Toxic metals in toenails as biomarkers of exposure: A review

This work was supported by FIS grants PI12/00150, PI17CIII/00034, PI18/00287 (Instituto de Salud Carlos III, State Secretary of R + D + I and European Union (ERDF/ESF, "Investing in your future") ) , P42ES010349 and P30ES009089 (National Institute of Environmental Health Sciences) .Toenails have been used as biomarkers of exposure to toxic metals, but their validity for this purpose is not yet clear and might differ depending on the specific agent. To evaluate this issue, we reviewed the literature on: a) the time-window of exposure reflected by toenails; b) the reproducibility of toenail toxic-metal levels in repeated measures over time; c) their relationship with other biomarkers of exposure, and; d) their association with potential determinants (i.e. sociodemographic, anthropometric, or lifestyle characteristics) or with sources of exposure like diet or environmental pollution. Thus, we performed a systematic review, searching for articles that provided original data for levels of any of the following toxic metals in toenails: aluminum, beryllium, cadmium, chromium, mercury, nickel, lead, thallium and uranium. We identified 88 articles, reporting data from 67 different research projects, which were quite heterogeneous with regard to population profile, sample size and analytical technique. The most commonly studied metal was mercury. Concerning the time-window of exposure explored by toenails, some reports indicate that toenail cadmium, nickel and lead may reflect exposures that occurred 7–12 months before sampling. For repeated samples obtained 1–6 years apart, the range of intraindividual correlation coefficients of aluminum, chromium and mercury was 0.33–0.56. The correlation of toxic metal concentrations between toenails and other matrices was higher for hair and fingernails than for urine or blood. Mercury levels were consistently associated with fish intake, while other toxic metals were occasionally associated with specific sources (e.g. drinking water, place of residence, environmental pollution, and occupation). The most frequently evaluated health endpoints were cardiovascular diseases, cancer, and central nervous system diseases. Available data suggest that toenail mercury levels reflected long-term exposures and showed positive associations with fish intake. The lack of standardization in sample collection, quality control, analytical techniques and procedures – along with the heterogeneity and conflicting results among studies – mean it is still difficult to conclude that toenails are a good biomarker of exposure to toxic metals. Further studies are needed to draw solid conclusions about the suitability of toenails as biomarkers of exposure to toxic metals.FIS (Instituto de Salud Carlos III, State Secretary of R + D + I) PI12/00150 PI17CIII/00034 PI18/00287FIS (European Union (ERDF/ESF, "Investing in your future") PI12/00150 PI17CIII/00034 PI18/00287 United States Department of Health & Human Services National Institutes of Health (NIH) - USA NIH National Institute of Environmental Health Sciences (NIEHS) P42ES010349 P30ES00908

Toenails as biomarker of exposure to essential trace metals: A review

Health problems associated with essential trace metals can result from both inadequate (i.e., low intake) and excessive exposures (i.e., from environmental and/or occupational source). Thus, measuring the exposure level is a real challenge for epidemiologists. Among non-invasive biomarkers that intend to measure long-term exposure to essential trace metals, the toenail is probably the biological matrix with the greatest potential. This systematic review collects the current evidence regarding the validity of toenail clippings as exposure biomarker for trace metals such as boron, cobalt, copper, iron, manganese, molybdenum, selenium, silicon, vanadium and zinc. Special attention was paid to the time-window of exposure reflected by the toenail, the intraindividual variability in exposure levels over time in this matrix, and the relationship of toenail with other biomarkers, personal characteristics and environmental sources. Our search identified 139 papers, with selenium and zinc being the most studied elements. The variability among studies suggests that toenail levels may reflect different degrees of exposure and probably correspond to exposures occurred 3–12 months before sampling (i.e., for manganese/selenium). Few studies assessed the reproducibility of results over time and, for samples obtained 1–6 years apart, the correlation coefficient were between 0.26 and 0.66. Trace metal levels in toenails did not correlate well with those in the blood and urine and showed low-moderate correlation with those in the hair and fingernails.This work was supported by FIS grants PI12/00150, PI17CIII/00034 & PI18/00287 (Instituto de Salud Carlos III, State Secretary of R + D + I and European Union (ERDF/ESF, "Investing in your future"))

Investigating metal exposure on the general populace of the copper mining town of Kitwe, Zambia

Zambia’s economy is highly dependent on the copper mining industry for foreign export. It is well established that copper mining contributes to significant elevated metals in the environment, hence increasing the risk of metals exposure to human populations living in and around mining areas. Recent research has focused on the environmental impact of copper mining byproducts such as lead, cadmium, copper, cobalt and arsenic, found at elevated concentrations in the Copperbelt region of Zambia. Although the environmental metal concentrations are well known, the human health risks of exposure have not been fully addressed. Few studies have addressed metal exposure in communities adjoining mining activities in Zambia. The health impacts of increased metals exposure include: immune disturbance, gastrointestinal effects, various diseases such as: cancer, cardiovascular disease, neurological disease and decreased fertility in women. This study has investigated the relationship between environmental concentrations of metals and non-occupational human exposure and the most significant contributors to human exposure. A cross sectional human exposure study of 45 copper-mining town residents in Kitwe (exposed) and 48 non-mining town residents in Livingstone (unexposed) of similar socioeconomic and demographic characteristics aged between 20 and 30 years was undertaken to quantify metals exposure levels of Kitwe residents. This age group is most vulnerable to HIV/AIDS and is therefore at an increased risk of adverse health impacts from elevated metal concentrations. Metal concentrations were determined for arsenic, cadmium, cobalt, copper, lead, nickel, selenium and zinc in environmental (drinking water, indoor dust and soil) and biological (toenail) samples provided by participants. A questionnaire was also administered to establish potential factors influencing exposure including environment, behaviour and lifestyle factors. The results from this study suggest that residents in Kitwe are at an increased risk of metals exposure and consequently are also at an increased risk of being impacted by the adverse health effects associated with exposure. This study found elevated environmental and personal exposure concentrations shown to be comparable with, and in some cases higher, than other heavily industrialized regions. Metal concentrations were generally higher in Kitwe. Lead and arsenic in drinking water were most concerning having concentrations above acceptable health standards. Arsenic, copper and lead were elevated in soils. Copper and lead in dust were also high relative to other studies. The personal exposure concentrations (in toenails) most concerning were: cadmium, lead, copper and cobalt whose concentrations were either comparable or higher than studies reporting adverse health effects. Demographic and lifestyle characteristics did not show to be important contributors to exposure. This study found environmental metal concentrations to be the most important contributors to human exposure concentrations. The project has provided current information on personal exposure concentrations resulting from non occupational exposure to copper mining, as well as establishing background environmental and exposure concentrations which can be used as a reference for future studies. It is hoped that these findings will prompt further research into metals exposure and the associated health risks in order to provide a safer environment for Kitwe residents

Environmental and toenail metals concentrations in copper mining and non mining communities in Zambia

Copper mining contributes to increased concentrations of metals in the environment, thereby increasing the risk of metals exposure to populations living in and around mining areas. This study investigated environmental and toenail metals concentrations of non-occupational human exposure to metals in 39 copper-mining town residents and 47 non-mining town residents in Zambia. Elevated environmental concentrations were found in samples collected from the mining town residents. Toenail concentrations of cobalt (GM 1.39. mg/kg), copper (GM 132. mg/kg), lead (21.41. mg/kg) selenium (GM 0.38. mg/kg) and zinc (GM 113. mg/kg) were significantly higher in the mining area and these metals have previously been associated with copper mining. Residence in the mining area, drinking water, dust and soil metals concentrations were the most important contributors to toenail metals concentrations. Further work is required to establish the specific pathways of exposure and the health risks of elevated metals concentrations in the copper mining area