The distinct, sharp odor of a swimming pool is often mistakenly attributed to a high concentration of chlorine actively disinfecting the water. This assumption is incorrect; the characteristic pool smell is not clean, active chlorine. Chlorine does not simply “kill” urine and other bodily fluids. Instead, it enters a chemical reaction with the nitrogenous compounds present in these wastes. The resulting chemical byproducts create the unpleasant smell and the physical discomforts swimmers experience, while consuming the effective disinfectant and compromising water quality.
The Chemical Reality: Reaction Between Chlorine and Organic Waste
The nitrogenous waste introduced by swimmers, primarily through urine and sweat, acts as a chemical reactant for the pool’s sanitizer. Urine contains high concentrations of urea and uric acid, which are the main precursors for negative chemical reactions. Free chlorine, existing as hypochlorous acid, is the active disinfectant intended to neutralize pathogens like bacteria and viruses.
When urea, a nitrogen-rich compound, enters the water, it begins a slow process of chlorination by the free chlorine. This reaction involves multiple steps, forming intermediate compounds known as N-chloroureas. The reaction with uric acid is particularly efficient at forming harmful volatile byproducts.
This chemical interaction is not simple disinfection but a complex transformation that consumes beneficial free chlorine. Hypochlorous acid is diverted away from its primary role of killing germs to reacting with the nitrogen compounds. This diversion effectively lowers the amount of free chlorine available for true sanitation, reducing protection against waterborne illnesses.
Chloramines: The True Cause of Pool Smell and Irritation
The reaction between free chlorine and nitrogenous waste produces combined chlorine compounds called chloramines. These compounds are responsible for the strong, irritating odor incorrectly associated with a “clean” pool. Paradoxically, a powerful chlorine smell indicates a dirty pool with poor water chemistry, signaling a buildup of these reaction byproducts.
Three main types of inorganic chloramines form sequentially: monochloramine, dichloramine, and trichloramine. As free chlorine reacts with nitrogen compounds, the molecules progress through these stages, becoming increasingly volatile. Monochloramine is the least volatile, while trichloramine (NCl3) is highly volatile and readily off-gasses into the air.
This gaseous trichloramine causes common physical complaints like irritated, red eyes and respiratory discomfort, especially in indoor pools with limited ventilation. Trichloramine is a known respiratory irritant, and exposure can lead to asthma-like symptoms and other acute lung injuries. The reaction between chlorine and uric acid can also generate toxic compounds like cyanogen chloride (CNCl), which poses health risks upon inhalation.
The combined chlorine compounds still possess some disinfecting ability, which is why they are sometimes used in municipal drinking water systems. However, they are significantly slower and less effective than free chlorine. The presence of these chloramines locks up available chlorine, making the pool environment less sanitary and more irritating.
Pool Maintenance Strategies to Eliminate Chemical Byproducts
Maintaining proper pool chemistry requires actively managing chloramine levels. The most effective method for eliminating chloramines is “breakpoint chlorination,” commonly known as “pool shocking.” This involves adding a large, calculated dose of chlorine to overwhelm the combined chlorine compounds.
When the chlorine dose reaches the breakpoint, it provides enough oxidizing power to break the chloramine bonds. This process converts the nitrogen-containing compounds into inert, gaseous nitrogen, which then escapes harmlessly into the atmosphere. Operators must typically add chlorine at a ratio that is at least ten times the measured combined chlorine level to achieve this chemical threshold.
The effectiveness of free chlorine and breakpoint chlorination depends highly on the pool’s pH level. Free chlorine exists in equilibrium with the less potent hypochlorite ion. The ideal pH range of 7.2 to 7.8 ensures a sufficient amount of the more powerful hypochlorous acid is available, allowing chlorine to react quickly and efficiently with pathogens and nitrogenous waste.
Source Reduction
The most proactive strategy for minimizing chloramine formation is source reduction, preventing waste from entering the pool in the first place. Encouraging swimmers to shower with soap before entering removes organic matter, including sweat and residual personal care products. Requiring swimmers to use the restroom drastically reduces the introduction of urea and uric acid, thereby preserving the free chlorine for its intended purpose of sanitation.