Why Do Our Eyes Sometimes Turn Red When We Get In The Pool?​

Why Do Our Eyes Sometimes Turn Red When We Get In The Pool?

Especially on summer days, we prefer to enter the pool to cool off or to do sports. However, sometimes we feel redness or stinging in our eyes when we stay in the pool for a long time. This uncomfortable feeling is actually a signal that we need to get out of the pool. By the end of the article, we will learn why we should run away from the pool as soon as our eyes start to turn red!

Analysis of Pool Water: Pool Chemistry

First, let’s take a look at the chemistry of the pool. It is an expected result that there are microorganisms in an environment where many people constantly go in and out. Among these microorganisms, there are also harmful ones and they need to be inactivated regularly. Chlorine tablets are used to remove these harmful microbes or unwanted liquids such as sweat and urine in the pool, that is, to sterilize them. Chlorine is a highly reactive nonmetal in the group of halogens (Group 7A) in the periodic table. That is, it is so aggressive that it can react with almost any substance. For this reason, when added to the pool, it reacts with water to form hypochlorous acid (HClO).

Cl2 + H2O → HOCl + HCl

pool image

Hypochlorous acid has a bleach-like effect by breaking down microorganisms in the pool. The acid, which provides sterilization by ionizing, is also called active chlorine due to this feature.

Over time, nitrogen-containing substances such as sweat and urine begin to accumulate in the water. Active chlorine ions react with accumulated ammonia-containing substances to form chloramine. Chloramine, a derivative of ammonia, is the reason behind the redness and stinging sensation in our eyes at the end of the time we spend in the pool.

2 NH 3  + Cl 2  ⇌ NH 2 Cl + NH 4 Cl

The More Pee The More the Pool Smell!

In addition, the reason for that smell, which we call the pool smell, is the chloramine molecules formed. The formation of hypochlorous acid decreases as the basicity of the medium increases. That is, the disinfecting effect of chlorine decreases as the pH increases. Since the chloramine molecule formed is slightly basic, it will slow down the formation of hypochlorous acid after a while and prevent the removal of microorganisms in the environment. For this reason, when we detect the smell of the pool, we need to sterilize the environment by adding new chlorine tablets.

The chemicals in our urine (or pee) turn to ammonia (NH3) if it awaits some time. As mentioned above, ammonia combines with chlorine and forms the chloramine compound that causes the odor we call pool odor. In other words, if we reduce this event to a more daily language, we can say exactly “the more pee, the more pool smell“. In fact -I’m sure you have noticed too- this pool smell is felt more in children’s pools. The reason for this is that children have little accidents more than adults.

Children's Pool Image

Note: You must take a shower before entering pools that are open to many people, such as summer or Olympic swimming pools. The reason for this is to minimize the formation of chloramine in the pool by ensuring that our body fluids are removed with a shower.

References and Further Readings

Chloramines: Understanding “Pool Smell”. (n.d.).

U.S. National Library of Medicine. (n.d.). Chloramine. National Center for Biotechnology Information. PubChem Compound Database.

Water Treatment Solutions. Lenntech Water treatment & purification. (n.d.).,are%20present%20in%20the%20water.

Wikimedia Foundation. (2021, May 25). Monochloramine. Wikipedia, the free encyclopedia.

You can access the sources of the images used by clicking on the images.

The proofreading has been done by Asu Pelin Akköse and Mete Esencan.

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Mete Esencan

Hello everyone! I'm Mete Esencan. I am a graduate student in the Department of Chemistry at METU. I was planning to establish a platform by combining the research knowledge I gained during my basic science education and the management experience I gained in the METU Chemistry Society, which I was in charge of for three years. For this purpose, in February of 2021, I took the first step and established the OkButWhy, a platform where we can write articles as if to chat about science, art and philosophy. I wish everyone a pleasant reading!

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