In the present community-based study, we screened for the existence and distribution of HMs in the serum of Popokabaka's children. Arsenic was detected in almost all children (95.6%), of which more than half (59.7%) had quantifiable values. Mercury was detected in 66.0% of children, with a few (8.2%) having quantified values. Lead and cadmium had relatively low detection rates of 13.1% and 19.4%, respectively. Arsenic was negatively related to Zn, while mercury was positively related to selenium levels, both statistically and geographically. These findings reveal the existence of environmental exposure that must be discussed.
Since the arsenicosis crisis report from Bangladesh 31 due to As contamination in drinking water, there has been an increasing public health interest in this metal. Recently, two papers 32,33 reported that in Africa, As is spatially abundant in water, soil, sediment, fish, and vegetation and advocated for human exposure and health effect assessment. Accordingly, our results suggest that As is widespread in Popokabaka and that communities would be exposed to a permanent source. For example, it is easy to understand that, as in any rural conditions, untreated contaminated water used for drinking, cooking, and irrigating crops may be an important source. Consumption of contaminated seafood also results in elevated As concentrations. The literature 34 supports that predatory fishes such as shellfish, sea mammals, and other predatory fish (shark, swordfish, mackerel, tilefish from the ocean) increase the As level. However, as shown in Table 2, the consumption of fish and sea food in Popokabaka is quite low. The inaccessibility of this area excludes any importation source of such foods. Arsenic exists in different forms in nature, toxic in acute and poisoning conditions (artsenate, arsenite), nontoxic when metabolized by the body (monomethylarsine, dimethylarsine), and nontoxic in food (arsenobetaine, arsenocholine). Inorganic As is the most lethal carcinogen. However, we have reported in this study total arsenic that need complementary analysis to specify the form.
Geospatial analysis has shown that the As high spot was located in children north of Popokabaka and on the eastern side of the Kwango River. This may also confirm the As variability in the soil of Popokabaka. In addition, arsenic was highly quantified in children's blood with more profound Zn deficiency. This interaction between As and Zn is supported in the literature. Kader et al.35 reported a chelation interaction of the two minerals in the soil. The authors concluded that Zn uptake in plants was significantly reduced in As-containing soils. Arsenic was found to be highly concentrated along the Kwango River. The washout rain effect from highly situated villages to those close to the river valley should be considered.
Lead, Hg and Cd usually share the same geogenic or industrial sources and have been more frequently reported from mining or rural regions23, 25–27,36,37. They leach from geogenic granite rock or industrial pollution and enter the food chain by contamination. Popokabaka is known as a nonmining region. The low proportion of quantification we have found may find a justification. However, Hg detection may be of interest. Further exploration may continue.
The literature supports inverse relationships between these three metals and children's IQ and growth. Our study found no relationship between the growth and detection of HMs. A review from Lancet 38 pointed out that the relationship between coexposure to multiple metals and increased neurotoxicity leads to a decline in neurocognitive development during early life.
We assessed the four HMs in blood under fasting conditions (> 8 hours from the last meals). Studies have reported that these four metals have blood concentrations elevated only for a short time after ingestion (4–6 hours). They are rapidly metabolized by the liver, accumulate in specific tissues (keratins), and are excreted by the kidney. Under this condition, the HM levels that we reported could be underestimated. Additionally, it was not possible to capture the chronic exposure and accumulation history. Studies that use matrices such as nails, hair, and urine should complement such pieces of information. The selection of a spatial interpolation method may impact the distribution map of HMs and their associated minerals. Sophisticated spatial interpolation methods such as kriggring could have been applied because they provide the best linear unbiased estimates and highlight local variations 39. However, it was very complex to fit the semivariogram, probably due to the distribution of sample points (rand cluster). The inverse distance seems to suit this study and its outcomes kriging 39 in some circumstances. In addition, the result of the bivariate spatial association may also be affected by the number of neighbourhoods, which is set to four (standard) in this study.
Despite these limitations, we have reported a picture of heavy metals in a representative rural community for the first time in DRC. Most of the studies are hospital-based and low-scale. The lab analysis method used in this study, ICP–MS, is the most accurate and indicated in the case of trace elements. Statistical analysis was confirmed and completed by advanced geospatial techniques to better describe the distribution and variability of these HMs.