What is the ideal pH for swimming pools?
Pool industry textbooks and certification courses tell us that swimming pools should be maintained ideally between 7.4 to 7.6 pH. 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.
Covered in this article:
- Ideal swimming pool pH
- pH and chlorine
- pH and LSI
- pH and swimmer comfort
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.
- Effective sanitization/disinfection and oxidation for the cleanliness of the pool
- Water balance (LSI), as a function of calcium carbonate saturation
- Swimmer comfort
Contain pH, don't try to control it
Instead of pursuing an "ideal pH", we strongly recommend setting a minimum that allows you to maintain LSI balance. Since we have physics on our side, we know pH will naturally rise thanks to CO2 loss. That ceiling caps out around 8.2 pH, depending on your carbonate alkalinity. So set a minimum of, say, 7.5 or 7.6, and let it naturally drift up to its ceiling. The closer the pH rises toward its ceiling, the slower the pH will rise. The only exception to this is when pH is forced to rise, like a rebound from overcorrecting with too much acid.
This is why we recommend containing pH instead of trying to control it.
pH and chlorine
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.
Source: 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.
Related: Chlorine, pH and CYA Relationships
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 and LSI
Orenda is among the loudest voices when it comes to teaching 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:
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.
Related: Total Alkalinity vs. pH and their roles in water chemistry
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.
Related: Why CO2 Controls pH in Water: Henry's Law
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.
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. The bather comfort argument is just one of many misunderstandings about 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) are 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.
Is there even such a thing as an "ideal pH"? We have to take LSI, chlorine efficiency and bather comfort into consideration, which is why we recommend a containment strategy, not a strict pH range. In pools with CYA in them, pH impact on chlorine strength is negligible. 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. Keep in mind, the pH will rise naturally anyway.
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.
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.