A fresh manicure, a new hair colour or a sleek straightened style may be part of a regular salon visit. But some of the chemicals and equipment used to achieve these looks have come under scrutiny in research. ALSO READ: Health and safety procedures required for beauty salons Studies have reported skin reactions, breathing problems and, in some cases, serious health effects associated with particular salon chemicals. Repeated exposure to high heat can also damage the hair. Here are some of the substances and salon practices researchers have examined. Trimethylbenzoyl diphenylphosphine oxide and fertility Trimethylbenzoyl diphenylphosphine oxide, commonly known as TPO, is among the chemicals used in some gel manicures that have prompted health concerns. TPO is a photoinitiator, a chemical that helps gel polish harden when exposed to ultraviolet or LED light, giving the manicure its durable, glossy finish. The chemical is now subject to new restrictions in the United Kingdom. From August 15, new cosmetic products containing TPO can no longer be placed on the market in England, Scotland and Wales. Products already placed on the market before that date can continue through the transition period until February 14, 2027. The European Union prohibited TPO in cosmetics from September 1, 2025, after classifying it as a Category 1B reproductive toxicant, a classification used for substances that may harm reproductive health. A 2025 study published in “Contact Dermatitis” found TPO in 90 percent of 30 gel nail polishes tested, with concentrations ranging from 1.42 to 6.43 percent. The researchers also found photoinitiators that were not listed on the products’ labels. Another 2024 study found high levels of photoinitiators in dust collected from nail salons, suggesting that salon workers may be exposed to these chemicals in the workplace. Animal studies have found potential effects of TPO on reproductive health. However, they do not establish that ordinary use of gel nail products causes infertility or illness in people. Glyoxylic acid in straighteners and kidneys Some hair-straightening products contain glyoxylic acid, which is used to straighten the hair as an alternative to formaldehyde in some products. Cases of acute kidney injury — a sudden decline in kidney function, have been reported in women after using some straightening products containing glyoxylic acid. A 2024 report in the “New England Journal of Medicine” documented the case of a 26-year-old woman who developed acute kidney injury three times after using a product containing glyoxylic acid. Researchers found that the chemical can pass through the skin and contribute to the formation of crystals that can damage kidneys. A 2025 review identified 36 cases of acute kidney injury in 34 women following exposure to hair-straightening products. Twenty of the products were labelled “formaldehyde-free”. The reported cases involved specific hair-straightening products containing glyoxylic acid. Some other straightening treatments use formaldehyde, which can irritate the eyes, skin and respiratory system, mainly among salon workers who handle these products regularly. Persulfates in bleach and breathing Hair bleaching powders often contain persulfates, chemicals that help lighten hair. A 2022 systematic review of 42 studies identified persulfates as a major cause of work-related asthma and rhinitis among hairdressers, who may be exposed when mixing and applying bleaching products. Rhinitis is inflammation inside the nose and can cause sneezing, congestion and irritation. The review also found evidence of breathing problems and reduced lung function. One study included in the review found that hairdressers exposed to bleaching powder had up to 20 times the risk of respiratory symptoms compared with people who were not exposed. Experts say persulfates can sensitise the skin, making some people more likely to develop allergic contact dermatitis, which causes itching, redness and inflammation. Methylisothiazolinone in shampoo and skin Some shampoo ingredients can cause skin reactions, particularly in people who are sensitive to them. A 2022 systematic review of 99 studies involving 3,185 patients found that shampoos accounted for 28 percent of reported cases of allergic contact dermatitis associated with hair and scalp products. The review identified methylchloroisothiazolinone and methylisothiazolinone, preservatives used to prevent the growth of bacteria and fungi, among ingredients associated with reactions. The review also identified fragrances and cocamidopropyl betaine, a cleansing ingredient used in some shampoos. Sodium lauryl sulfate, which is used to remove oil and dirt and create foam, can cause dryness and irritation in some people. High heat and damaged hair The products used in salons are not the only source of potential damage. Repeated exposure to high heat can weaken hair. A 2011 study published in “Annals of Dermatology” found that repeated heat exposure damaged the hair cuticle, the outer layer that protects the hair shaft. The damage became greater as temperature increased, with very high heat causing cracks and lifting of the cuticle. Researchers also found that holding a hair dryer about 15 centimetres from the hair and keeping it moving caused less damage than concentrating the heat on one section. A 2025 study found further structural changes in hair following prolonged heat exposure. Repeated high-temperature drying can therefore leave hair weaker and more prone to breakage, especially when the dryer is held too close or in one place for too long.
