What is the ideal pH for swimming pools?

September 17, 2019
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Pool industry textbooks and certification courses tell us that swimming pools should be maintained with a pH between 7.2 and 7.8, but ideally 7.4 to 7.6. But why? Is it for sanitizer efficiency or overall water balance? Or perhaps it is because of swimmer comfort. Everyone we have asked in the pool industry makes mention of the pH of human tears...but a simple online search debunks that myth. So let's challenge conventional wisdom and get to the truth.

Ideal Swimming Pool pH

pH impacts just about everything else in water chemistry. It determines the percentage of bicarbonate to carbonate alkalinity, and without a doubt, it is vital to understand and measure. But rather than just sticking to a textbook range chemistry recommendation, we believe the ideal pH depends on what you're trying to accomplish. It depends on your pool's circumstance, bather load, CYA level, and more. So is 7.4 to 7.6 pH always ideal? No.

Consider all of these things we must satisfy in real-world pool operations.

pH's impact on sanitization and oxidation

In the absence of cyanuric acid (CYA, or stabilizer), pH has a significant impact on the strength of chlorine. Look at the chart below. The red line shows that lower pH means a higher percentage of Hypochlorous Acid (HOCl), which is the active killing form of chlorine. The smaller that percentage, the weaker the chlorine.

HOCl CYA relationshipSource: Cyanuric Acid - CMAHC Ad Hoc Committee Report, Slide 6. - Richard A. Falk

Also, notice how that percentage of HOCl drops dramatically with the use of CYA. It shows the pH has virtually no impact on the strength of chlorine with 10+ ppm of CYA; a point studied and published by Dr. Stanley Pickens. So for indoor pools without CYA, pH makes an enormous impact on chlorine efficacy, but pH makes a much smaller impact on stabilized pools.

Chlorine strength is probably the most common argument we hear when it comes to choosing an ideal pH, but as mentioned earlier, it is not the only one. The next one is...

pH's impact on the LSI

Orenda is among the loudest voices when it comes to the LSI and overall water balance. We teach it every day and even developed the Orenda App specifically to have a dosing calculator that measured it. Of the six factors that go into the LSI formula, pH stands on top as the most impactful. In fact, pH is so influential in the LSI formula that it does not even have a factor on the reference chart...it just plugs right into the equation. Look:

LSI Equivalents Chart

Since it has no factor to look up, pH itself has a direct impact on the LSI number. If you go from 7.2 to 7.8 in pH, that +0.6 increase raises the LSI exactly +0.6. Considering the LSI safe zone is only 0.6 (-0.30 to +0.30), it is clear that pH has a tremendous influence on the LSI. 

We like the 7.4-7.6 pH range for the LSI most of the year, but in the winter, it's difficult to keep the water balanced when the temperature drops. In that case, a higher pH, like 7.8 to 8.0 is appropriate. Furthermore, pH naturally wants to be about 8.2 anyway, based on carbon dioxide equilibrium with the air. The risk is that a high pH will be more apt to drive calcium carbonate out of solution and form scale. In fact, pH has more of an impact on scale formation than calcium hardness itself, which is contrary to conventional wisdom on the pool business.

It must be accounted for, while not trying to keep it too low against its will. pH is a funny thing...it goes where it wants to be, and fights back when we force it to be where we want it to be. The Orenda way to control pH is to adjust other factors that can narrow its range, such as calcium hardness and alkalinity.

If we go too low with pH, the LSI may go too low, etch cement, and the pH will be driven up again. This is common during a traditional pool startup and especially hot starts

tears in the pool, pH of tears, ideal pH in pool, swimmer comfort

pH and Swimmer Comfort

Myth: The pH of the water should be 7.4-7.6 because human tears (and eyes) have a pH of 7.4.

Truth: Human tears can range from 6.5 - 7.6 pH. The notion that everyone's eyes are about 7.4 is false. That said, human blood is about 7.4 pH.

This myth is widely believed in the pool business, and while it is possible to have tears that are 7.4, the truth is there is a wide range. According to the research we read, people's eyes tend to get more alkaline throughout the day. So you could start in the morning with a 7.0 pH in your eyelids, and by the evening it could be 7.3 or 7.4. Very seldom does the pH of an eyeball go higher than 7.5.

But what about the pH of human sweat? Irritation to our skin might be related to that, right? Actually, not really. According to this study, human sweat ranges from 4.2 to 7.0 pH. 

With all that knowledge in mind, the myth that pH should be 7.4-7.6 because of swimmer comfort should be debunked. If swimmer comfort is about matching the pH of the human eye, it should be lower than 7.4, and possibly raised up to 7.5 in the afternoon. If it were supposed to match human sweat, it would be far lower. But come on, that's not practical at all.

A swimmer's opinion

I personally do not think pH is the leading cause of eye and skin irritation in swimming pools. I believe chlorine is–specifically combined chlorine, like chloramines. Bromine pools too, and their disinfection byproducts (DBPs) called bromamines. I trained every day in a bromine pool for many years, and while it was less irritating to me than chlorine, it could still make my eyes burn. I cannot tell the difference between a 7.2 pool and a 7.8 pool.

Ideal pH depends on circumstances

Here are our recommended pH ranges, depending on certain circumstances. For instance, if your pool is serviced weekly and your primary chlorine is trichlor, you may want a higher pH because trichlor will lower it, and CYA will further lower the LSI. If you're using cal hypo or liquid chlorine, you will want a slightly lower pH, because those chlorines raise the pH. 

The name of the game is optimizing chlorine's strength while also maintaining LSI balance. As mentioned earlier, pools with over 10 ppm CYA can afford a higher pH because the impact on chlorine is so much less. 

As far as patron comfort goes, the pH will not always correspond to human eyes, tears, or sweat. And that's okay. The evidence shows that unless pH is way out of whack, eye and skin irritation are not likely. And in my opinion as a competitive swimmer, the more likely culprit for swimmer discomfort is the sanitizer and its disinfection byproducts like chloramines.

Related: Understanding Breakpoint Chlorination

Over-treating swimming pools and relying on chlorine to do the lion's share of bather waste removal (via oxidation) is part of the problem...at least as far as I can tell. So if you want your water to be comfortable for swimmers, focus on that.

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