The Hidden Truth Behind The “Chlorine Smell” In Swimming Pools And Why We Need A Safer Water Treatment Solution

Swimming is widely regarded as one of the best forms of exercise for cardiovascular and respiratory health, yet few people realize that the swimming pool environment especially chlorine-treated pools, can pose far more health risks than we imagine.

According to a comprehensive analysis by the European Academy of Allergy and Clinical Immunology (EAACI), chlorine-based disinfectants react with organic matter introduced by swimmers (such as sweat, urine, cosmetics, and dead skin cells) to form chlorination by-products (CBPs). These CBPs, not chlorine itself, are the real cause of the characteristic “strong chlorine smell” and the irritation of the respiratory tract, eyes, and skin.

Figure 1: Chlorination by-products detected in the water and air of pools disinfected with chlorine (image sourced from a study published by EAACI)

High levels of CBPs accumulate not only in the water but also volatilize easily into the air just above the water surface, especially in indoor pools. Swimmers are exposed to CBPs through three primary routes:

• Inhalation
• Dermal absorption
• Accidental ingestion

The Most Concerning Chlorination By-Products

Trichloramine (NCl₃): The real culprit behind pool odor and respiratory irritation

The “chlorine smell” often detected at pools is not from chlorine itself but from Trichloramine (NCl₃), a highly volatile chloramine gas. The World Health Organization (WHO) recommends strict control of NCl₃ concentrations in pool air due to its ability to cause:

• Irritation of the eyes, nose, and throat
• Coughing and breathing discomfort
• Increased permeability of the airway epithelium
• Greater penetration of allergens, facilitating allergic reactions

Prolonged exposure to NCl₃ can also contribute to oxidative stress and promote airway inflammation.

Monochloramine and Dichloramine: Irritants of the mucous membranes

These chloramines form when hypochlorous acid (HClO) reacts with nitrogen-containing compounds. They strongly irritate the mucous membranes of the eyes, nose, and throat, often causing red eyes, runny nose, and persistent coughing after swimming. Such symptoms frequently indicate a high organic load in the pool water.

Trihalomethanes (THMs): A long-term concern

THMs, especially chloroform (CHCl₃), have long been studied in public health research. Although drinking water regulations primarily assess ingestion exposure, scientific evidence shows that inhalation and dermal absorption are the dominant exposure pathways for swimmers.

Epidemiological data have linked THM exposure (during bathing, showering, and swimming) to an increased risk of bladder cancer, underscoring the need for stricter control of water quality, and particularly air quality in swimming pools.

Documented Health Effects

A growing body of clinical and molecular studies describes the health impacts of CBPs across three levels: acute symptoms, biological changes, and chronic effects.

Acute effects

These symptoms appear immediately after swimming and include:

• Eye stinging and redness
• Sore throat and dry cough
• Nasal congestion and breathing difficulty
• Chest tightness during deep inhalation

These effects are not caused by “strong chlorine.” They indicate elevated CBP concentrations in the water and air, often associated with high levels of organic matter introduced by swimmers.

Biological changes (Biomarker responses)

CBP exposure can induce measurable cellular and molecular alterations, including:

• Changes in FeNO, a marker of airway inflammation
• Changes in serum CC16 protein (Clara cell protein), indicating epithelial damage or increased epithelial permeability
• Alterations in mucosal barrier integrity

These findings demonstrate that CBP exposure affects not only surface-level comfort but also the structure and function of the respiratory system.

Chronic effects

Repeated exposure over time may lead to:

• Chronic airway inflammation
• Higher risk of asthma and allergies, especially in children with atopy
• Increased bronchial hyper-responsiveness (BHR)
• Tissue remodeling patterns similar to mild asthma

Groups at Highest Risk

Pool staff and swimming instructors

Daily prolonged exposure places this group at significant risk of occupational asthma, even when NCl₃ levels remain below allowable limits.

Professional swimmers

Athletes show markedly higher rates of asthma, often presenting:

• Elevated BHR
• Signs of airway inflammation resembling mild asthma

Chemical exposure becomes an unavoidable part of their training routine.

Children and frequent swimmers

Infants and young children (particularly under age three) with atopic predisposition face higher risks of wheezing, bronchiolitis, and increased respiratory allergen sensitivity. Their developing respiratory systems make them the most vulnerable.

Why BWT Becomes a Necessary Solution

CBPs form as an inherent result of chlorine disinfection, not due to poor hygiene. Therefore, the only effective way to reduce CBPs is to adopt advanced water treatment technologies that minimize chlorine demand and prevent the formation of harmful by-products.

BWT provides solutions that:

• Use depth filtration and advanced materials to reduce organic matter and limit CBP formation
• Reduce the amount of chlorine needed to maintain disinfection, thereby lowering the formation of NCl₃, chloramines, and THMs
• Ensure consistently pure, stable water quality to help protect the eyes, skin, and respiratory system—especially for children and sensitive individuals

Conclusion

Chlorine remains an effective disinfectant, but as scientific evidence continues to highlight the health risks posed by its by-products, it is time for pool operators and water treatment professionals to adopt a higher safety standard.

BWT’s intelligent and modern water treatment solutions offer a pathway to a cleaner, safer swimming environment—one that protects health without compromising hygiene.

Source: Couto, M., Bernard, A., Delgado, L., Drobnic, F., Kurowski, M., Moreira, A., ... & Quirce, S. (2021). Health effects of exposure to chlorination by‐products in swimming pools. Allergy, 76(11), 3257-3275.