Swimming pools require constant sanitation to protect swimmers from waterborne pathogens such as E. coli and Salmonella. Disinfection is maintained through chlorine-based compounds, which neutralize bacteria, viruses, and other microbes. A common misconception is that this involves dumping household bleach into the water. While the chemical basis is similar, pool maintenance products are specifically engineered for water chemistry and safety, differing significantly from the bleach used for laundry or surface cleaning.
The Chemical Difference Between Pool Sanitizers and Household Bleach
Pool sanitation products and household bleach share the active ingredient, which is a form of hypochlorite, but they differ significantly in concentration and formulation. Household bleach is typically a liquid solution of sodium hypochlorite at a concentration of about 5% to 6% by weight. Liquid pool chlorine, which is chemically similar, is sold at a much higher concentration, often ranging from 10% to 12.5% sodium hypochlorite. The primary difference is that household bleach often contains additives such as fragrances or thickeners, which are unsuitable for pool water and can cause foaming or other imbalances.
Many common pool sanitizers are sold as stabilized solids, such as trichloroisocyanuric acid (trichlor) or calcium hypochlorite. These solid forms contain a much higher percentage of active ingredient; calcium hypochlorite, for example, contains around 65% active ingredient by weight. Pool-specific products like trichlor are often stabilized with cyanuric acid, which acts as a “sunscreen” to protect the chlorine from being rapidly broken down by the sun’s ultraviolet rays. Household bleach lacks this stabilizer, meaning its disinfecting power would dissipate quickly in an outdoor pool, requiring constant replenishment.
How Pool Sanitizers Work to Kill Germs
Regardless of the source chemical, the disinfection process begins when the compound dissolves in water. This reaction releases a weak acid called hypochlorous acid (HOCl) and its conjugate base, the hypochlorite ion (OCl\(^{-}\)), which together are known as free available chlorine. Hypochlorous acid is the faster-acting disinfectant, working to destroy pathogens within seconds. It neutralizes microorganisms by attacking the lipids in their cell walls and destroying internal enzymes, effectively oxidizing the cells into harmless compounds.
The effectiveness of disinfection is highly dependent on the water’s pH level. A pool’s pH needs to be maintained within a narrow range, typically between 7.2 and 7.8, to ensure the maximum concentration of the more potent hypochlorous acid is present. If the pH level rises too high, the chemical equilibrium shifts, and a greater proportion of the less effective hypochlorite ion is formed. This slows the rate of germ destruction significantly, meaning the water is not being sanitized efficiently.
Why Pools Smell Like Chlorine
The strong odor associated with swimming pools is not the smell of the chlorine disinfectant itself, but rather the smell of chemical byproducts called chloramines, also known as combined chlorine. These compounds form when active hypochlorous acid reacts with nitrogen-containing organic materials introduced by swimmers. Common contaminants include sweat, urine, skin cells, and cosmetic residues. The reaction between chlorine and ammonia, which is present in sweat and urine, creates different types of chloramines, such as monochloramine, dichloramine, and trichloramine.
The strong, irritating odor is most often caused by the more volatile dichloramine and trichloramine. A pool with a pungent “chlorine smell” is actually poorly sanitized because the active chlorine has been used up reacting with contaminants. This reaction reduces the amount of free available chlorine needed to kill new germs. Chloramines are significantly less effective at disinfection than hypochlorous acid and are responsible for eye and skin irritation.
To eliminate the smell and restore sanitizing ability, pool operators perform a process called “shock treatment” or superchlorination. This involves adding a large dose of chlorine, raising the free available chlorine concentration high enough to break down the chloramine molecules. This process effectively converts the undesirable combined chlorine back into a form that can continue to disinfect the water, improving both air quality and the hygienic condition of the pool. A well-managed pool with adequate free available chlorine should have very little to no odor.