Parents, swim teachers, lifeguards – we’ve all heard it from kids regularly when instructing them to open their eyes underwater: “The chlorine hurts my eyes”, or its variation, “The water stings my eyes.” I’ve even heard it from adult friends at pool parties: “That’s a lot of chlorine in this water”.
Most everyone (at least in the United States) knows - and usually learn as kids - that swimming pools of all kinds use chlorine to sanitize the water, so it’s not surprising the chemical becomes the first thing people blame for that underwater stinging sensation. But there’s a lot more that goes into pool chemistry than just chlorine. Today, we’re debunking that myth and exploring the real culprit.
Let’s discuss the chlorine myth first. To start, chlorine in swimming pools is counted in the parts per million. Most state codes require that chlorine levels in public swimming pools be between 1 and 5 parts per million (ppm), with many jurisdictions finding 2 ppm to be the 'sweet spot'. In other words, for every million parts of water, there are only two parts of chlorine to be found. The chances of that finding its way into your eyes to make them sting are literally one (maybe two) in a million. Your chances of getting struck by lightning are far greater (1 in 15,300).
While it’s true that extended exposure to chlorine may cause slight irritation, it won’t cause the sensation most refer to when coming up after opening your eyes underwater for a few seconds.
So if not chlorine, what causes it? There’s another major component to pool chemistry that the layperson might not know or consider: pH balance.
In case you don’t remember 7th-grade chemistry class (at least, that’s when I took it in middle school), let’s recap what pH is:
pH is the measure of acidity or basicity of a liquid. The abbreviation, pH, stands for “potential of hydrogen” and, according to the Model Aquatic Health Code (MAHC), that means the “negative log of the concentration of hydrogen ions”. pH is measured on a scale of 0 to 14, with numbers less than 7 being increasingly acidic and numbers greater than 7 being increasingly basic. 7 is neutral, or balanced. In chemistry class, we were taught that water is also a perfect 7; however, that’s not usually true (sorry Mr. Brostrom!).
Many of the water sources we use - ground water and lake water – typically hover around a pH of 8. According to the National Institute of Health, the normal pH of your eyeballs is about 7.1 with a tolerance of up to 7.4. So, to make the water more comfortable for swimmers (and their eyes), pool operators need to bring the pH of the source water down closer to 7.1, because even a slight imbalance in pH can cause discomfort to the human body.
To do this, operators introduce acid into the swimming pool to lower the pH to somewhere around 7.2 to 7.6, depending on local or state codes. (The MAHC states that the pH of the water should be maintained between 7.0 – 7.8.) Further, pH is uniformly distributed across an entire body of water, so it’s highly unlikely that pool chemistry tests taken at opposite ends of a pool would yield different results for pH.
Consequently, when your eyes sting when you swim, it’s because the water is more basic than your eyeball usually tolerates, which means that the pH of the swimming pool may be too high. The same case applies when you shower or bathe.
So, the next time you’re in the water and feel that sting in your eyes, don’t worry and it won't harm you: It’s just a difference in pH balance between your body and the water.
Curious to learn more about chemical treatment options for pools? Check out this blog here.