Geochemistry. Группа авторов
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Rare earth elements are often excluded in literature on the geochemistry and medical geology of serpentinitic geological systems. Exceptions are cases where they are used as geochemical or environmental tracers [13]. Yet, evidence shows that rare earth elements may undergo enrichment in serpentinites such as peridotite and dunite, as well as their ultramafic protoliths [32, 33]. The enrichment of rare earth elements have been attributed to various processes occurring during the hydrothermal transformation [32, 34]. First, a decline in the solubility of rare earth elements is attributed to a rise in the pH of hydrothermal solutions during the transition from acidic to alkaline conditions after interacting with alkaline ultramafic rocks. Second, the interactions between the ultramafic rocks and hydrothermal fluids/melts and sea water rich in rare earth elements promote enrichment of rare earth elements. Therefore, rare earth elements may occur in high concentrations in serpentinitic geological systems, where they may undergo simultaneous release and dissemination with other toxic contaminants. The dissemination of rare earth elements may occur via contaminated sediments, and solid wastes and wastewaters from mining and mineral processing [4, 10, 35].
1.3 Human Exposure Pathways
1.3.1 Occupational Exposure
Toxic contaminants in serpentinitic geological systems may occur in various environmental compartments, including soils, wild plants, crops, and animals. Once in these environmental compartments, toxic contaminants may enter the human body via occupational and non-occupational exposure [4]. Occupational exposure to toxic contaminants may occur via inhalation in industrial production systems [36]. Typical industries promoting occupation exposure are: (1) mining and mineral processing, including quarries; (2) production of frictional materials such as brake pads, textiles, gas masks, cement, and asbestos; (3) agriculture; (4) construction; and (5) sculpturing, engraving, and carving [4, 37, 38]. Occupational exposure to chrysotile asbestos has been linked to human health risks such as kidney and ovarian cancers, respiratory diseases, and mesothelioma [38].
1.3.2 Non-Occupational Exposure Routes
1.3.2.1 Inhalation of Contaminated Particulates
Non-occupational exposure may occur via inhalation of contaminated air-borne particulates among populations living close to mining, construction, and agricultural activities [4]. For example, several abandoned chrysotile mines and waste dumps in Australia [39], South Africa [40], and Zimbabwe [41, 42], posing non-occupational exposure risks to communities. Non-occupational exposure may also occur via the transfer 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