LONDON (AP) — Authorities said 29 people were taken to the hospital with breathing difficulties on Wednesday after a “high quantity of chlorine gas” leaked in a swimming pool at the London sports complex that hosted the 2012 Summer Olympics.

The London Fire Brigade said around 200 people were evacuated after the chlorine gas was discharged inside the Aquatics Center at the Queen Elizabeth Olympic Park in east London due to a “chemical reaction.”

The brigade said it took 29 people to the hospital and assessed another 48 people at the scene. Most of those affected reported minor breathing difficulties, it said.

The fire service declared a “major incident” and sent a large team of emergency workers including 13 ambulance crews and members of its hazardous area response team. Surrounding roads were cordoned off and members of the public were denied access to the park.

Local residents were asked to close their doors and windows while workers ventilated the affected area.

While the chlorine that is added to swimming pools to kill bacteria is safe, chlorine in gas form is highly toxic.

The Aquatics Center’s management said the chlorine gas release occurred “when the facilities management company that operates the plant room took delivery of pool chemicals.”

It added that it was waiting for guidance on when the pool building can safely reopen.

Queen Elizabeth Olympic Park, which was built for and hosted the 2012 Olympic Games, first opened to the public in 2014.

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