Updated

Nearly three dozen people, including some children, were hospitalized in California on Thursday after a chlorine gas cloud formed at a swimming pool during a class, a fire official said.

San Jose firefighters were dispatched to the Shadow Brook Swim Club at around 12:10 p.m. after someone accidentally mixed pool chemicals, Santa Clara County Fire Capt. Bill Murphy said.

The cloud was created after someone in the pump room mistakenly mixed chlorine with muriatic acid, according to KGO-TV.

Murphy said 35 people were hospitalized and most of the patients’ symptoms were minor, according to KTVU-TV. Officials initiated a mass casualty response and decontaminated the kids and adults by hosing them down with water, KGO-TV reported.

The swim club remained open, but emergency personnel were standing by if anyone experienced symptoms again. No shelter in place was ordered.