This week’s question comes from Amy, who asks “I am a swim instructor and I’m curious what the long term effects of pool chlorine are.”
Well Amy, this is a very interesting question that I am sure most people who use pools regularly don’t consider. Before I get into the known effects, let’s first go over what chemicals they use.
Pools, as most people know, are most often sanitized by chemicals that contain chlorine. The unique, familiar smell of a pool in the summer comes directly from this and chloramines, products of a reaction between the chlorine and some organic molecules. But most people outside of the pool maintenance industry (or a particularly active do-it-yourself-er) aren’t aware of the variety of chemicals used to keep the pool clean and sanitary.
As far as I can find, most pools are sanitized by one of three salts: trichlor (sodium trichloroisocyanurate), dichlor (sodium dichloroisocyanurate), and cal-hypo (calcium hypochlorite). These salts are all used effectively as carriers for chlorine. Trichlor and dichlor react with water to release chlorine (Cl2) and cyanuric acid, which is the base chemical from which trichlor and dichlor are made. The cyanuric acid produced by this breakdown also protects the free chlorine from UV breakdown from sun exposure by weakly bonding to it. Unfortunately, this process also reduces the chlorine’s effectiveness and thus cyanuric acid levels must be monitored to ensure they don’t get too high. Trichlor and dichlor more slowly dissolve in water and are put in the floating canisters present in many home pools.
Cal-hypo is an older method of chlorination that is still used for both primary chlorination and for “shock” treatments that bring the chlorine levels up to required levels. It is less sophisticated than trichlor and dichlor and has no built in UV protection for the produced chlorine. This compound is used most often to “shock” pools with chlorine when they get too low because dichlor and trichlor dissolve too slowly.
On the whole, it appears that most professionally maintained outdoor pools one would encounter would more likely be primarily chlorinated with trichlor or dichlor. The choice between the three is not particularly important, but the attached sources should give a good idea if you’re interested. Regardless of the choice, chlorine levels in the pool should remain above roughly 1.5-3.0 parts per million (ppm) in order to effectively sanitize. This is approximately the same level as chlorinated drinking water.
Insofar as it relates to health, these chemicals are a mixed bag.
- Cyanuric acid is considered non-toxic, with an LD50 of oral exposure of over 10,000mg/kg. LD50 is the dosage necessary to kill 50% (hence the 50 in LD50) 0f the subjects. Thus it would take over 10g per kg of weight to have a 50-50 chance of killing you; in other words, a standard 165lb (75kg) person would require a direct dose in excess of 750g or over 1.5lb. Longer exposure studies on rats show that it does not accumulate in the body and is effectively eliminated from the body within in a few hours. At the huge tested doses, far beyond the rate of any possible human pool exposure, the only reported complications were from cyanurate crystals forming in the bladder and causing an obstruction. There was no shown increase in cancer risk, birth defects, or organ damage.
- Dichlor and trichlor (collectively tested and referred to as chlorinated isocyanurates), because they release cyanuric acid and chlorine, have effectively the same toxicity profile as cyanuric acid in water. Because the chlorine level is very small relative to the amount of water, skin irritation is the only significantly reported complication of long term exposure. According to a German environmental protection agency study, the margin of exposure rating, which measures the overall toxicity rating regardless of route of exposure, for each is over 8. For reference, triclosan (the anti-bacterial agent in hand soap) is a 9.6, sodium hypochlorite is 0.040, and the toxic fixative (allows for long term storage of organic material, ie. formaldehyde) glutardialdehyde is 0.49. Thus, it is reasonable to conclude these chemicals are theoretically safe even at levels much higher than present in a pool.
- Hypochlorite salts (calcium, sodium, etc. hypochlorite) have a much different toxicity profile and thus present a much more risky exposure. As I mentioned above, the margin of exposure of sodium hypochlorite is over 200 times lower than the chlorinated isocyanurates and over 10 times lower than a toxic fixative. This means that they have a toxic exposure limit across all routes (ingestion, inhalation, and skin) that is much lower than even a toxic fixative. It is most dangerous through skin exposure, as it is corrosive and an irritant in higher concentrations. Luckily, these salts are used in very small amounts in pools, but it stands to reason that long term exposure to these compounds present a greater risk, especially to the skin, than with the chlorinated isocyanurates.
- It is also worth noting that the formation of chloramines by any of these compounds have been thought to contribute to respiratory irritation and asthma in swimmers. However, this effect is normally associated with indoor pools, especially ones with poor ventilation.
In sum, it is clear that the use of chlorinated isocyanurates presents a much safer method of pool sanitation. Experimental evidence of minimal long term toxicity from doses much larger than would ever be found in a pool certainly supports this conclusion. It seems, then, that a properly treated and professionally maintained pool should not present a long term health hazard for those who use it, even those who do every day for multiple hours a day. If any, the greatest known risks are of skin irritation and, in the case of indoor pools, respiratory tract irritation.
Hope this helps, Amy! As always, feel free to submit your questions at the link above.
Till next time, abyssus abyssum invocat