How to handle chlorine incidents at swimming pools

Authors: CANUTEC Emergency Response Advisors

For immediate help, call 911 and refer to the first aid section.
For technical help, call CANUTEC at 613-996-6666 and refer to the spill response section.

Each summer, CANUTEC receives many calls involving chlorine incidents at swimming pools. When different pool chemicals accidentally mix, or water seeps into a pails of pool chemicals, a severe reaction can occur which creates an irritating odour.

This article summarizes information on this topic gathered by CANUTEC Emergency Response Advisors.

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Chlorine and chlorine derivatives have been used to sanitize swimming pool water for close to 100 years. One of the most common pool sanitizers is called calcium hypochlorite. Its solid form makes it easier to handle than liquid or gas and it will form chlorine under the right conditions. Sanitizing alternatives like bromine, salt chlorine generators, UV light systems, ozone systems, biguanide, and silver ions can also be used.

Acid or alkaline materials are used to adjust pH levels, algaecides are used to reduce algae growth, and flocculants can be added when pool water becomes cloudy. A normal pH level for pool water is between 7.2 and 7.6, and the amount of chlorine found in the water can range from 1 to 3 ppm (parts per million). These values are often checked with colorimetric kits provided by the pool manufacturer.

Although many products are used to control water quality, if there’s a fire or toxic vapour release, it’s likely that chemicals that form chlorine will be on site. Sanitizers like calcium hypochlorite will quickly break down when exposed to the sun and are mostly used as shock treatments because they work quickly.

This sanitizing effect can be slowed by adding a stabilizer to produce stabilized chlorines. This is why stabilized chlorine is used during normal operations and pool shock is recommended as a shock treatment ideally done at night. Chemicals that release chlorine gas are classified as oxidizers which are a class 5.1 dangerous goods. If a product is mainly corrosive, like bleach, it will be classified as a class 8 dangerous good.

Products used as chlorinating agents fall in 2 categories: rapid-release chlorine products and slow-release chlorine products.

Rapid release chlorine products include but are not limited to:

  • calcium hypochlorite, dry with > 39% available chlorine (UN1748 / Class 5.1)
  • calcium hypochlorite, dry with 10 - 39% available chlorine (UN2208 / Class 5.1)
  • calcium hypochlorite, hydrated with 5.5 - 16% water (UN2880 / Class 5.1)
  • sodium hypochlorite (bleach) with > 7% available chlorine (UN1791 / Class 8), and
  • lithium hypochlorite, dry (UN1471 / Class 5.1)

Slow-release chlorine products include but are not limited to:

  • dichloroisocyanuric acid, dry (UN2465 / Class 5.1) and
  • trichloroisocyanuric acid, dry (UN2468 / Class 5.1)

Potential hazards

Adding a small amount of water

Pool chemicals are meant to be added to a large amount of water. When a small amount of water is added to pool chemicals, it can result in a strong reaction that releases a lot of heat. This increase in temperature can cause toxic gases like chlorine gas or nitrogen trichloride gas to be released. These gases are very toxic and can burn your airways if inhaled.

Something as simple as putting a wet scoop into a container can be enough to trigger this reaction. High humidity, like leaving a container partly open on a humid summer night, can also cause pool chemicals to react but in this case any toxic gases produced will be released at a much slower rate.

Improper mixing

Most common pool chemicals react with each other. Many incidents occur because pool chemicals were mixed, accidentally or not, by using the same scoop for different products. This cross-contamination can cause a reaction. Many incidents occur when products are mixed by accident. This causes chlorine, an acrid gas, to form.

A large amount of heat can be released when incompatible chemicals are mixed and this means any flammable materials nearby can ignite. Mixing can also release toxic gases like chlorine gas or nitrogen trichloride gas. Fires have also started when an old batch and new batch of the same product were mixed. Make sure to not mix your pool chemicals with material that could be lingering around your containers, like sawdust, oily rags, or other flammable substances because they could start a fire.

“Incompatible materials” include:

  • flammable materials
  • herbicides, fertilizers and algaecides
  • ammonia
  • urea
  • acids and alkalis
  • ethanol
  • methanol
  • turpentine
  • tile cleaners
  • paints
  • rust, or
  • charcoal


As pool chemicals decompose, they can generate heat and ignite any flammable materials that are nearby. Once started, the reaction of wet or mixed chlorinated pool chemicals can continue generating heat, unless the material is cooled or until all chlorine is used.

  • Oxidizers do not burn but will increase the risk of fire or the intensity of the fire by releasing oxidizing agents like chlorine and oxygen
  • Avoid heat sources like lit cigarettes, matches, open flames from barbecues or hot lawn mower motors

First aid

For moderate to serious exposures, call 911 or the Poison Control Centre in your area and seek medical treatment.

  • Concentrated pool chlorinating agents can irritate the skin and cause serious eye burns. Remove any contaminated clothing and make sure to carefully wash them before reusing. The affected skin or eyes should be flushed with tap water for at least 15 to 20 minutes
  • A solution with a concentration of more than 15% calcium hypochlorite can be deadly if swallowed, but lower concentrations can still cause severe damage to the digestive tract. If ingested, rinse the mouth and do not induce vomiting
  • Someone exposed to corrosive chlorine gas or oxidizer dust should be moved to fresh air and monitored for delayed pulmonary edema symptoms by trained professionals for the next 48 hours


  • In cases where the product instruction includes mixing, make sure the chemicals are mixed outside or in a well-ventilated area
  • Make sure the tools used to handle one chemical are cleaned and dried before handling a different chemical
  • Anyone working with pool chemicals should wear proper protective equipment like coveralls, boots, dust mask, gloves and goggles


  • Store in a cool, dry, well-ventilated area, out of direct sunlight and away from other heat sources
  • Protect from moisture and humidity to limit the chemicals from decomposing
  • Do not store liquid containers above your pool chemical containers
  • Store away from incompatible materials like:
    • flammable materials (gasoline, diesel, oil, paint, solvents, etc.)
    • herbicides, fertilizers, algaecides
    • ammonia
    • urea
    • acids and alkalis
    • ethanol
    • methanol
    • turpentine
    • tile cleaners
    • paints
    • rust or
    • charcoal
  • Close containers properly or cover damaged packaging
  • Store chemicals away from doors and windows
  • Keep containers off the floor and store on shelves.

Remember all these points when storing pool chemicals together in your shed.

Fire response

In most cases, contact the fire department and evacuate the area.

  • Do not use dry chemical fire extinguishing agents that contain ammonium compounds (such as some A:B:C agents), since they can create an explosive compound
  • Do not use carbon dioxide, dry chemical powder or other extinguishing agents that smother flames. They aren’t effective in extinguishing fires that involve oxidizers
  • From a distance, apply a large quantity of water to the fire to douse the heat

Spill response

For technical advice, contact CANUTEC at 1-888-226-8832 (Canada and US only), 613-996-6666, or *666 (cellular only).

For small spills, start to ventilate the area by opening windows and doors if possible. If a chlorine smell is present, do not enter the area until it’s properly ventilated. Keep any flame or flammable material away from the spilled product. Prevent any moisture or liquid from coming into contact with it.

Cover small spills using plastic sheets to protect against the rain, if needed. Wear proper protective equipment like dust masks, gloves, and goggles before scooping the chemicals with clean tools and transfer them into a clean container. Do not seal the container to avoid any pressure buildup if a decomposition reaction has started

For large spills, contact the fire department and evacuate the area. A dangerous goods contractor should be contacted to clean-up the spill.


The spilled material is waste and should be disposed of according to local or provincial regulations. Waste management companies are aware of the regulations that apply and using their services is an effective way to have a spill handled safely and comply with all regulatory requirements.

  • For small spills, dispose of the containers at a local hazardous waste site
  • When disposing of used containers, make sure the residues do not mix
  • For large spills, contact a dangerous goods contractor for disposal


When mishandled, swimming pool chemicals can be hazardous enough to require medical attention and can require an area to be evacuated. CANUTEC often provides technical advice on these types of incidents.

As such, the intention of this article is to improve the public’s understanding of the dangers of swimming pool chlorine chemicals and to provide guidelines when handling a swimming pool chlorine incident.

Remember that these chemicals wouldn’t do such a good job sanitizing your swimming pool if they weren’t so reactive.


  1. K. Olson, "Clear waters and a green gas: A history of chlorine as a swimming pool sanitizer in the United States," Bulletin for the History of Chemistry, vol. 32, no. 2, pp. 129-140, 2007.
  2. G. Fitzgerald and M. DerVartanian, "Pseudomonas Aeruginosa for the Evaluation of Swimming Pool Chlorination and Algicides," Applied Microbiology, vol. 17, no. 3, pp. 415-421, 1969.
  3. EPA, "Safe Storage and Handling of Swimming Pool Chemicals," [Online]. Available:
  4. E. Canelli, "Chemical, Bacteriological, and Toxicological Properties of Cyanuric Acid and Chlorinated Isocyanurates as Applied to Swimming Pool Disinfection - A Review," American Journal of Public Health, vol. 64, no. 2, pp. 155-162, 1974.
  5. American Chemistry Council, "Pool Treatment 101: Introduction To Chlorine Sanitizing," [Online]. Available:

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