patrons allowed back in the building shortly after.

Zulick said he didn’t expect anyone would suffer long-term effects from the exposure, and the situation was handled swiftly.

“It certainly could have been much worse,” he said.