Swimming pool chemical mixing causes toxic gas

WorkSafe is issuing a safety alert on the hazards and risks associated with automatic chemical dosing systems at swimming pools.



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.

Safety issues

Chlorine is a chemical that inhibits bacterial growth in water. It's an active ingredient in several cleaning products and to disinfect swimming pool water. Chlorine is poisonous when it comes into contact with the skin, ingested or inhaled.  Chlorine poisoning can be fatal and causes symptoms through the body, including coughing, difficulty breathing and fluid inside the lungs.

When dosed into a swimming pool, pool chlorine and pool acid should be added separately to the filtered water re-entering the pool.

Recommended 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.

Legal duties

Under the Occupational Health and Safety Act 2004, employers must, so far as is reasonably practicable, provide and maintain a working environment that is safe and without risks to the health of employees and independent contractors. Where the risk cannot be eliminated, it must be reduced so far as is reasonably practicable.

Employers must also ensure, so far as is reasonably practicable, that persons other than employees are not exposed to health and safety risks arising from the employer's undertaking.

Employers must provide employees with the necessary information, instruction, training and supervision to enable them to do their work in a way that is safe and without risks to health.

Employees must take reasonable care for their own health and safety at work, and the health and safety of others who may be affected by their acts or omissions at a workplace.

The Dangerous Goods (Storage and Handling) Regulations 2012 also sets out legal duties for occupiers of premises where dangerous goods are stored or handled. The Code of Practice for storage and handling of dangerous goods provides practical information on how to comply.

Please note the Dangerous Goods (Storage and Handling) Amendment (Notification) Regulations 2021 come into effect on 1 July 2021 (Amended Notification Regulations).

This information has not yet been updated to reflect the changes introduced by the Amended Notification Regulations. Complying with the guidance after 1 July 2021 may not necessarily mean compliance with a duty under the Amended Notification Regulations. Information on the Dangerous Goods Act 1985 and associated regulations can be found on:

Australian Standards

AS3780-2008 The storage and handling of corrosive substances