It is understood that the route of exposure from soil-to-human can lead to human health problems. However, different management regimes in this sub-region result in a mixed pattern of exposure. The following are some well-documented examples of how soil contaminants may be responsible for the deterioration of human health in this region.
Arsenicosis is considered a “water-related disease” by the World Health Organization because the main route of human exposure to arsenic is through the consumption of groundwater (WHO, 2020). However, arsenic in soils in paddy fields and industrial and mining areas (see section 6.2.1 and 6.3.1), cannot be excluded as a source of human exposure based on the bioavailability of arsenic from soil to humans (Rodrigues and Römkens, 2018).
There are a few case studies of arsenic poisoning of residents of Hyderabad, India (Figure 16). Industrial sites were responsible for soil, groundwater and food pollution. Chandra Sekhar et al. (2003) studied the risks associated with various pathways of exposure to arsenic in 14 villages in the Pathancheru area. Samples of water, soil, fodder, milk and vegetable were collected for analysis. Soil arsenic concentrations ranged from 0.87 mg/kg to 12.8 mg/kg, with mobile fractions of arsenic ranging from 4.6 percent to 10.1 percent between sites. Although the concentration of arsenic in soil is low, bioaccumulation by crops grown on these soils is of great concern. Overall, concentrations of arsenic were higher in human samples (blood, milk, hair, nail, etc.) from those who had resided at contaminated sites; however, many other factors come into play, including nutritional and economic status. Older people (> 40 years of age) were reported to be more poisoned by arsenic, possibly due to long-term exposure.
Although selenium is an important dietary component for humans, overexposure causes selenosis (Chawla et al., 2019). Cases of selenosis have been identified in the Asia–Pacific region, such as in India and China (Chawla et al., 2019; Long and Luo, 2017). However, it should be noted that human selenosis from environmental origins is rare but is caused by the ingestion of selenium from food (Plant et al., 2003).
Human selenosis has been reported by Chawla et al. (2019) in Punjab, India, where repeated deposition of selenium-rich sediments has resulted in a seleniferous area. In the surface soil (0-24 cm), total selenium was 1.4-3.2 mg/kg, while the sub-surface soil contained 0.3-1.7 mg/kg of selenium. Selenium in blood and hair was detected and was found to be an exposure biomarker for selenium toxicity. Although this study is not comprehensive and did not conclude an epidemiology of selenosis, there may be links to the consumption of selenium from locally grown foods by the human population.
Cui et al. (2017), in a study of seleniferous area of Shuang’an in Ziyang, China, reported the incidence of human selenosis. One severely polluted site was the village of Naore. The soil contained selenium in the concentration range 0.21-36.1 mg/kg, and 60 percent of the soils studied exceeded the selenium toxicity threshold (>3 mg/kg). Plants, such as eggplants and garlic, grown in the polluted area bioaccumulated selenium at concentrations ranging from 0.02 mg/kg to 17 mg/kg, with 40 percent exceeding the toxicity standard (1 mg/kg). Among the human population of the village of Naore, the concentration of selenium in blood was reported as 2.8 mg/litre; furthermore, 78.8 percent of the population exceeded the toxicity threshold of selenium in hair (>3 mg/kg). Daily intake of selenium from locally grown vegetables could be a source of toxicity.
Inhalation of lead-contaminated dust can reduce children’s intellectual abilities through short- and long-term cognitive impairment (Bihaqi and Zawia, 2013; Dong et al., 2015). For example, studies of children near Australian lead mining sites have detected negative impacts on the intellectual abilities of school children (Dong et al., 2015). Soils around the mining premises contained approximately 8 900 mg/kg of lead. The survey showed that the primary school located near the contaminated premises performed poorly compared to a school located away from the mine (Dong et al., 2015).
Children’s exposure to lead may not affect their health immediately, but it may trigger the development of cognitive disease later in life (Bihaqi and Zawia, 2013; Masoud et al., 2016). Research indicates that various levels of lead exposure could induce the expression of genes attributed to Alzheimer’s disease, according to experiments using mice (Bihaqi et al., 2014; Masoud et al., 2016; Wright et al., 2018), a primate model (Bihaqi and Zawia, 2013), and human-derived cells (Bihaqi, Eid and Zawia, 2017).
Cadmium is another element to which humans could be exposed through contact with contaminated soils and the consumption of grains grown in these soils (Wang et al., 2017). In a study of rice and vegetable samples and their soils of origin in Xiangtan, China, Chen et al., (2018) reported that the non-carcinogenic health impact of cadmium exposed via rice and vegetables was significant. Observing that among the total sample studied, 87.5 percent of the rice sample and 28.9 percent of the vegetable sample exceeded the Chinese environmental standard soil quality limit. These two cases were positively correlated with the cadmium concentration in the soil (81.9 percent exceeded the national standard threshold). In particular, children (4-11 years of age) were found to be more vulnerable to the health risk posed by cadmium, as identified by the Hazardous Quotient (HQ, where >1 of HQ indicates a health risk) (Chen et al., 2018). In a metal recycling village in Bac Ninh province in Viet Nam, cadmium potentially from soil and rice was also reported as the non-carcinogenic health hazard (Minh et al., 2012). The study utilized the Mean Estimated Dietary Intake (MEDT) tool and found that the HQ value was greater than 1 in more than 80 percent of the studied population, while the value was <1 in more than 85 percent of the population residing outside metal recycling areas. The study concluded that the cadmium, contained in the rice grains that the local people consumed the most, was the factor causing a high health hazard to the local population. Although, the study was limited by the fact that other trace elements reported from these recycling areas, such as lead, were not included in the HQ calculation, the cadmium concentration in the soils in this region would have been high. The exact value is not known for soils, but more than 20 percent of the rice samples contained cadmium concentrations above the accepted limit of 0.4 mg/kg, which could be due to contamination from pigmented metals and plastic substances (Minh et al., 2012).
Trace elements-linked cancer risk (CR) has been most reported in the Asia–Pacific region (Deng et al., 2019; Fei et al., 2018; Sharma, Nagpal and Kaur, 2018). Sharma, Nagpal and Kaur (2018) studied the content of various trace elements (cadmium, chromium, cobalt, copper) in soils and food crops grown in Punjab, India, and reported that daily intake of food crops grown in the study area could lead to an intake of chromium in the body with a cancer risk of 9/1000 cases. The calculated cancer risk was also reported to be high due to exposure to cadmium in the city of Xiangtan, China. This study reported that the total cancer risk from the exposure to cadmium was 1.7 × 10-4 (Deng et al., 2019). The ingestion pathway via crops and vegetable consumption was the largest contributor (CR of 1.69 × 10-4). Recent studies by Fei et al. (2018) of 9 378 newly-diagnosed stomach cancers revealed that an elevated risk of cancer in humans may be caused by a mixture of trace elements. Similar and more direct evidence emerged when a “cancer village” cluster was identified in Guangdong Province of China, where soil polluted with mainly trace elements was linked to cancer incidence (Guiford, 2013).
Pearce, Dowling and Sim (2012) reported that arsenic levels in soils at the former mine site in Victoria, Australia, were recorded in the range of 1.4 mg/kg to 1 857 mg/kg. The results indicated that exposure to arsenic in soil was linked to an increased number of cancer cases in individuals, particularly those residing in socio-economically disadvantaged areas.
In Myanmar, villagers living near mines may be exposed to toxic elements (e.g. arsenic and lead) leached from mine waste. Considering the soil pollution levels from a mining area in the Tanintharyi region as a case study (see Table 5), the three main scenarios of exposure to trace elements are 1) working outdoor on contaminated farmland, 2) working on contaminated sediment and surface water (such as in artisanal or small-scale miner), and 3) ingesting contaminated drinking water. In the case of soil pollution, using the outdoor work scenario for agricultural activity, the risk of exposure to trace elements resulting from working in unaffected soil using the US-EPA’s Risk Calculator is acceptable (i.e. excess carcinogenic risk <1x10-6 and non-carcinogenic hazard index (HI) <1). Nevertheless, conducting agricultural activity on mine waste deposited on farmland may result in an unacceptable risk (i.e. excess carcinogenic risk >1x10-6 for affected land). Therefore, deposited mining waste should be removed from affected farmland for health protection purposes (Phenrat, 2020).
Soil organic contaminants, including PAHs, VOCs, POPs and emerging organic contaminants, are reported to be risk factors for both non-carcinogenic and carcinogenic diseases in human (Ali et al., 2020; Cao et al., 2019; Sunderland et al., 2019). However, their bioavailability in soil and the exposure route from soil to humans are determinants of health risk (Semple, Morriss and Paton, 2003). Below, a few examples from published research are used to showcase the health impacts of frequently reported soil organic contaminants in various parts of the Asia Pacific region. However, there is little research that reports human health risk factor associated with soil-bound contaminants.
Regarding non-carcinogenic health risk, endocrine disrupting chemical phthalate esters (PAEs) have been detected in soil (mean 0.2 mg/kg) and in locally grown vegetables (0.54 mg/kg) in the Yangtze River Delta region of China (Wei et al., 2020). This study expressed health by estimating HI (i.e., the sum of HQs) and indicated that the risk was only 6.5 percent and 7.8 percent for adults and children, respectively. Because the risk was low, the study warned that continued exposure to PAEs through soil could increase the risk factor in the study region (Wei et al., 2020). In assessing 123 agricultural soils from 31 provinces in China, Niu et al. (2014) reported that, although the concentration of one group of PAEs was 0.075-6.4 mg/kg, the HQ was below 1, indicating an acceptable health risk. However, in another study conducted in the suburbs of Nanjing, China, Ma et al. (2015) indicated that although there is no health risk to adults, children (<6 years old) were found to be more at risk from exposure to types of PAE, such as bis(2-ethylhexyl) phthalate primarily through soil ingestion.
In many countries in the Asia–Pacific region, agricultural practice with traditional systems requires that farmers and laborers come into direct contact with the soil where these compounds may have resided. Although cultivation and ploughing practices may have been mechanized, the legacy of pesticides use and its subsequent transfer from the soil, could lead to chronic diseases such as diabetes. Among the wide range of chemicals in the environment, pesticides, bisphenol A and other POPs have been reported to be causative factors for diabetes (Jeon et al., 2015). A study conducted in Thailand on the potential link between direct or indirect soil pollution and diabetes is reported by Juntarawijit and Juntarawijit (2018). The target group of farmers was selected in Bang Rakam District, Phitsanulok Province, Thailand. This case study was conducted with 866 participants with diabetes, while an additional 1 021 healthy persons were included as controls. After adjusting for gender, age, smoking, alcohol and family history, the study found that pesticides, including organochlorine, organophosphate, carbamate and benlate, were responsible for diabetes in farmers who had been exposed to pesticides throughout their lives (Juntarawijit and Juntarawijit, 2018).
The prevalence of chronic kidney disease of unknown aetiology (CKDu) in the North Central province of Sri Lanka is suspected to be related to over-application of agrochemicals in crop fields (Kulathunga et al., 2019). The disease, first recognized in the mid-1990s in the Anuradhapura and Polonnaruwa districts of Sri Lanka (Ranasinghe and Ranasinghe, 2015; Wimalawansa, 2014), has now spread to some villages in the Central, Northern, Eastern and North-Western provinces (Wijetunge et al., 2015) of the country. The victims are mainly paddy rice farmers, and the number of male patients is higher than that of female (Jayasumana et al., 2015; Mendis, 2011; Wijetunge et al., 2015). The number of patients is increasing rapidly, with CKDu incidences doubling every four years (Wimalawansa, 2014). Many scientists postulate that this localized incidence of the disease was related to exposure to salt water and trace element(loid)-contaminated irrigated runoff from crop fields (Bandara et al., 2008).
In another recent study, Ali et al. (2020) assessed 30 soil sites near the OCPs destruction facility in Pakistan and reported that the calculated risk to human health was within the acceptable limit (HQ <1). In this case, the residual soil concentration of the sum of the OCPs was found to be 0.036-0.567 mg/kg. However, it should be considered that repeated exposure and bioaccumulation of these OCPs could increase this health risk (Ali et al., 2020).
Regarding carcinogenic health impacts, PAHs and pesticides are reported to be important soil contaminants in the Asia–Pacific region (Jia et al., 2017; Qu et al., 2015; Singh and Agarwal, 2018). Based on research published between 1990 to 2017, Singh and Agarwal (2018) reported that the presence of PAHs in many food items such as oilseeds or grains is related to PAHs in soil and could lead to higher cancer risks for consumers. Jia et al. (2017) studied the human health risk from PAHs associated with soil near the premises of a coking plant in Beijing, China, and reported that the cancer risk could range from 5.92 × 10-3 to 4.9 × 10-2, which is well above the acceptable limit of 1 × 10-6. The authors further determined that soil-to-human exposure occurred via the oral intake, contributing 99.3 percent of the risk. However, while this risk was significant within 100 m of the coking plant, it decreased with increasing distance (Jia et al., 2017).
Another case study on the cancer risk associated with exposure to agricultural pesticides was reported by Qu et al. (2015). In assessing the agricultural soil in Ningde, southeast China, the authors concluded that organochlorine pesticides such as DDTs, HCHs and endosulfans were present in large quantities in the soil and that this could lead to a high cancer risk, particularly for the farmer and for people working on cropland in this area. Tang et al. (2014) reported a case study on the link between cancer and DDT in Jiaxing of Zhejiang, China, after comparing 78 females with breast cancer and a similar number in a control group. Dietary ingestion of DDT from soil-to-food might be linked, as the average daily intake of p,pʹ-DDE (the main bioactive constituent in DDT) was estimated to be 0.34 μg/kg. Based on the population attributable fraction estimated with a median value of 0.6 percent (interquartile range 0.23–2.11 percent), the annual excess incidence rate of breast cancer attributable to DDE exposure averaged 0.06 × 10−5 with significant spatial variations ranging from 0.00021 × 10−5 to 11.05 × 10−5 in the sampled women. Exposure to DDT was observed to be a contributing factor to breast cancer, although the low risk factor and limited representations of participation warrants further detailed investigation.
As mentioned in previous sections, nuclear weapons testing in the Pacific has had a significant impact on the health of local populations. Between 1946 and 1958, there were 66 nuclear detonations on the Enewetak and Bikini atolls in the Marshall Islands (US Department of Energy, 1978). The population of Alilinginae and Rongelap was evacuated (80-100 km east of Bikini) and could not return until three years later. The main routes of transfer of radioactivity were horizontal distribution by wind, precipitation, runoff and wash-overs, and vertical distribution by percolation and absorption in the soil, contaminating food and drinking water sources. Contaminated food and water, as well as air, caused internal exposure to ionizing radiation. The immediate effects of radioactivity, such as exposure to gamma rays emitted by radioactive fallout, included skin burns, gastrointestinal problems, hair loss, and blood complications. Absorption of radioactive elements from food, water and air led to longer term cases of thyroid nodules and fatalities due to thyroid and stomach cancer and leukaemia, which also affected the second generation (US Department of Energy, 1978). However, there is considerable uncertainty regarding future health impacts and the safety of nuclear weapon test sites for local populations. Other nuclear weapon test sites in the Pacific include Johnson Atoll (USA), Christmas Island (Australia), Maralinga (Australia) and Mururoa (French Polynesia).
The use of Agent Orange containing polychlorinated dibenzo-p-dioxins (PCDDs) during the Viet Nam War between 1962 and 1971 reportedly resulted in a non-carcinogenic and carcinogenic health risk to local civilians and military personnel involved in the war (Mossanen, Kibel and Goldman, 2017; Stellman and Stellman, 2018; Yi et al., 2014). However, the health risk associated with PCDDs pollution from Agent Orange and their bioavailability in soil has only been investigated in a very small number of studies due to technical difficulties. In a systematic study conducted between 1996 and 1999, Dwernychuk et al. (2002) reported that the Aluoi Valley region of Viet Nam could be considered as a reservoir of the most toxic PCDD isomer, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), that was associated with Agent Orange. The authors established an exposure pathway for TCDD from contaminants in the soil through to the food chain. However, it was not clear from this study whether soil contaminated with TCDD was the only source that posed a risk to human health (Dwernychuk et al., 2002). In another study, Banout et al. (2014) reported that although the low concentration of PCDD was found in soil (0.05-5.1 ng/kg TEQ (Toxic Equivalents), bioaccumulation of Agent Orange-related PCDD from soil to food items could be linked to an increased risk of cancer and non-cancerous diseases in the local population. The authors estimated the HI to be 13.3 to 17.7 for the non-carcinogenic risk from food production. Using the same sources of exposure, it was further established that the carcinogenic risk could be in the range of 28 to 150 individuals per one million people (Banout et al., 2014).