Geochemistry. Группа авторов

oxygen radicals that cause severe oxidative stress [4, 66].

Controversy and misconceptions exist with respect to the toxicity of chrysotile asbestos. Some studies suggest that chrysotile is less biopersistent, thus less toxic and safer than other types of asbestos such as amphiboles [67–69]. These sentiments are largely driven by sectoral interests, and are meant to promote the production and use of asbestos [4]. For example, one of the studies claiming that chrysotile is non-toxic was funded by the Asbestos Institute, Montréal, Canada, and the Government of Québec, raising potential conflict of interest [70]. Moreover, a series of studies by Bernstein and co-workers were conducted for short exposure periods (i.e., 1 year) [67, 70, 71]. Such periods are far lower than the latent period of 40 years required between time of exposure and expression of human health outcomes [36]. In Zimbabwe, where large chrysotile deposits exist in Shabani-Mashava Mine in Zvishavane [72], the Minerals Marketing Corporation of Zimbabwe claim that “If inhaled, chrysotile or white asbestos is moved from the lung while the amphiboles persist. Therefore it is the blue asbestos, amphiboles that causes asbestosis, lung cancer etc., not chrysotile asbestos.” (http://www.mmcz.co.zw/products/industrial-minerals/). This claim is in total disregard of the evidence from Zimbabwe showing profound human health risks associated with exposure to chrysotile asbestos [4, 41, 42]. For example, Cullen and Baloyi [41] showed that several human health risks, including malignant mesothelioma, morbid asbestosis, non-malignant pleural disease, and lung cancer reported in other countries were also prevalent among workers in the chrysotile asbestos industry. The fact that chrysotile is carcinogenic just like amphibole asbestos is also shared by authoritative global health agencies, including the WHO and the International Agency for Research on Cancer [4, 73, 74]. The position of the WHO and International Agency for Research on Cancer is based on comprehensive reviews of recent literature comprising of about 100 studies.

       1.4.1.2 Toxic Metals

      Data tracing human health outcomes to toxic metals and rare earth elements in serpentinitic geological systems remain limited. Lacking that, the human health risks of toxic metals and rare earth elements are drawn from general literature on these elements [74]. For example, toxic metals including Cr, Co, Cu, Mn, Fe, and Zn are redox active; hence, they undergo redox reactions to generate highly reactive free radical species [4, 74]. Highly reactive free radicals cause oxidative stress, which damages lipid membranes and deoxyribonucleic acids [75, 76]. Other metals such as cobalt and its salts have been associated with genotoxicity and carcinogenicity [74]. Iron is often regarded as less toxic than other heavy metals partly because it tends to occur in high background concentrations especially in tropical environments [4]. However, high intake of iron may cause iron overload and human toxicity as reported in some native African communities with a high genetic predisposition for such a conditions [77, 78]. Thus, high iron intake in iron-rich foods, drinking water, and geophagic earths derived from serpentines may increase the risk of iron overload and toxicity. Moreover, the co-occurrence of chrysotile asbestos and toxic metals may result in synergistic interactions, which may result in adverse human health outcomes [4]. For example, evidence drawn from southern Italy suggest that the high incidences of lung cancer among human populations living in serpentinitic geological environments were related to the synergistic interactions between co-occurring asbestos and the toxic metals particularly Ni and Cr [21, 22].

       1.4.1.3 Rare Earth Elements

Rare earth elements are highly reactive elements with unique physicochemical properties [10]. In cases where the enrichment of rare earth elements occurs in serpentinitic geological environments, they are likely to be simultaneously taken up by humans together with other toxic contaminants. Evidence on the human health risks of rare earth elements derived from serpentinitic environments are scarce. However, the human health risks of rare earth elements are well-known, and have been reviewed in an earlier paper [10]. Examples of health risks include, damage to the human nephrological system, pneumoconiosis, dysfunctional neurological disorders, and anti-testicular effects and male sterility, among others [4, 10, 79, 80]. A detailed review of the health effects of rare earth elements were presented in earlier literature [10]. The bulk of the evidence on human health risks relate to the application of rare earth elements in agriculture, health care, and occupational exposure [10, 81]. By compassion, data relating to serpentinitic geological systems are scarce, highlighting the need for further research on this aspect.

      1.4.2 Mitigating Human Exposure and Health Risks

      Environmental and human health risk assessment and mitigation are required to safeguard human health against toxic contaminants in serpentinitic geological systems. A detailed discussion of the human health risk assessment protocol and mitigation for toxic contaminants in serpentinitic geological environments is presented in an earlier review [4]. Here, an overview of the risk analysis, evaluation, and mitigation, including specific interventions, is presented based on literature [4, 10].

       1.4.2.1 Risk Analysis

      This step entails the identification and characterization of the nature of human exposure and health risks. Key activities include (1) determining the concentrations of the toxic contaminants, (2) identifying the human population at risk, (3) determining exposure pathways, and (4) estimating daily intakes of toxic contaminants relative to permissible maximum guidelines.

       1.4.2.2 Risk Evaluation

      Risk evaluation involves the determination of the likelihood of occurrence and the associated human health consequences of exposure to toxic contaminants. A risk evaluation framework based on a combination of likelihood and consequences can be used to rank the risks as follows: “extremely high”, “high”, “moderate”, and “low/negligible”. Such a framework can be used to identify human health risks warranting mitigation and for prioritization and allocation of scarce resources.

       1.4.2.3 Risk Mitigation

      Risk mitigation involves identifying the mitigation interventions and evaluating their potential to address the human health risks, and in terms of feasibility and cost. This is then followed by the implementation of the mitigation strategy, and subsequent monitoring and evaluation and feedback to improve performance.

       1.4.2.4 Overview of Mitigation Interventions

      Several mitigation measures have been highlighted for safeguarding human health in serpentinitic geological environments [4]. These include (1) proper land use planning to avoid human settlement in hotspot areas, (2) the use of appropriate barriers and safety procedures (e.g., special respirators) to isolate humans from occupational exposure to toxic contaminants, (3) soil conservation practices and use of soil amendments such as biochar to reduce erosion, mobility, and dissemination of toxic contaminants [82, 83]; (4) restoration and stabilization of post-mining landscapes to reduce emissions of toxic contaminants [84], (5) the application of low-cost methods for the removal of toxic contaminants in drinking water sources such as biochar-based filters [85], and (6) in the case of chrysotile asbestos, the development and use of alternative substitutes which are non-toxic in order to ultimately stop the mining, processing, and application of chrysotile [4, 36]. In most cases, depending on the severity of the human exposure and health risks, a combination of several interventions may be required to safeguard human health.

1.5 Future Perspectives

      The medical geology of serpentines is an emerging topic of research that requires an integration of earth sciences, public health and environmental sciences. As pointed out in an earlier paper, further research is required to address several knowledge gaps [4]. Addressing these knowledge gaps is key to risk assessment and mitigation of the human health risks associated with serpentine geological systems.

      1 (1) Understanding the occurrence