mariakray/Shutterstock
While a pool is a beloved refuge on a scorching summer day, it often carries an unmistakable aroma—commonly labeled as the “chlorine smell.” In reality, this scent is not the direct result of chlorine itself but rather a more pungent byproduct formed when chlorine reacts with organic matter present in the water.
Chlorine in pool water is typically supplied as hypochlorous acid (HOCl), a powerful disinfectant that also gives bleach its characteristic odor. When this disinfectant encounters ammonia (NH3)—a compound released through sweat and urine—reactive species called chloramines form. These include monochloramine, dichloramine, and trichloramine, with the latter two responsible for the distinctive “pool smell.”
Adam Pretty/Getty Images
Urine is ubiquitous in swimming environments; even Olympic athletes have been reported to urinate in the pool during the 2024 Games, as highlighted by a Wall Street Journal investigation. A 2017 study by researchers at the University of Alberta examined 31 pools and hot tubs, measuring traces of the artificial sweetener acesulfame potassium—a marker that cannot be metabolized and therefore reflects urine content. They found that approximately 0.01% of the water volume was urine. While this percentage seems minuscule, it significantly contributes to chloramine formation when combined with sweat and skin oils.
When chlorine is first added, it exists as free available chlorine (FAC). As FAC binds to ammonia, it becomes combined available chlorine (CAC). The greater the CAC fraction, the less FAC remains to disinfect the water. Temperature, sunlight, and high usage can further deplete FAC, leaving a mixture of chloramines that not only smell strong but may also pose health risks.
New Africa/Shutterstock
Emerging evidence links chloramine exposure, particularly trichloramine (nitrogen trichloride), to eye irritation, airway discomfort, and respiratory conditions. While pure trichloramine is a highly reactive gas that can be explosive in its dry form, its presence in water is typically harmless in terms of explosiveness but can irritate mucous membranes. A 2007 study in Occupational and Environmental Medicine found associations between indoor pool attendance and asthma or hay fever symptoms, underscoring the importance of adequate ventilation and maintaining sufficient FAC levels.
Practical prevention focuses on cleanliness: showering before entering a pool removes sweat and reduces ammonia load. Avoiding urination in the water is essential—despite the prevalence of the practice among elite swimmers, it is a primary source of chloramine production. Regular monitoring of FAC, CAC, and pH, along with proper circulation and filtration, helps keep chloramine levels low and ensures a safer swimming environment.
