You are the health and safety advisor for a charitable trust that runs four leisure centres for the local council. You have just been asked to look at the largest centre, which has a 25-metre pool, a learner pool, a gym and a café. It is open from 06:30 to 22:00, seven days a week, and has about 60 staff, many of them part-time lifeguards aged 17 to 21.

The pool water is treated in a plant room below the pool hall. Two chemicals are pumped into the water automatically from bulk tanks: sodium hypochlorite, which disinfects the water, and an acid, which controls its pH. If the two are mixed, they react and give off chlorine gas. The tanks are topped up by hand from 25-litre drums. The plant room has a chlorine gas detector linked to an alarm, but staff say it was switched off several months ago because it kept giving false alarms.

Two months ago, the trust changed its chemical supplier to save money. The new supplier’s drums of acid and sodium hypochlorite are the same size and colour, and differ only in their labels. Nobody obtained the new supplier’s safety data sheets, and the centre’s COSHH assessment still refers to the old supplier’s products. On delivery days, the driver leaves the drums in the corridor outside the plant room, because the plant room door lock is broken and the room cannot be secured.

At the same time, the trust introduced a new rota. The evening duty manager post was removed, and the most senior person on site after 18:00 is now a lifeguard supervisor. Topping up the chemical tanks used to be done by the duty manager, who had been trained by the old supplier. It is now done by whichever lifeguard supervisor is on shift. The supervisors were not asked about the change. Each one shadowed the day manager for one hour before taking it on.

Last Tuesday at 19:40, a lifeguard supervisor who had been in the role for three weeks went to top up the acid tank. They picked up a drum of sodium hypochlorite from the corridor by mistake and began to pour it into the acid tank. There was a strong smell, the supervisor began coughing and their eyes started streaming, and they left the plant room, leaving the door open. Chlorine gas spread up the stairs into the pool hall, where an evening swimming lesson for children was taking place.

The pool was evacuated by the lifeguards, but it took several minutes because the alarm in the pool hall was not set off. Twelve swimmers, including eight children, were coughing, and three of the children were taken to hospital by ambulance and kept in overnight. The supervisor was treated by paramedics at the scene and went home. The fire and rescue service ventilated the building, and the centre reopened the next morning.

The centre manager wrote “chemical smell in plant room, pool evacuated, reopened next day” in the incident log. The manager believes the incident does not need to be reported to the enforcing authority because “nobody was badly hurt and the children went home the next day”. The drums have been moved back into the corridor. Nobody has spoken to the supervisor about what happened, other than to tell them to “be more careful”. The centre’s CCTV keeps recordings for seven days.

The trust’s board meets next month. One board member has suggested that the trust should adopt a formal health and safety management system and seek certification to ISO 45001, because a neighbouring trust has done so. You have been asked to brief the board.