Occupational Exposure to Resorcinol and Thyroid-Disrupting Effects: Protocol for an Exploratory Field Study in French Hairdressers.

Background: All around the world, the hairdressing sector constitutes a major occupational group, including about 90% women, most of whom are of reproductive age. Hairdressers are continuously exposed to numerous chemicals used in hair products, including endocrine-disrupting compounds such as resorcinol, parabens, phthalates, and UV filters. Few biomonitoring studies have explored occupational exposure to endocrine disruptors in hairdressers, and no data were found on their impact on the thyroid hormone system. Resorcinol is an oxidative hair dye with thyroid-disrupting properties that decrease thyroid hormone synthesis and could alter neurodevelopmental functions during fetal and perinatal stages in case of maternal exposure. Objective: This study aims to assess the occupational exposure to resorcinol in French hairdressers and analyze the relationship with biological thyroid parameters, taking into account the occupational exposure to other potential thyroid disruptors (parabens and UV filters like benzophenone and cinnamates). Methods: An exposed-unexposed cross-sectional study is proposed involving female hairdressers aged 18 to 45 years (working in hair salons) compared to occupationally unexposed controls (employed in office activities), who are recruited within 14 French occupational health centers. The hairdressers are followed during a 5-day working week to assess exposure data at both the individual level and the salon level. Urinary samples for the measurement of thyroid disruptors (resorcinol, parabens, metabolites of ethylhexyl methoxycinnamate, and benzophenone-3) are collected at 6 time points (before the day 1 shift, before and after the day 3 and day 4 shifts, and before the day 5 shift). Daily work tasks and use of hair products are self-reported within the workplace, and a complete inventory of hair products within the salon is carried out. Thyroid disruption effects are assessed by measuring blood thyroid parameters: triiodothyronine, thyroxine, thyroid-stimulating hormone, thyroglobulin, thyroperoxidase, and thyroglobulin antibodies. To assess nonoccupational exposure to thyroid disruptors and other confounding factors, information on sociodemographic data, place of residence, food and tobacco consumption, personal use of care products, professional career, and medical history is collected through questionnaires. Regarding statistical analysis, urinary samples from hairdressers and controls will be compared, and adjusted multivariable models will be used to analyze health outcomes. Results: The study duration extends from 2022 to 2027. As of December 2025, 9 occupational health centers have enrolled 66 hairdressers (employed in 54 hair salons) and 30 occupationally unexposed participants. Conclusions: The results will represent the first data on occupational exposure to resorcinol in France and its relationship with thyroid hormones in hairdressers. Following a multidisciplinary approach that includes biomonitoring, epidemiology, and exposure data collection at both the hairdressers and salon levels, this study enables an in-depth assessment of exposure to the thyroid disruptors in the workplace. Together with the inventory of hair products, these results may enhance the tools for chemical risk assessment and prevention in hair salons.