The strong, unmistakable odor at indoor swimming facilities is often attributed to a large amount of chlorine in the water. This belief is inaccurate, as the smell is not caused by active, disinfecting chlorine, but by chemical compounds created when chlorine reacts with organic waste. The true nature of the smell indicates a water chemistry problem rather than a properly sanitized environment. This pervasive scent signals that the water is unclean and that the disinfectant is actively being consumed.
The Chemical Culprit
The specific compounds responsible for the pungent odor are known as chloramines, also referred to as combined chlorine. These are distinct from free available chlorine (FAC), which is the active form added to water to destroy pathogens. In a properly balanced pool, the smell of free chlorine should be minimal or virtually undetectable. Chloramines form when free chlorine is consumed while attacking contaminants, bonding with nitrogen-based compounds to create these less-effective, odorous byproducts.
How Chloramines Form
The nitrogen compounds necessary for this reaction are introduced into the pool by swimmers themselves, making chloramine formation an ongoing process. Sources of this nitrogen include organic waste such as sweat, saliva, dead skin cells, and cosmetics. A significant contributor is uric acid from urine, which contains large amounts of nitrogen. When hypochlorous acid (the active component of free chlorine) encounters these nitrogenous materials, it forms a series of chloramines.
The reaction progresses through several stages. It starts with monochloramine, which is mildly odorous and retains some disinfecting ability. As more free chlorine is consumed, this progresses to dichloramine and finally to trichloramine, also known as nitrogen trichloride. Trichloramine is highly volatile and readily escapes from the water surface into the air. A strong, irritating smell indicates high levels of this volatile compound, meaning the pool contains excess combined chlorine and insufficient active free chlorine.
Why the Smell Matters
The smell is more than just an unpleasant condition; it is a direct indicator of poor air quality and potential health risks, particularly in enclosed spaces. When trichloramine off-gasses into the air, it acts as a strong irritant to the mucous membranes of the eyes and respiratory tract. Swimmers often experience symptoms like red, burning eyes, skin irritation, and the characteristic “pool cough.”
Prolonged exposure to high concentrations of trichloramine in poorly ventilated indoor pools has been associated with serious respiratory issues. Studies suggest that exposure may contribute to the development of asthma and allergies, especially for pool staff and competitive swimmers. The odor serves as a physical warning sign that the concentration of these harmful chemical byproducts is elevated.
Getting Rid of the Odor
Paradoxically, the most effective way to eliminate the smell and destroy the chloramines is by deliberately adding a large dose of chlorine, a process known as superchlorination or shocking. This procedure raises the free chlorine level high enough to completely break apart the chemical bonds of the existing combined chlorine. This goal is achieved by reaching the “breakpoint,” a specific chemical threshold where the ratio of free chlorine to combined chlorine is sufficient to fully oxidize the nitrogen compounds.
Once the breakpoint is reached, the chlorine breaks down the chloramines into harmless byproducts, primarily nitrogen gas, which dissipates into the air. This restores water clarity, eliminates the odor, and re-establishes the pool’s free chlorine residual. Maintaining this balance through regular testing and proper hygiene, such as showering before swimming, ensures the pool water is clean and the air is fresh.