The strong, distinct odor often associated with swimming pools leads many people to believe the pool contains too much chlorine. This is a misconception, as a clean and properly maintained pool should have no noticeable chemical smell at all. The pungent, irritating scent is not a sign of high chlorine concentration; rather, it is evidence that the chlorine is actively reacting with contaminants. This odor is caused by a group of chemical byproducts known as chloramines.
The True Cause: The Formation of Chloramines
The chemical reaction that produces the characteristic pool smell begins when the active disinfectant, called free chlorine, encounters organic materials introduced by swimmers. Free chlorine is the germ-killing form of chlorine and is highly reactive. When added to water, it forms hypochlorous acid and the hypochlorite ion, which are the agents responsible for sanitation.
The problem arises when free chlorine reacts with nitrogen- and ammonia-containing compounds, which are abundant in human waste products like sweat, body oils, skin cells, and urine. Swimmers introduce these contaminants into the water, and the chlorine immediately begins to oxidize them. This reaction creates a series of chemical compounds called chloramines, which are also known as combined chlorine.
Three main types of inorganic chloramines can form: monochloramine, dichloramine, and trichloramine. Trichloramine, also called nitrogen trichloride, is the most volatile byproduct, meaning it readily escapes the water and becomes airborne as a gas. This gaseous trichloramine is responsible for the pungent odor often mistaken for pure chlorine. The presence of chloramines indicates that the pool water has been contaminated and that the disinfectant is being consumed.
Why the Smell Causes Irritation
The airborne chloramines, particularly trichloramine, are potent chemical irritants, not just a nuisance odor. As these gaseous compounds accumulate, especially in poorly ventilated indoor pool environments, they cause physical discomforts in swimmers and spectators. The chloramine gas interacts with the body’s mucous membranes, leading to immediate irritation.
A common symptom is red, stinging eyes, often mistakenly attributed to the chlorine itself. Chloramines also irritate the delicate tissues of the respiratory tract, which can trigger nasal irritation, coughing, and even wheezing. For individuals with existing conditions like asthma, exposure to these airborne irritants can trigger an attack. Skin irritation and dryness are also common when chloramines come into prolonged contact with the skin.
Eliminating the Odor
The most effective way to eliminate the chloramine odor is to prevent the formation of these irritating compounds in the first place. Swimmers play a significant role by practicing pre-swim hygiene, as a brief shower rinse removes the majority of sweat, oils, and other organic matter. This action reduces the contamination entering the water, which in turn reduces the chemical demand on the free chlorine.
For pool operators, removing existing chloramines requires a process called superchlorination, or “shocking” the pool. This process involves intentionally adding a large dose of chlorine, typically reaching a concentration ten times the amount of combined chlorine. This high level of free chlorine oxidizes and destroys the nitrogen-chlorine bonds in the chloramines. The chloramines are then converted into harmless nitrogen gas that vents into the atmosphere.
Proper pool chemistry management, including maintaining the water’s pH level between 7.2 and 7.8, allows the chlorine to work effectively. For indoor pools, robust ventilation systems are necessary to draw fresh air over the water surface and exhaust the volatile chloramine gases. Regular superchlorination, coupled with good swimmer hygiene, ensures the free chlorine remains available to disinfect the water. This prevents the creation of irritating byproducts that cause the distinctive odor.