Swimming Pool Chemicals

by Jacques Savard, Ph.D. Chemist


Last summer, as usual during summertime, CANUTEC received dozens of calls from individuals and fire departments requesting information on pool chemicals. Most of the time a reaction had occurred when adding the content of an old container to a new one or when scooping from the latter. An acrid gas evolved and, frequently, someone had to be taken to hospital for medical attention. On most occasions, the premises had to be evacuated.

This article summarizes information CANUTEC gathered on the matter.

Most swimming pools recirculate their water in a closed loop. New water is added from time to time but refilling is rather unusual. A subtle balance must be obtained between many different chemical processes to maintain the quality of the water and the required health standards. Several biological and chemical phenomena will occur and must be controlled by adding different chemicals. Algae may readily form from airborne spores. Bacteria, brought in by the swimmers themselves may prosper. Bacteria will be controlled by adding "chlorine" to the water. Algae will be controlled using an algaecide. The pH should also be adjusted for the swimmers comfort and the effectiveness of the chlorinated agents.

Chemically, the required chemicals often belong to very different and non-compatible chemical families: quaternary ammonium salts (algaecides), inorganic salts, acids or bases (pH adjusters), oxidizers, etc... To destroy bacteria (microbes), chlorine and chlorine derivatives are the most widely used biocides in commercial or public water treatment facilities. These are also often involved in dangerous materials incidents. For private pools and spas, solid derivatives susceptible to yield "chlorine" are used. As solids, they are easier to handle and store than gases and liquids. Unfortunately, they are probably the most hazardous chemicals to be found in your backyard when improperly handled. Most often they are part of CANUTEC's summer avalanche of pool chemical incidents.

Two Types of Chlorinating Agents

There are two types of chlorinators: the "organic" and the "inorganic". The inorganic chlorinating group contains mainly one chemical product: Calcium hypochlorite, also called "Cal hypo" for short. It is shipped under two forms: anhydrous (less than 5.5% water) or hydrated (5.5% to 10% water). A very closely related product, Lithium Hypochlorite can also be found on the shelves and should be handled with the same precautions. These products belong to the same chemical family as bleach (Sodium Hypochlorite solution). All chlorinators show the same chemical behaviour and are classified as Oxidizers. Class 5.1 by the TDG Regulations (Bleach is Class 8(9.2)). On a shipping document they will appear as:

Lithium hypochlorite, dry , or Lithium hypochlorite mixtures, UN 1471
Calcium hypochlorite, dry, or Calcium hypochlorite mixtures, UN 1748
Calcium hypochlorite hydrated, or Calcium hypochlorite hydrated, mixtures, UN 2880
Calcium hypochlorite mixtures, dry, UN 2208

Note: The entry UN 1471 is an unusual UN identification number.

As a pure product, or as a concentrated solution, Cal hypo has a very strong oxidizing power which means it will provide the oxygen needed to light and sustain a fire. The US National Fire Protection Association (NFPA) defines four classes of oxidizing power, from 1 to 4. The highest is the most potent. Calcium Hypochlorite is rated 3. In addition it might become unstable and dangerous when mishandled or contaminated by other chemicals like acids, alkali or easily combustible materials. Almost anything may start a self-decomposition of chlorinating agents, even moisture. In some incidents where Calcium Hypochlorite drums were flooded by water, containers released suffocating fumes and bulged while heating.

Pool chlorinating agents may also belong to second class of pool "chlorine", the organic "chlorine" made of several chemically related products called "chlorinated isocyanurates" (also known as Isos...) and brominated Hydantoins.

They are:

Trichoroisocyanuric acid UN2468 Thrichlor
Potassium dichloroisocyanurate UN2465 Dichlor
Sodium dichloroisocyanurate, anhydrous UN2465 Dichlor
Sodium dichloroisocyanurate, dihydrate UN2465 Dichlor
Brominated Hydantoin UN1479 ------

These organic chlorine materials are also known under a large variety of trade names most often followed by a number that indicates the percentage of chlorine available from the product.



Organic and inorganic "chlorine" (Isos and Cal Hypo) are totally non-compatible with each other. Many incidents occurred because there was a mixture of both categories, done intentionally or not, when using the same scoop or when adding one product after the other in a pail or in the pool chlorinator.


Pool chlorinating agents (both organic and inorganic) are not combustible but may be decomposed by warming. Calcium hypochlorite decomposition is self-sustaining. Getting rid of the source of heating will not stop the formation of fumes and the reaction will proceed until the material is no longer available.

Dichlor decomposition not only will generate enough heat to sustain a thermal decomposition but it may generate enough heat to ignite easily combustible material such as wood, paper or oily rags.

Trichlor ceases decomposition when the heat source is removed. The decomposition products may include Oxygen and/or chlorine gas. Here are some examples of hot spots likely to trigger the decomposition process:

  • Lighted cigarettes or matches
  • Any flames (e.g., portable gas stove)
  • Welding rods or molten particle from welding operations.


Organic pool chemicals are also strong oxidizers. Sodium Dichloroisocyanurate dihydrated is rated 1 by NFPA. Trichloroisocy-anuric Acid is rated 2 and Potassium (or Sodium) Dichloroisocyanurate, anhydrous, are rated 3. As oxidizers, they are incompatible with most materials and all of them should be kept away from any type of chemical, including pool algaecides, sweeping compounds, alcohol's, petroleum products, solvents, paints, etc... They strongly react with all of these releasing much heat along with dangerous and choking fumes. Remember this when storing together herbicides, fertilizers, algaecides, pool conditioners, tile cleaners, turpentine, paints and chlorinators in your garden shed.

Contamination may cause a spontaneous combustion at room temperature. Consequently, spilled material should never be put back in its original container since it may be contaminated by dirt, grass, grease, etc. Remember that anhydrous materials are more sensitive to decomposition by contamination than hydrated ones. Dispose of the spilled material and keep it away from any reactive product. Keep an eye on it since it may start reacting later when you believe it is safe. Special care should be taken to store them away from any type of chemical.

Pool chlorinating agents should be protected from water since a powerful reaction may occur releasing Chlorine and Oxygen gas (don't drop a cup of coffee in your pail!). Too often someone will bring back a wet scoop to the pail and trigger a reaction. Keep them away from any moisture sources. Never store liquids above any oxidizer. The only exception to using water will be for fire fighting (see below).



Do not use dry chemical extinguishers or foams. Use only water. Call the fire department, even if the area has a sprinkler system. For large fires, the area should be flooded from a distance. Do not get water inside the containers. Be careful, runoff may contain enough material to make it react with combustibles. It should be kept for later disposal. In some cases where it mixes with some other chemicals, it may release chlorine and oxygen. Self-contained breathing apparatus should be worn at all times.


Any spill larger than 50kg should be handled as an emergency: the fire department should be called in. Immediately instruct everyone to clear the area over at least 15 to 25m depending on the size of the spill. If contamination occurs or is suspected, or when fumes are released, the site should be cleared from 100 to 250 m and the fire department called in immediately. Ventilate indoor areas. Keep any combustible 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 rain.

Since pool chlorinating agents may be sold as solid materials, or as powders, be aware of dust. Gas/dust cartridge respirators can be worn but remember they will not provide any skin protection (front, neck...) For that purpose, goggles, gloves and coveralls should be used. CANUTEC may help in recommending the best protective gear.

Any spill should be dealt with immediately. Make sure there is no contamination by water or any combustibles. If there is no contamination, you may transfer the spilled product to a clean poly-ethylene-lined container using a clean shovel. If a drum is leaking, it too should be transferred into an overpack drum. Do not seal the container. This is to avoid any pressure build-up in case a reaction starts. If there is any sign of reaction, contact the fire department.


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


Calcium Hypochlorite and Dichlor can be "neutralized" in small quantities only (less than 5kg) by pouring the spilled material into a drum full of water. Never add water to Dichlor or Cal Hypo: add these to the water. This will produce heating and some choking gases may evolve. If there is less than 1 ppm (use a pool water test kit to control the concentration) it probably may be flushed down the drain. Check your local environmental regulations first. But remember this: trying to neutralize 1 g of pure (100%) calcium hypochlorite will require using 1000L of water to reach the 1 ppm level. For 6 kg it will be ....5 million litres! CANUTEC's experience is that neutralization is to be undertaken only when nothing else is workable and it should be attempted only on very small quantities (traces). The neutralization procedure should never be attempted in private dwellings. Only industrial size areas can allow such an experience on a very small scale with the supervision of technically skilled personnel and with the agreement of Ministry of Environment inspectors.

Trichlor should always be kept dry and disposed of without any neutralization attempt.

First Aid

Pool chlorinators are skin and eye irritants. When contamination occurs, flush with lukewarm water for 15 minutes. A short 2-3 minutes washing is not enough, particularly for eye contact. Remove any contaminated clothing and make sure they are carefully washed before reuse.

If the victim inhaled dusts or fumes, move him or her to fresh air immediately. Apply artificial respiration if not breathing. Obtain immediate medical attention.

Household chemicals are unfortunately often mishandled because we are not aware of their real chemical potency. It is easy to respect things we don't use frequently. However "Familiarity breeds contempt". Over time we tend to forget and start to believe "there's no real hazard here". CANUTEC answers questions on such situations often and for this reason felt that a reminder on the hazards would provide improved safety. There is no such thing as a 100% safe household chemical. These pool chemicals are also used in industrial processes where health and safety regulations have been implemented to protect the employees. At home, the same safety rules should apply. Swimming pool chlorinating agents may become some of the most unstable and hazardous materials when mishandled. They would not be so efficient if they were not so active.

Publication: TDG Dangerous Goods Newsletter, Vol. 12, No. 3, Fall/Winter 1992.
* Jacques Savard is a former Emergency Response Advisor with CANUTEC.