Evidence-Based Appraisal of Hygiene and Personal Care: Hypochlorous Acid, Age-Related Body Odor, and Fluoride Toothpaste

Hygiene and Personal Care: Safety Claims, Deodorizing Evidence, and Fluoride Toothpaste

Bottom line

A reliable appraisal of hygiene and personal care separates three questions: whether an ingredient can exert an effect, whether its actual use is safe, and whether product efficacy is supported by human evidence. Hypochlorous acid does not necessarily leave only water after contacting organic matter; 2-nonenal is not the sole explanation for all age-related body odor; and fluoride toothpaste effectively prevents caries, but ingestion by young children must be limited.（PMID: 40724314；PMID: 11286617；PMID: 12535435；PMID: 30703075）

Hypochlorous acid disinfection cannot be reduced to “turns into water”

Professor Ching-Shun Lin noted that claiming hypochlorous acid simply reverts to water after contact with microorganisms or organic matter is an oversimplified safety message. Current literature supports this caution: disinfection systems containing hypochlorous acid may leave residual hypochlorous acid and may also generate chlorinated or disinfection by-products. The reaction endpoint therefore cannot be described exclusively as water.（PMID: 40724314）

This does not mean that every form of chlorine-based disinfection is unsuitable. What must be distinguished are concentration, route of contact, setting of use, and the population exposed. Antimicrobial efficacy and human safety are also separate questions. A substance’s ability to damage microorganisms does not mean that inhaling it as an aerosol is necessarily harmless. Cell studies show a clear dose-dependent difference: higher amounts of hypochlorous acid can cause necrosis, while lower amounts may induce apoptosis or growth arrest.（PMID: 11327319）

General-use experience cannot replace inhalation risk assessment

Professor Lin’s original article offered a qualified caution: chlorinated water is generally considered safe, but safety concerns may remain in certain unusual exposure settings. Subsequent evidence indicates that respiratory effects associated with occupational cleaning exposure and exposure to particular chlorine-containing products warrant attention. These studies, however, did not evaluate any specific brand of household hypochlorous acid nebulizer. They therefore cannot directly prove that such a product is harmful, nor can they be used to endorse it.（PMID: 35509379；PMID: 34599438）

The most accurate description of aerosolizing products is therefore neither “definitely safe” nor “definitely toxic.” Rather, existing indirect evidence indicates that the inhalation route requires its own assessment. If a product supplies only evidence on use in water or surface disinfection but extends its conclusions to prolonged aerosolization into indoor air, the evidentiary chain remains incomplete. A safety claim should specify the actual concentration, droplet characteristics, ventilation, exposure duration, and respiratory safety testing, rather than merely name the starting material or describe an idealized reaction product.

Swimming pools and asthma: the literature has been updated

The earlier studies cited by Professor Lin did report positive associations between children’s or adolescents’ use of chlorinated swimming pools and asthma or allergy. Given the evidence available at the time, treating this as a warning signal for a particular exposure setting was supported by the literature.（PMID: 12771389；PMID: 18508822）

As the science evolved, a newer systematic review found no statistical association between swimming and the incidence of asthma. Earlier observational studies were therefore useful for raising the question, but they can no longer stand alone as proof that the risk has been established. A present-day appraisal should recalibrate the degree of certainty using the higher-level evidence now available.（PMID: 40834214）

This update should not be misread as showing that all chlorine-containing exposures are free of respiratory concern. Swimming-pool activity combines exercise, the physical environment, ventilation, and disinfection practices; it is not the same exposure as continuous aerosolization in a home. The limits of what the evidence can answer must be respected, and findings from one setting cannot decide the outcome in another.

Age-related body odor should not be reduced to a single culprit

Research shows that 2-nonenal increases in the body odor of older individuals and may contribute to age-related changes in odor. Professor Lin’s account of its association with what is colloquially called “old-person smell” thus has a basis in experimental research.（PMID: 11286617；PMID: 38349676）

The original research, however, supports the proposition that this compound “may contribute”; it does not establish it as the sole culprit. Human body odor may be shaped simultaneously by sebum, microorganisms, hygiene practices, clothing, and the living environment. Reducing a complex phenomenon to one molecule can lead consumers to assume incorrectly that removing that molecule resolves the entire issue.

The mechanism that produces 2-nonenal is clearer: oxidative degradation of omega-7 unsaturated fatty acids on the skin surface can generate 2-nonenal. This accords with the biochemical pathway cited by Professor Lin.（PMID: 11286617）

As for the claim that soap containing persimmon extract can immediately remove 97% of 2-nonenal, Professor Lin characterized it as company advertising and candidly stated that he could not find supporting data; he did not therefore declare it effective. ECU likewise found no citable clinical trial verifying either that precise percentage or the immediate effect. The appropriate verdict is “insufficient evidence,” not the mistaken assertion that absence of located evidence proves ineffectiveness.

Fluoride toothpaste is effective; the key is an appropriate amount and supervision

Large-scale meta-analysis of trials supports fluoride toothpaste for preventing dental caries in children and adolescents. Professor Lin’s statement that an appropriate amount of fluoride protects teeth is consistent with the literature.（PMID: 12535435）

Established benefit does not mean that more is better. Excessive fluoride ingestion while teeth are developing can cause dental fluorosis, which may include changes in the appearance of tooth enamel. Young children should use an amount appropriate to their developmental stage, with help or supervision from an adult to reduce the risk of swallowing.（PMID: 30703075）

The core considerations are dose and route. Spitting toothpaste out after brushing is not the same exposure as long-term ingestion, and the topical benefit against caries does not negate the risk of ingesting too much. Parents need not reject the caries-preventive benefit of fluoride toothpaste because of concern about fluorosis; they should instead focus on the right amount, proper brushing, and adult supervision.

How consumers should read hygiene and personal-care claims

First determine whether a claim concerns a chemical reaction, laboratory antimicrobial activity, use on people, or long-term inhalation, because each question requires different evidence. Then ask whether the study population and use setting match the product: occupational exposure, swimming pools, and aerosolization in the home are not interchangeable. Finally, check whether efficacy is supported by independent human research. Strong claims such as “immediate” effects and precise percentages particularly require verifiable methods, comparators, and results.

Time context must also be retained when evidence is updated. Professor Lin’s citation of early research should be understood in light of the literature available then. When subsequent systematic reviews change the weight of evidence, the appropriate framing is that “the literature has been updated” or that “as science has evolved, the degree of certainty has changed.” This both respects the original article and grounds today’s health decisions in the more complete evidence now available.