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San Francisco JCC officials were trying to determine this week how to avoid another mishap after chlorine gas seeped through the building from a health club inside the site.

About 500 children and adults were evacuated Friday morning of last week from the Jewish Community Center of S.F. at 3200 California St. after the gas wafted through the structure.

Twenty adults, including eight firefighters, went to hospital emergency rooms after experiencing burning eyes, irritated throats or breathing problems. All were released after checkups, but at least one asthmatic JCC worker remained at home Monday.

The accident happened about 10:30 a.m., when a worker at Pinnacle Fitness Center, a private health club that rents space at the JCC, mistakenly poured two gallons of muriatic acid into a 30-gallon vat of chlorine in the basement boiler room.

Both chemicals are used to help balance the pool's pH level, Pinnacle general manager Denise Van Alstine said, but they are typically released into the water separately in small amounts.

When the muriatic acid hit the chlorine, the worker apparently smelled something strong but quickly left the boiler room without realizing what he had done.

The mixture created hydrochloric acid and released toxic chlorine gas, S.F. Fire Department Battalion Chief David Haberlin said. The JCC's ventilation system pulled the fumes upstairs to the main level of the building.

"We couldn't smell it in the club at all," Van Alstine said.

The cause of the accident seemed clear to both the fire department and Pinnacle officials. But Zev Hymowitz, JCC executive director, said Monday that center officials were still working with Pinnacle officials to understand how such a mistake could have occurred.

Haberlin also questioned how a worker could have mistaken muriatic acid for chlorine. The gallon containers look similar, he said, but are clearly marked.

And Van Alstine said Pinnacle workers are taught how to handle hazardous materials.

"It was human error," she said. "It is fairly rare, but it is a fluke that can happen."

Hymowitz said JCC officials hadn't determined yet whether to file a claim against Pinnacle for damages. For example, all the kosher food sitting out in the kitchen for lunch had to be thrown away because of the gas contamination.

At the time, about 500 people were in the JCC. That included up to 130 kindergym and preschool children, 100 JCC and Pinnacle workers, 120 seniors in day care or waiting for kosher lunch, 150 emigres in English classes, and at least 20 Pinnacle members who were working out.

Ronnie Jacobs, who runs the JCC's preschool, called 911 at about 10:30 a.m. when teacher Annette DeVille reported the smell in the outdoor playground. They first brought the children inside where there was no odor.

Lenore Naxon, the JCC's development director, said that at first she smelled something that was simply "malodorous."

"Then it began to smell like when you're standing over a wash machine and pouring in Clorox — and get a big whiff," Naxon said.

A small group of firefighters arrived from the fire station around the corner. They immediately decided to evacuate the entire building.

"Once we got the building evacuated, it wasn't dangerous at all," Haberlin said.

Menorah Park, an elderly housing complex next to the JCC, was alerted but residents were not evacuated. The S.F.-based Jewish Family and Children's Services immigrant department and its parental resource center Parents Place, which are also adjacent to the JCC, were temporarily evacuated. The immigrant department later decided to shut for the day.

Police and firefighters cordoned off California Street. Paramedics treated about 30 people at the scene. "Most of it was fairly minor," Haberlin said of the injuries.

Firefighters, wearing gas masks, added a neutralizing chemical to the chlorine vat and brought in powerful fans to air out the building.

By 2:30 p.m. Friday, Pinnacle staffers returned to work. But the JCC and Pinnacle, which opened in mid-January and now has 2,800 members, remained closed until Saturday morning.