Twenty-nine people were taken to hospital after a high quantity of chlorine gas was released in an accident on Wednesday at the aquatics centre at London’s Olympic park. Some of the injured included swimmers in a parent and toddler group who were in the children’s pool at the time of the incident, which was first reported by the centre just after 11am. About 200 people were evacuated from the centre during the incident, firefighters said. At least nine construction workers on a neighbouring site were also treated at the scene. Darren Farmer, London ambulance service gold commander, said: “Our crews took 29 patients to hospital and assessed a further 48 patients at the scene. The majority of patients were reporting minor breathing difficulties.” He added: “We sent a significant number of resources including 13 ambulance crews, advanced paramedics, two medics in fast response cars, two incident response officers, a medical incident adviser and members of our hazardous area response team. We also dispatched medics from London’s air ambulance.” The London fire brigade (LFB) said a “high quantity of chlorine gas” was released inside the centre due to a “chemical reaction”. “While we ventilate the premises we would ask those residents in the immediate vicinity to close their doors and windows,” it said. Former Guardian journalist Tess Riley, 37, who is 15 weeks pregnant, was among those taken to hospital with her toddler daughter. She told the Guardian: “The pool was full of toddlers and babies as it’s the time of day when the classes happen. My view is they should not have been taking a chlorine delivery at that time of day while open to the public – if that’s what caused the problem.” She said that she was staying in hospital as a precaution but her husband was about to take her daughter home. The LFB said it had dispatched 12 fire engines and about 80 firefighters to the incident. Swimmers were told to evacuate immediately without picking up their clothes. Some of them were treated in a cafe opposite the centre. They included a woman in a swimming costume whose baby son was being checked by paramedics as a precaution. Among them was Nick Milcoy. Wrapped in a foil blanket with his goggles still on his head, he told Guardian: “I was on my way through the changing room when I could smell chlorine gas. I’m fine but I need to get my clothes.” Graham Redman, 42, said: “I just got in to go for a swim and the alarm went off halfway down my first length. They said there was a gas leak of some kind.” He added: “I’m fine, but there were some people who were struggling with breathing and coughing and London ambulance service were treating. I don’t think anyone was really badly hurt but some went off in ambulances. The staff have been lovely – they had blankets for us straight away. It took a little while for them to work out what to do but that’s understandable. All the emergency services turned up very quickly.” Lydia Nicola, 32, whose 26th-floor flat overlooks the centre, said she saw several emergency workers enter the building wearing gas masks and breathing apparatus. Texting the Guardian as the incident unfolded, she said: “I live opposite the aquatic centre. It is hard to tell exactly how many are currently are in gas masks. There were only eight or so earlier but it seems to have increased.” A construction worker who did not want to be named said he was told the wrong chemical had been pumped into the pool causing a chemical reaction.