Can chlorinated pools be dangerous?

Q: I’ve heard that chlorinated pools can be dangerous. Is that true?

A: The answer is “possibly”. Any substance, even pure water, can be dangerous or even deadly when used improperly. Chlorine is no exception.

Chlorine is the most commonly used sanitizer for pools. When used properly it is safe, effective, and economical. Unfortunately, its popularity also means it is the most frequently misused chemical in the industry. This misuse can lead to unhealthful conditions.

We’ve all walked into a health club or public pool and choked at the smell of chlorine in the air. The likelihood is that this is an example of chlorine misuse. Chlorine doesn’t have to make your eyes burn in order to work.

About Chlorine

Chlorine kills bacteria, viruses, protozoa, and algae – all things we DON’T want in our pool. It also works to oxidize the organic matter, effectively removing it from the water.

In pure form, chlorine is a gas. Contact with your skin or eyes can cause burns, and breathing it can be fatal. Chlorine gas was used as a weapon in WWI!

On the other side of the coin, chlorine is probably the principal sanitizer for your drinking water. Before public water supplies were sanitized, millions of people died from water-borne diseases such as cholera, typhoid, dysentery, and hepatitis.

Just as it keeps our drinking water safe, chlorine also allows us to swim without worrying about disease and infections.

Chlorine for Swimming

Here are a few things you need to know about using chlorine:

  • It must be present in sufficient quantities. Too little chlorine actually leads to other problems.
  • It must remain in contact with the contaminants for long enough to do the job.
  • The amount of chlorine needed depends on how much the pool is used, not on how big the pool is.
  • If your water pH is too high or too low, chlorine effectiveness can be greatly reduced.
  • Chlorine is very volatile, and breaks down rapidly. Sunlight causes it to break down even faster.

Depending on what you buy, the bucket of sanitizer you bring home from the pool supply store may contain more than just chlorine. Cyanuric acid (often called a conditioner) is often added in order to keep the chlorine from dissipating before it has a chance to work. Since some chlorine formulas can affect pH, buffers may be included to offset this. You’ll also get compounds designed for time-release, anti-caking, dust control, flocculants, or even perfumes.

Mistake #1

The problem with this is that the chlorine is used up, but the other chemicals remain in the water. When more treatment is added, the other compounds accumulate. It doesn’t take long for the other things to reach levels where they effectively reduce the ability for the chlorine to do its job. These “left over” materials are often the cause of red eyes, green hair, whacked-out pH, cloudy water, and the choking smell. The unwary pool owner notices these symptoms and often dumps in more chemicals that just make the problem worse!

The only way to remove these unwanted materials is to do a water change. Of course, that’s expensive, time-consuming, and means lost business while the change is being done, so it’s often a last resort.

So the first chlorine mistake is not understanding exactly what is being added to the pool and why. Too many pool owners rely on recommendations from a clerk in a store, who may be a minimum-wage teenager and might not know anything other than “this one costs more, so it must be better”.

Mistake #2

There is no simple formula for knowing the right amount of chlorine to add to a pool. It has nothing to do with whether the pool is 500 gallons or 50000 gallons. Basically, you need enough chlorine to kill and oxidize the contaminants that are brought in. This means you MUST measure it regularly and frequently and make adjustments to ensure the correct level is maintained.

There are instruments (called ORP monitors) that can measure this for you automatically and add the appropriate amount of sanitizer. They work by measuring the “oxygen potential” of the water. However, they are very expensive to both buy and maintain, and are probably not cost-effective for our application. Please note that these are NOT the same as automatic feeders. The latter devices simply dispense a pre-measured dose of chemicals at regular intervals, and do not measure chlorine levels or adjust the amount.

So without an ORP system, in order to know that the correct amount of chlorine is present the water must be tested on a regular basis. Depending on the bather load, this may mean anything from twice a week to twice a day. More importantly, there are TWO chlorine levels that must be tested: Total chlorine and free chlorine.

Free chlorine measures the amount of chlorine available to kill the germs and remove organics. Think of this as the “good chlorine”. There must be sufficient free chlorine in the water. If chlorine is the only sanitizer, at least 3 ppm (parts per million) is needed. If chlorine is supplementing another sanitizer such as ozone or metals, as little as 0.5 ppm is needed.

Total chlorine is a combination of the good chlorine and the “used up” chlorine. When the total chlorine exceeds the free chlorine by more than about 1 ppm, it means that action must be taken (usually a shock treatment).

So mistake number two with chlorine is not measuring the actual chlorine levels, or not understanding what the measurements mean.

Keeping it Safe

If you’re using someone else’s pool, how do you know if it’s safe to swim in?

Use your nose. A properly chlorinated pool should NOT knock you over with chlorine smell. Interestingly enough, the smell could mean either too much or not enough chlorine. But if the odor is objectionable, it is a danger sign.

Talk to the owner or pool maintenance person. Ask about how the water is tested, how often, and what the latest test results were. If they’re not willing to talk with you about this, I would take my business elsewhere.

You can even invest in your own test kit. For about $10 you can buy 40 test strips that measure total chlorine, free chlorine, pH, and cyanuric acid. The strips also indicate the desired range. Note that a low cyanuric acid level is fine in indoor pools.

If the pool is yours, then I have a recommendation: Salt water chlorine generators are the greatest thing to happen to pools since heaters! They eliminate the need for all the other chemicals that can build up in the water. Although a bit expensive to purchase at the beginning, you make the money back by not having to buy chemicals. It is much easier to stabilize the chlorine levels. The water is soft, odor-free, and sanitary. My personal preference is to combine a salt water chlorinator with an ozonator for the ultimate in water quality.

If the generator is not an option, then take the time to understand the different chemical forms of chlorine and when each is appropriate for you system. For example, don’t use a form intended for outdoor pools in an indoor setup. Don’t depend on a feeder to maintain the proper levels of chlorine – use those test strips!

Regardless of what method you use to sanitize your pool (chlorine, bromine, metals, or ozone), regular water changes are a must.

Two Final Thoughts

There has been some publicity about chlorine causing the formation of trihalomethanes, or THMs, which have been identified as possible carcinogens. In general, these are not considered to be a health risk when proper levels of chlorine are maintained. However, I would recommend extra care when using chlorine shock treatments. During the shock treatment be sure to leave the cover off the pool, keep the pool house well ventilated, and ensure the chlorine has dropped to appropriate levels before use. Another alternative would be to use non-chlorine shock treatments.

Finally, some people reject the idea of salt water chlorinators because their equipment is not compatible with salt water. The water in these pools is not technically “salt water” – it is water to which a small amount of salt has been added. In reality, the amount of sodium in these pools is no different that the sodium content of a pool sanitized with chemical chlorine or bromine.

Happy swimming!

Answer courtesy of:

The ACWT Pool Guru:
Karen Hunter
22609 102nd Ave SE
Woodinville , WA 98077
425 487-3078