When a swimming pool lacks the necessary chemical balance, it transforms into a potential breeding ground for harmful microorganisms. Chlorine is the primary defense against pathogens and organic contaminants. When its concentration drops below safe levels, the water’s ability to sanitize is compromised. Swimming in this environment can expose a person to various health risks and infections collectively known as Recreational Water Illnesses (RWIs). Ensuring a safe and hygienic swimming experience requires understanding the function of this chemical and the consequences of its absence.
The Role of Chlorine in Pool Safety
Chlorine serves two distinct functions in pool water: sanitization and oxidation. As a sanitizer, it actively seeks out and destroys living organisms, such as bacteria, viruses, and algae introduced by swimmers or the environment. The active form, called Free Available Chlorine (FAC), works rapidly to inactivate these pathogens, providing a continuous chemical barrier against illness. The Centers for Disease Control and Prevention recommends maintaining an FAC concentration of at least 1.0 parts per million (ppm), alongside a pH level between 7.2 and 7.8, to maximize its effectiveness.
The second major role of chlorine is as an oxidizer, meaning it breaks down non-living organic matter. This includes residues like sweat, body oils, cosmetics, and urine introduced into the water. By oxidizing these contaminants, chlorine prevents them from accumulating and consuming the active sanitizer. When the FAC level is too low, the chlorine cannot keep up with this demand, which quickly leads to a breakdown in water quality.
Health Risks of Low Chlorine Water
The most common consequence of swimming in inadequately treated water is the transmission of Recreational Water Illnesses (RWIs), primarily causing gastrointestinal distress. Pathogens like Escherichia coli (E. coli) and the parasitic protozoans Giardia and Cryptosporidium can survive and spread when chlorine levels are insufficient. Swallowing contaminated water can lead to severe diarrhea, vomiting, and stomach cramps.
The parasitic organism Cryptosporidium (Crypto) is a particular concern because it is highly resistant to chlorine and can survive for over seven days even in properly treated water. Low chlorine levels extend its survival window, greatly increasing the risk of infection. This parasite is responsible for the majority of waterborne disease outbreaks linked to swimming pools, often requiring hyper-chlorination to eradicate.
Beyond diarrheal illnesses, low chlorine levels also facilitate skin and respiratory infections. Folliculitis, commonly known as “hot tub rash,” is a skin infection caused by the bacterium Pseudomonas aeruginosa, which thrives in warm, under-chlorinated water. This infection presents as an itchy, bumpy red rash that can appear within a few days of swimming. Respiratory issues can also arise from inhaling fine mists or aerosols containing pathogens like Legionella bacteria.
Recognizable Signs of Low Chlorine
A clear sign that a pool’s sanitation system is failing is a change in the water’s appearance. Low Free Available Chlorine often results in a cloudy or murky look, indicating the sanitizer is not effectively destroying microscopic contaminants and organic debris. This cloudiness can also be an early indicator of algae beginning to take hold, as chlorine is no longer present in sufficient concentrations to inhibit its growth.
The presence of a strong chemical odor is another counterintuitive sign of low effective chlorine. Many people mistakenly believe the sharp, irritating smell means the pool has too much chlorine, but the scent is actually caused by compounds called chloramines. Chloramines are formed when chlorine binds with nitrogen and ammonia compounds found in swimmer waste, such as sweat and urine. This reaction means the chlorine has been used up and is no longer available to actively sanitize the water.
Chloramines are the true source of common complaints like red, irritated eyes and dry, itchy skin. To confirm a problem, pool water should be tested for both Free Chlorine (FAC) and Total Chlorine (TC). If the difference between these two readings—known as Combined Chlorine (chloramines)—is above 0.5 ppm, the pool is chemically unbalanced and requires immediate treatment.