Chlorine possesses a distinct odor, though the scent most people associate with it is often a misidentification. Chlorine is widely used as a disinfectant, and its presence can be detected by smell in various forms. While pure chlorine gas has a recognizable, pungent aroma, the familiar odor encountered in swimming pools is usually a byproduct of chlorine doing its job. Understanding the difference between the true smell of chlorine and its reaction products is important.
The Common Scent: Chloramines, Not Chlorine
The sharp, irritating smell near a swimming pool is not the disinfecting agent itself, but the result of chemical compounds called chloramines. Chloramines are formed when the free chlorine added to pool water reacts with nitrogen-containing organic materials introduced by swimmers. These contaminants include sweat, urine, body oils, and cosmetic products.
This process involves the active disinfectant, hypochlorous acid, combining with ammonia and urea to form mono-, di-, and trichloramine. The most volatile and pungent of these, trichloramine (nitrogen trichloride), readily off-gasses into the air above the water, creating the characteristic pool smell. A strong chloramine odor indicates that chlorine is being consumed by contaminants, signifying poor water quality rather than an over-chlorinated pool. In a properly managed swimming environment, the chloramine smell should be minimal or nonexistent.
The True Odor of Elemental Chlorine
The odor of pure, elemental chlorine gas (Cl2) is markedly different from the smell of chloramines, though both are described as sharp and bleach-like. At room temperature, elemental chlorine exists as a dense, yellow-green gas. Its scent is intensely pungent, highly irritating, and suffocating, even at low concentrations.
This gas is rarely encountered by the general public but is used heavily in industrial applications, such as manufacturing plastics and various chemicals. Chlorine is a powerful oxidizer, which gives it strong bleaching and disinfecting properties. The threshold for detecting the odor of chlorine gas is low, but continuous exposure quickly dulls the sense of smell, making the odor an unreliable warning sign for high concentrations.
What the Smell Indicates About Safety and Hygiene
The presence of a noticeable chloramine odor serves as a warning sign regarding the hygiene of a swimming facility. The volatile trichloramine that causes the smell is an irritant, responsible for the reddening of eyes, itchy skin, and respiratory discomfort experienced by swimmers. When the smell is strong, it suggests that the water is unbalanced and that the concentration of combined chlorine is too high.
The practical solution for pool operators is a process known as breakpoint chlorination or “shock treatment.” This involves adding a higher dose of chlorine to fully oxidize and destroy the chloramines and other organic compounds. This action eliminates the volatile byproducts, reducing the irritating smell and restoring the water’s sanitizing capacity. When using chlorine-based cleaning products, proper ventilation is necessary to prevent the buildup of fumes and reduce the risk of irritation to the airways and mucous membranes.