The cancer burden in Sub-Saharan Africa is projected to double between 2008 and 2030 (McCormack and Schüz, 2012). According to the list of confirmed and possible carcinogens issued by the International Agency for Research on Cancer (IARC), some of the main soil contaminants in the region are categorized as cancer agents for which sufficient evidence is available (Group 1) (IARC, 2020). These include arsenic and inorganic arsenic compounds, cadmium and cadmium compounds, hexavalent chromium compounds, PCBs and PCDD/Fs. Contaminants that fall in Group 2A (probably carcinogenic) and Group 2B (possible carcinogenic) include lead and lead compounds as well as the pesticides, aldrin, DDT, dieldrin and lindane. Statistics on cancer prevalence and the links to environmental risk factors in the region are lacking. In 2013, it was reported that Mauritius and South Africa have been the only regional countries that provided mortality data to the WHO database (Fasinu and Orisakwe, 2013).
Humans in sub-Saharan Africa may have higher risks of exposure to soil pollution caused by mining than those in developed countries. Populations in close proximity to mining areas are often unaware of or ignore the risks associated with their cultivation of food crops on polluted soil. They also spend more time outside in closer proximity to the pollution (Eijsackers et al., 2014). Soil ingestion can also happen accidentally through the inhalation of dust or when soil-covered hands or other items are inserted into the mouth (Simon, 1998). A study on carcinogenic and non-carcinogenic health risks around the iron ore mines of the Kogi State in Nigeria identified that exposure to trace element contamination of soil by ingestion was greater than both dermal and inhalation (Aluko et al., 2018). A study in the Niger Delta area of Nigeria has indicated that although the concentrations of individual trace elements may be non-carcinogenic, continuous exposure to cadmium, hexavalent chromium, nickel and lead are cumulatively carcinogenic (Olawoyin, Oyewole and Grayson, 2012).
Case study: Health impacts of geophagy of polluted soil
Geophagy is the practice of eating soil or other earthy substances. Geophagy in sub-Saharan has been identified as a cause for higher exposure risk of humans to soil pollutants (Eijsackers et al., 2014). It is a practice that occurs globally but is most frequently observed in countries in sub-Saharan Africa. Soil for ingestion can be bought on regional markets as well as from ethnic markets in the United States of America, Belgium, United Kingdom of Great Britain and Northern Ireland, and Austria (Kambunga et al., 2019b). Although different reasons for this practice have been offered (Huebl et al., 2016), several publications have shown it to be a source trace elements that have negative health consequences (Kambunga et al., 2019b). Pregnant and breast-feeding women are especially at risk from exposure to microbial pathogens through the ingestion of soil (Kutalek et al., 2010). Consumption of polluted soil can harm the unborn baby by causing growth retardation and premature births (Kambunga et al., 2019b). A study conducted in South Africa by Mathee et al. (2014), confirmed that depressed haemoglobin levels in pregnant women are associated with geophagy.
Analysis of maternal and umbilical cord blood shows that unborn babies can be exposed to high concentrations of cadmium, lead, mercury and selenium that can be correlated with the environmental pollution in the areas where their mothers live (Röllin et al., 2009). The prevalence of geophagy during pregnancy was first summarized in a review of four studies (Kambunga et al., 2019b). These four studies identified that pregnant women are exposed to negative health impacts by voluntary ingestion of soil with high concentrations of aluminium, antimony, arsenic, iron, lead and manganese. The studies focussed on the trace element concentrations in soil and not on the trace element concentrations in human milk of the women practicing geophagy. The specific trace elements that pregnant women and their unborn babies are exposed to as a result of voluntarily soil ingestions, is summarised in Table 2.
According to global data, pesticides are widely used in sub-Saharan Africa (FAOSTAT, 2019). Data for some countries also show the continued agricultural use of pesticides that should be banned under the Stockholm Convention. Nigeria and the United Republic of Tanzania have reported such use. Gonzalez et al. (2005) have shown that organochlorine pesticide residue can hyperaccumulate in vegetables at levels four to forty five times higher than those in the soil in which it is grown. This was confirmed by a study in Togo that linked the presence of low contaminant concentrations in soil to the accumulation of lindane and alpha-endosulfan in carrots and peppers (Kanda et al., 2012).
In addition to pesticides, trace elements have been shown to enter the food system through the application of fertilizer to soil. An example of this is the high level of cadmium that were present in zinc-phosphate fertilizer that was applied unknowingly to pineapple fields in the Eastern Cape province of South Africa (Figure 11) (Hill, Fraser and Baiyegunhi, 2012a). Although the main economic loss was the rejection of the canned fruit on the international market, it was never determined what the human health implications of local fruit consumption was. Apart from consumption by communities around the pineapple fields, the non-exported fruit was also sold at nearby beaches to tourists.
While the regulations of the export market prevented exposure of its consumers in Europe to high cadmium levels in this instance, high concentrations of trace elements can be ingested by eating food produced in polluted soil that are not controlled by strict regulations. A study on trace elements in the blood of immigrants from region that now live in Spain, found extremely high levels of vanadium (Henríquez-Hernández et al., 2017). The source and pathway of this exposure was thought to be ingestion of food crops that had been cultivated on soil polluted with vanadium.
Soil pollution by petroleum products can reduce plant diversity by affecting the seed bank as well as natural plant succession. This is demonstrated by a study conducted on polluted soil around a petroleum storage facility in Nigeria (Akande, Ogunkunle and Ajayi, 2018). The results show that the seed bank and standing vegetation of the surrounding soil consists of only one species, Eleusine indica while the seed bank of unpolluted soil from a reference area consists of 17 species from 14 plant families. The study illustrates that, unless soil polluted by petroleum products is remediated, it will remain sparsely vegetated by only a very limited number of plant species that are tolerant of these contaminants. It also suggests that biodiversity will have to be re-introduced through ecological restoration techniques.
The iSimangaliso Wetland Park in South Africa is a biodiversity hotspot that consists of four internationally important wetlands that are listed under the Ramsar convention (Buah-Kwofie and Humphries, 2017a). Although this conservation area is of high importance, the concentrations of organochlorine pesticides in sediment sampled all over the park are the highest ever reported in South Africa and amongst some of the highest concentrations reported globally, ranging from 26.29 to 282.5 ng/g for HCHs and 34.49 to 262.4 ng/g for DDTs (Buah-Kwofie and Humphries, 2017b). The highest concentrations were attributed to agricultural use and IRS of malaria vectors. The presence of p,p’-DDT congener, HCH and drin-residues indicated a recent use, while the presence of endosulfan was attributed to historical application (Buah-Kwofie and Humphries, 2017a).