Not just splitting hairs—monitoring environmental chemical exposures, their biological effects and their social determinants with hair shafts and follicles: a narrative review

Humans encounter a wide range of natural and synthetic chemicals throughout their lifespan and these exposures have a complex impact on health outcomes. Epidemiological studies provide compelling evidence linking chemical exposure to neurological disorders and various cancers. However, establishing causality between specific pollutants, or their mixtures, and health outcomes is challenging due to long disease latencies. Functional biomarkers in exposed subjects provide rapid predictions of the impact of environmental exposures. Notably, genome alterations, including DNA breaks and epigenetic modifications, are hallmark features of cancers, premature aging, and neurological diseases. Biomonitoring studies show that individual chemical exposures are specific. These differences highlight disparities across socioeconomic groups. They also present opportunities to associate specific environmental factors and disease development, as well as socioeconomic determinants of exposures and their consequences. This review focuses on biomonitoring environmental chemical exposures and their effects with hair and hair follicles. Pollutants detected in the hair matrix reflect cumulative internal exposures over specific time periods, while biomarkers can be analyzed in plucked hair follicles. These mini organs contain proliferating and stem/progenitor cells, which are critical to understanding the onset of various diseases. We propose an integrated approach that combines socioeconomic indicators, chemical analyses in hair, and biological assessments of matching hair follicles to evaluate the exposome and its effects on the genome. We discuss the state of the art of these approaches, their current limitations, and the perspectives they offer.