A chemical reaction caused a release of poisonous chlorine gas at a swimming pool in London today, when the employees of the London Aquatics Center at Queen Elizabeth Olympic Park in Stratford were taking delivery of chemicals.

According to the London Fire Brigade called on Wednesday morning after a gas release occurred, “a high quantity of chlorine gas was released inside the center due to a chemical reaction.”

About 200 people were evacuated from the pool complex and the residents of buildings were ordered to close their windows and doors as firefighters ventilated the toxic fumes.

A number of people were hospitalized with breathing difficulties in London‘s hospitals. According to the pool management, “a number of casualties with breathing difficulties” were being treated by the London Ambulance Service.

Chlorine is commonly used to disinfect swimming pools, and is usually transported in liquid or crystal form. However, chlorine becomes a gas when mixed with an acid such as ammonia or hydrochloric acid, and this yellow-green fog can cause vomiting and respiratory damage when inhaled.