Zulfiqar, HiraLlompart Vizoso, María del PilarCastiñeira Landeira, AnaDuque Villaverde, AndrésFabbri, Daniele2026-04-102026-04-102026-02-03Zulfiqar, H., Llompart, M., Castiñeira-Landeira, A., Duque-Villaverde, A., & Fabbri, D. (2026). Assessment of UV filter ethylhexyl methoxycinnamate photoisomerization in aquatic environments, cosmetics and in vitro skin by (micro)extraction GC–MS analysis. Microchemical Journal, 222, 117149. 10.1016/j.microc.2026.117149https://hdl.handle.net/10347/46646This work examines the photoinduced isomerization of the UV filter ethylhexyl methoxycinnamate (EHMC) in different real samples, including environmental waters, cosmetic products, and human skin. Water samples (river, sea, and swimming pool water) spiked at environmentally relevant concentrations were irradiated with UV light and analyzed by solid-phase microextraction (SPME) followed by GC–MS. Natural waters showed minimal total degradation of EHMC, reaching an (E/Z, trans/cis)-photostationary state with slight predominance of the Z isomer. In contrast, swimming pool water exhibited rapid EHMC loss, likely due to disinfectants, reducing concentration below 20% within 10 min. Cosmetic products (sunscreen cream, lip balm, and hair oil) were irradiated under the same UV conditions after application onto glass slides, and EHMC was extracted using vortex and ultrasound assisted extraction (UAE) followed by GC–MS. All samples exhibited formation of Z-EHMC, representing 30–50% of total EHMC at photostationary equilibrium, with minimal total degradation. Similar behavior was observed under natural sunlight, confirming that cosmetic products undergo photoinduced transformation under real conditions. To simulate human exposure, cosmetic products were applied to in vitro human skin and irradiated, again resulting in rapid formation of Z-EHMC. This isomer is known to be less effective as a sun protector and potentially more toxic. These findings demonstrate that EHMC consistently converts into its Z isomer across environmental waters, cosmetic products, and human skin, highlighting potential implications for sunscreen efficacy as well as environmental and human health risks. Additionally, the applied extraction and analytical methods proved to be suitable for monitoring EHMC in complex matrices.eng© 2026 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by- nc-nd/4.0/).http://creativecommons.org/licenses/by-nc-nd/4.0/Personal care productsWater analysisEHMC isomerizationUV filtersGC–MSSPMEPhotoisomerizationjournal article10.1016/j.microc.2026.1171491095-9149open access