of toxic contaminants from clothes of workers to their family members.
1.3.2.2 Ingestion of Contaminated Geophagic Earths
The deliberate ingestion of geophagic earths such as clays and termite mounds, which is referred to as geophagy, may contribute to non-occupational exposure. Geophagy, which is practiced for various cultural and perceived health reasons, is common in several communities in Africa [10, 43]. For example, in Kenya, it is estimated that women consume 40 g per day of geophagic earths, contributing to iron intake of at least about nine times the maximum permissible daily intake [43]. High intake of toxic metals via geophagy has also been reported in other studies [44]. Although data pertaining to geophagy in serpentinitic geological systems are still missing, the intake of toxic contaminants could be higher in such environments compared to non-serpentinitic environments. This risk could be particularly higher among pregnant women and their unborn babies. This is because pregnant women have a high intake of geophagic earths, which is perceived to reduce anemia and nausea [45].
1.3.2.3 Ingestion of Contaminated Drinking Water
Toxic contaminants including chrysotile and toxic metals such as Cr and its highly toxic form Cr(VI) have been reported in aquatic systems in serpentinitic geological environments as early as the 1980s [46–48]. Cr(VI) exceeding the maximum permissible drinking water limit of 50 μg/L WHO [49] has also been detected in several aquatic systems, including groundwater, and surface water systems [50–52]. Thus, the consumption of untreated contaminated drinking water, a common practice in most developing countries may constitute a human exposure route to toxic contaminants.
1.3.2.4 Ingestion of Contaminated Medicinal Plants
Several herbal and medicinal plants have been reported to contain toxic metals exceeding permissible limits [4, 53]. For instance, a common medicinal plant (St. John’s Wort, Hypericum perforatum L.) growing on serpentinitic substrate had high concentrations of Cd, Ni, and Cr in dry plant material above the WHO permissible limits. Moreover, several medicinal and herbal plants in Africa (e.g., Senecio coronatus (Thunb.) and Datura metal L. (Solanaceae)) are known to be metallophytes and metal hyperaccumulators even under natural conditions [4, 54]. Hence, intake of herbal and medicinal plants constitutes a potential non-occupational exposure route especially for low income populations with limited access to modern health care.
1.3.2.5 Ingestion of Contaminated Wild Foods
Edible wild plants and animals foods such as mushrooms and honey harvested from serpentinitic geological environments have been reported to have high concentrations of toxic contaminants [4]. For example, wild edible mushroom species (e.g., Russula delica) harvested from serpentine had higher Cd, Cr, and Ni than those from volcanic sites [55]. Higher Al, Zn, and Pb were also observed in edible mushrooms in the Great Dyke (Zimbabwe), a well-known serpentinitic geological system than those from non-serpentinitic environment [56]. Wild honey harvested from the wild and apiaries in serpentinitic environments had high concentrations of toxic metals compared to that from the control [57, 58]. In Kosovo, the concentration of nickel in honey from serpentinitic flora (3.71 mg/kg) was twice that of the non-serpentine one (1.66 mg/kg) [58]. The same authors concluded that the high Ni in honey originated from Ni in dust from serpentine soils, and nectar collected by honeybees from Ni accumulating plants growing on serpentine soils.
As Gwenzi [4] pointed out, food crops, livestock products such as meat and milk, and edible rodents and insects derived from serpentinitic geological environments may also contain high concentrations of toxic contaminants. For example, paddy rice from serpentine soils had high total Ni concentration of 472 mg/kg and posed human health risks [59]. In Galicia (Spain), forage growing on serpentines accumulated Cr, Cu, and Ni, resulting in toxic concentrations of Ni in kidneys (1.296–1.765 mg/kg) and liver (257 mg/kg) [60]. In the same study, the concentrations of Ni and Cu in animal tissues were significantly correlated to concentrations in the soils and forage (r2 = 0.71–0.87). Insects and rodents occurring on metal contaminated environments have been reported to accumulate toxic contaminants such as metals [61–63]. Although data on toxic contaminants in edible insects and rodents on serpentinitic geological environments are still lacking, one may infer that such edible insects and rodents may also accumulate toxic contaminants [4]. Hence, the consumption of wild foods is a non-occupational exposure route for toxic contaminants.
1.4 Human Health Risks and Their Mitigation
1.4.1 Health Risks
1.4.1.1 Chrysotile Asbestos
The human health risks of chrysotile asbestos are the most documented among the three groups of toxic contaminants occurring in serpentinitic geological systems. Chrysotile is considered as a carcinogen and has been linked to incidences of human health conditions. High incidences of asbestosis, lung and ovarian cancers, and mesothelioma have been associated with chrysotile asbestos [4, 38]. Specifically, increased incidences of cancer in human populations inhabiting serpentinitic geological environments have been reported in Calabria in Southern Italy) [21, 22]. The toxicity mechanisms and carcinogenicity of chrysotile are quite complex and depends on the physico-chemical properties of the chrysotile [64, 65]. The toxicity mechanisms include (1) breakage of the deoxyribonucleic acid or gene structure and (2) the generation of highly reactive 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
