A Prospective Study of Toenail Trace Element Levels and Risk of Skin Cancer.

BACKGROUND: Few epidemiologic studies have investigated trace element exposure and skin cancer risk.
METHODS: Toenail levels of mercury, selenium, chromium, iron, and zinc were measured from 6,708 women in the Nurses' Health Study (1984-2012) and 3,730 men in the Health Professionals Follow-up Study (1986-2012) with data from prior nested case-control studies. Participants were free of skin cancer at toenail collection and followed for incident basal cell carcinoma (BCC), squamous cell carcinoma (SCC), and melanoma. Cox proportional hazards models were used to compute hazard ratios (HR) and 95% confidence intervals (CI) of skin cancer associated with the elements in each study. We calculated pooled multivariable HRs using a fixed-effects model. During 26 to 28 years of follow-up, 2,433 BCC, 334 SCC, and 130 melanoma cases were documented.
RESULTS: Higher toenail mercury levels were associated with risk of BCC [pooled HR for top vs. bottom quintiles = 1.34 (95% CI, 1.18-1.52), P trend < 0.0001]. Similar direct associations were found with risks of SCC [pooled HR for top vs. bottom quartiles = 1.41 (95% CI, 1.03-1.94), P trend = 0.04] and melanoma [pooled HR for top vs. bottom quartiles = 1.88 (95% CI, 1.12-3.16), P trend = 0.02]. Chromium was positively associated with BCC in women only. No associations were found between other metals and skin cancer risk.
CONCLUSIONS: Our prospective data found that increased toenail mercury concentrations were associated with increased skin cancer risk. IMPACT: If our novel findings are confirmed, mercury may play a role in skin carcinogenesis. ©2019 American Association for Cancer Research.