The familiar scent often associated with swimming pools, commonly referred to as “chlorine smell,” is a widespread experience. Many people believe this strong odor signifies a clean, well-chlorinated pool. However, this common assumption is a misunderstanding of pool chemistry.
The Chemical Reaction Behind the Smell
The smell frequently identified with swimming pools is not from the chlorine disinfectant itself, but from a chemical reaction occurring within the water. Chlorine, typically added to pools as hypochlorous acid, serves to kill germs and bacteria. When this active chlorine encounters nitrogen-containing compounds introduced by swimmers, it creates new substances. These nitrogenous compounds primarily come from human bodily fluids and organic matter, such as sweat, skin cells, and urea found in urine.
The interaction between chlorine and these nitrogenous wastes leads to the formation of compounds known as disinfection byproducts. This process consumes the active chlorine, making it less available for sanitation.
Understanding Chloramines
The specific chemical compounds formed from the reaction between chlorine and nitrogenous substances are called chloramines. These are the actual source of the strong, unpleasant odor often mistaken for “clean” chlorine. Chloramines are categorized into three main types: monochloramine, dichloramine, and trichloramine, which form sequentially as more chlorine reacts with ammonia or nitrogen compounds.
While monochloramine is a less effective disinfectant, dichloramine and trichloramine are responsible for the irritating effects commonly experienced by swimmers. These effects include red, itchy eyes, dry or irritated skin, and respiratory issues such as coughing, nasal irritation, and asthma-like symptoms. Trichloramine is particularly volatile and off-gases into the air above the pool water, contributing significantly to the strong smell and airborne irritants.
Free chlorine is the active, available disinfectant in the pool water, working to kill pathogens. Combined chlorine refers to chloramines, which are chlorine molecules that have already reacted with contaminants. A healthy, well-maintained pool should have a minimal level of combined chlorine, ideally below 0.2 parts per million (ppm). Therefore, a strong “chlorine smell” indicates a high concentration of these irritating chloramines, signifying a problem with water chemistry rather than optimal cleanliness.
Minimizing Chloramine Formation and Exposure
Reducing the formation of chloramines and limiting exposure involves both swimmer behavior and proper pool maintenance. Swimmers can reduce the amount of nitrogenous compounds entering the water by showering thoroughly with soap before entering the pool. This helps wash away sweat, skin oils, and other bodily contaminants. Using the restroom before swimming also prevents urea from entering the pool water.
For pool operators, maintaining proper water chemistry, including adequate free chlorine levels, is essential to manage contaminants. Effective ventilation, especially in indoor pools, helps remove airborne chloramines, improving air quality and swimmer comfort. Regularly “shocking” the pool, which involves adding a large dose of chlorine, can break down existing chloramines and convert them into harmless nitrogen gas. This practice aims to achieve “breakpoint chlorination,” oxidizing the combined chlorine and restoring the effectiveness of free chlorine.