The Gross Science Behind Chlorine And 'Pool Smell'

The local pool is a welcome sanctuary on hot summer days, but it's certainly not the best smelling of places. From homes to hotels, and from resorts to rec centers, every pool seems to have that signature aroma that we've all come to know as chlorine smell. But here's the thing ... that's not really the smell of chlorine. It's something a lot more unpleasant.

Chlorine itself has a very mild bleach-like odor. In fact, pool chlorine is a stronger relative of bleach. However, it can't account for the overwhelming odor that comes from many swimming pools, especially poorly-ventilated indoor pools. What you're actually smelling there are chloramines, chemical compounds that form when chlorine disinfectants react with human sweat and urine. So when you smell pool odor, it means somebody (or many bodies) probably peed in there.

Pool chlorine contains hypochlorous acid (HOCl), which reacts with ammonia (NH₃), a chemical found in sweat and urine. In these reactions, one, two, or even all three of the hydrogen ions in ammonia are replaced by chlorine ions, creating monochloramine, dichloramine, or trichloramine respectively. Monochloramine isn't a problem; in fact, it's sometimes used to disinfect drinking water. Dichloramine and trichloramine are the compounds that directly cause "pool smell." The more ammonia present in the water, the more these compounds will form, which raises the unsettling question: Just how much are people peeing in pools?

Pretty much every pool in the world has urine in it. Even Olympic swimmers pee in the pool, as a viral report from The Wall Street Journal revealed amidst the 2024 games. But how much pee is in the average pool?

In 2017, a research team from the University of Alberta published a study in Environmental Science & Technology Letters, wherein they tested pool water for traces of an artificial sweetener called acesulfame potassium. It's used in many foods and beverages, including sodas, condiments, and desserts, and it's not readily absorbed by the body. Whatever goes in, gets peed out, making its presence a useful indicator of urine content. Based on analysis of 31 pools and hot tubs, they determined that around 0.01% of pool water is pee. That's very little, but when compounded with sweat and body oils, it can have a big effect.

When chlorine disinfectant is initially added to pool water, it is known as free available chlorine (FAC). However, when chlorine ions bind with ammonia, they become combined available chlorine (CAC). The more chlorine gets converted into CAC, the less remains available as FAC. Heat and sunlight also affect chlorine, further reducing FAC. So, if you have a crowded pool on a hot summer day, the combined effects of weather and peeing patrons can burn through a lot of FAC, leaving behind a stew of byproducts that aren't just smelly; they can also pose a risk to your health.

A growing body of evidence suggests that chloramine compounds may have harmful effects on swimmers. The highest concern is focused on trichloramine, also known as nitrogen trichloride. A highly reactive compound, pure trichloramine is actually explosive, but that's fortunately not the case when it's mixed with water. However, the compound has been linked to irritated eyes and airways. Many swimmers complain of getting red and itchy eyes, and blame it on the pool having too much chlorine, but this is actually an indicator that there is not enough free available chlorine.

Respiratory issues are the biggest risk associated with chlorine and chloramines. The effects of chlorine inhalation pose a danger not just to swimmers, but also to lifeguards, maintenance workers, and other pool employees, especially at indoor pools where gasses released from the water don't have a place to escape. A 2007 study published in Occupational and Environmental Medicine conducted a broad analysis of swimmers across Europe, which indicated a link between indoor pool use and breathing problems like asthma and hay fever.

How do you keep your pool from becoming saturated with chloramines? The simple answer is to be clean. Despite what Olympic swimmers do, it's not a good idea to pee in the pool. That should be obvious, but one safety step a lot of people miss is to shower before entering a pool. That will clean off sweat, which is likely the most abundant source of the issue to begin with.