This post was written by  IU undergraduate students Alexa Akers and Amanda Lawson. It is the second in a series of featured stories written for a ScIU in the Classroom collaboration with Dr. Cathrine Reck in the Department of Chemistry.

There are some smells that are universal. They transcend region and culture, and can be recognized instantaneously. There’s no better example for this than the smell of chlorine.

As a former swimmer, chlorine has always been one of my favorite smells. It brings me back to summer, pool parties, and splashing around with my friends. However, that smell got a little less enjoyable after I learned what was really causing it. And to give you a hint — it’s not the chlorine.

When a chlorine disinfectant is added to water to destroy germs, two products are formed: hypochlorous acid (HOCl) and a hydrochlorite ion (OCl-). These compounds kill germs by attacking lipids in the cell walls through an oxidation reaction, ultimately resulting in the cell’s death. When treating pool water, we want as many of these pathogenic cells to be destroyed as possible.

However, adding these disinfectants to the pool water doesn’t create that iconic “pool smell.” Something much more disgusting does.

HOCl is a very strong acid. When exposed to a base, like ammonia, a reaction occurs and new products are formed. This reaction causes HOCl to reduce into water through the loss of it’s chlorine atom. Ammonia picks this up to form monochloramine.

Ammonia is a very common molecule. Biologically, it’s found in a lot of systems within our bodies. One place where it’s especially common? Our waste products.  

In case you haven’t caught on yet, let me be a bit more specific. That “pool smell” we know and love is primarily due to urine. And while you might think that’s bad enough, the full story only gets worse. The reaction can happen many times, depending on how much ammonia is in the water. After one reaction, monochoramine is formed. Another reaction creates dichloramine, a third produces trichloramine. With each reaction, the molecule becomes more unstable.

When a chloramine molecule reaches the surface of the water, its unstable structure causes it to form a gas. Thus, the more ammonia HOCl is exposed to, the more gas it forms, and the stronger the smell.

What’s even worse is the long-term side effects to those who are exposed to pool water for long periods of time. One example is competitive swimmers. Swimmers have the highest asthma rates of any type of athlete, which is concerning considering how much of the sport involves holding your breath. We also know people who work in natatoriums — which are buildings that contain a swimming pool — have a higher percentage of airway issues than the general population. This could be a direct result of chloramines. However, we don’t know for sure because there has been no large-scale research on how this gas affects the lungs of people who are exposed to it for long periods of time. When we look at the large amount of health conditions that could be linked to this unstable molecule, it’s worrying.

While all of this is important, there is one main message you should take away from reading this article. Be aware of the chemicals that surround us and their effects on our bodies, and most importantly, do not pee in the pool!

Edited by Jennifer Sieben