Comparative evaluation of hair, fingernails, and toenails as biomarkers of fluoride exposure: A cross-sectional study

Background: The increased prevalence of fluorosis has led to a search for biomarkers of fluoride exposure. Among the biomarkers of sub-chronic exposure to fluoride, hair, fingernails, and toenails have the advantage of being noninvasively collected, easily transported, and stored. Objective: The objective of this study was to comparatively evaluate coronal hair, fingernails, and toenails as biomarkers of fluoride exposure from drinking water; the study was designed as a population-based observational cross-sectional study. Materials and Methods: A population-based observational cross-sectional study was conducted in 60 children (20 subjects per group) of ages 12-17 years in three villages of Nilakottai block, Dindigul district, Tamil Nadu, India (Thomaspuram, Bangalapatti, and Singampatti). The fluoride concentration in the household drinking water was analyzed and compared with the fluoride content in the coronal hair, fingernail, and toenail clippings, which was estimated by potentiometric method (fluoride-ion-selective electrode) and expressed in ppm (parts per million). A two-tailed probability value of P < 0.05 was considered significant. Results: The mean fluoride concentration in drinking water was 0.63ppm in Thomaspuram, 1.63ppm in Bangalapatti, and 2.92ppm in Singampatti. The mean fluoride content in hair samples was 2.84ppm, 4.67ppm, and 6.53ppm; fingernail clippings was 2.99ppm, 4.94ppm, and 6.84ppm; and toenail clippings was estimated as 3.13ppm, 5.10ppm, and 7.24ppm in Thomaspuram, Bangalapatti, and Singampatti residents, respectively. The mean fluoride content in the hair, fingernails, and toenails was significantly higher as compared to the mean fluoride content in the drinking water (viz., toenail fluoride > fingernail fluoride > hair fluoride). Conclusion: Coronal hair, fingernails, and toenails are useful biomarkers for both sub-chronic and chronic fluoride exposure from drinking water. Due to ample sample availability and the highest fluoride content, toenails are the most suitable biomarkers of fluoride exposure from drinking water.