Safety alert issued after swimming pool chemical mix causes toxic gas

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Sunday, 16 May, 2021 - 12:30
Policy & legislation

WorkSafe Victoria recently issued a safety alert on the hazards and risks associated with automatic chemical dosing systems at swimming pools following an incident in which employees and swimming pool patrons were injured when a toxic gas was released into the pool area at an aquatic centre.

The incident occurred after an earlier electrical fault caused the automatic dosing system, used to administer chemicals to the pool water, to inadvertently mix sodium hypochlorite (pool chlorine) and hydrochloric acid (pool acid) together which resulted in a toxic chlorine gas.

The electrical fault caused the water filtering pump to shut down, resulting in no water flow within the pipeline. The pool chlorine and pool acid were automatically added into the pipeline that would normally return filtered water back into the swimming pool. Adequate water flow within the pipeline is necessary to prevent chlorine gas.

The safety alert recommended a number of ways to control risks:


  • In the event of power failure, all pumps associated with the filter water and automatic dosing system are fail-safe (need to be manually restarted when power is reinstated).
  • Dosing of chemicals must be interlinked with readings from the filtered water line that returns to the pool.
  • Dosing system should be interlocked with the filtered water pump. No dosing should occur if the filtered water pump is not in operation.


  • Sodium hypochlorite and hydrochloric acid must be stored at least 3 metres apart in a separate bunds/spill trays.
  • A physical barrier needs to be placed between the two containers to prevent mixing in the event of dosing line failure (split line or a disconnection).

Personal Protective Equipment (PPE)

  • When handling containers and charging dosing lines for these pool chemicals, follow the recommended use of PPE provided in the manufacturers' safety data sheets (SDS).


  • Pool chemical containers must always be clearly labelled.
  • Workers must always be provided with clear instructions for the safe storage and handling of pool chemicals.
  • Safe work procedures need to be provided for the restart of dosing/water filtration system after a shutdown.
  • Adequate supervision and reassessment for competency as is necessary in the use of this system.
  • Immediate clean-up of chemical spills, using recommendations provided in the SDS.