Worldwide, the main reason for decreased Hb synthesis in children is iron deficiency . Other causes include folic acid and/or vitamin B12 deficiency and thalassemia . In order to explore the effect of e-waste pollution on Hb synthesis in children, it is necessary to exclude these common nutritional and genetic factors. Out of this concern, normal levels of ferritin, folic acid, and vitamin B12 were required for our study, and there was no significant discrepancy in these levels between groups (P > 0.05). There is a relatively high incidence of thalassemia in Guangdong Province, as one study found an incidence of approximately 8.53% . In order to exclude any effect of thalassemia, we confirmed that no children who participated in our study had thalassemia by Hb electrophoresis screening.
It was necessary to understand the general characteristics (including social demography and cultural information) and nutritional status of children in the two regions. Families with higher maternal education may have better economic resources or engage in behaviours protective of child health, such as iron supplementation. The results showed that there was no significant difference in age, sex, family income, or parents’ education level distribution between the two study groups (P > 0.05). This means that families in both places were unlikely to have differences in the rate of insufficient dietary intake due to economic difficulties.
As shown in Table 2, the average BLL in the exposed group was higher than that in the control group (P < 0.01). In addition, the rate of children with lead poisoning in the exposed group was significantly higher than that in the control group (P < 0.01). These results indicate that e-waste pollution had an impact on BLL in the study population. Whether lead can inhibit Hb synthesis is closely related to the child's lead load level. Research by Chen and Kutllovci-Zogaj showed that blood lead could interfere with Hb synthesis [8, 9]. The United States Centers for Disease Control (CDC) has recommended that BLL can be used for the assessment of the lead load in the body and the degree of lead poisoning. Lead poisoning has been classified as mild, moderate, or severe at BLLs of 10–19.9 µg/dL, 20–44.9 µg/dL, and 45.0–69.9 µg/dL, respectively . At present, a BLL of 20–40 µg/dL is considered the critical range at which normal Hb synthesis is affected. When the BLL is 40–60 µg/dL or > 60 µg/dL, Hb synthesis will be inhibited, and there is an 18–40% probability of anaemia at the two former levels . In this study (Table 2), the majority of the children in both the exposed and the control groups had BLLs of less than 20 µg/dL. This indicates that low blood lead concentrations may also inhibit Hb synthesis. According to the NHANES III survey in the United States, Hb synthesis can begin to be inhibited even at a BLL of 5.0–9.9 µg/dL . Data suggested that lead-induced anaemia is mediated by the inhibition of Hb synthesis .
This study showed that Hb levels in the exposed group decreased significantly with increasing blood lead load, but not in the control group (Table 3). Moreover, the prevalence of anaemia in children with lead poisoning in the exposed group was significantly higher than that in the control group (Table 4). This indicated that lead exposure in Guiyu is different from that in the non-e-waste disposal area, and there may be coexisting factors that enhance lead blood toxicity. In addition to lead, e-waste contains more than 700 substances, 50% of which may endanger human health, such as cadmium, mercury, hexavalent chromium, polyethylene, polystyrene, brominated flame retardants and surface coatings . It has been reported that 58 kg of mercury, 24.6 kg of cadmium 340.5 kg of arsenic, and several other substances can be separated from 1 ton of randomly collected electronic cards . These toxic substances may interact with lead or cause lead to inhibit Hb more strongly than when acting alone.
Studies have shown that lead can occupy iron ion sites in the intestinal mucosa or haematological system, thus increasing the absorption or reducing the excretion of lead. Therefore, with the increase in lead load in children, iron content will correspondingly decrease, which can lead to an increase in the incidence of iron deficiency anaemia [24, 25]. Unexpectedly, there was no significant difference in the incidence of iron deficiency anaemia between the exposed group and the control group (P = 0.65). The possible reasons might be that few children had BLLs > 20 µg/dL, and normal ferritin status does not necessarily indicate iron sufficiency because ferritin is an acute-phase reactant and may be elevated by infection or inflammatory disease . Thus, some iron-deficient children may have been misclassified as iron-replete on the basis of ferritin level, which would bias the data.
This investigation also showed that the rates of non-lead-poisoning and non-iron-deficiency anaemia in the exposed group were significantly higher than those in the control group (P = 0.03). This finding suggests that toxic substances in e-waste besides lead may inhibit Hb synthesis and contribute to anaemia pathogenesis. Our previous study in this investigation group showed that there was a negative correlation between blood cadmium and Hb level in children in Guiyu , which is consistent with other findings that the Hb level in the high-cadmium group was lower than that in the low-cadmium group . Other substances contained in e-waste, such as aluminium and benzene derivatives, have also been reported to inhibit Hb synthesis [28, 29, 30]. In terms of heavy metals only, the analysis of river water samples and river sediments collected in Guiyu showed that these had high contents of silver, chromium, mercury nickel, copper, lead, zinc and other heavy metals. Chromium, mercury nickel, copper, lead, zinc and other heavy metals have common target organs and molecular targets, such as affecting the haematopoietic system of bone marrow, which would result in anaemia. There are often interactions between metals due to their similar chemical properties . Therefore, further study of the effects of other heavy metals on anaemia in children is required.
In conclusion, lead exposure (excluding the effects of common nutritional factors and thalassemia) more significantly inhibits Hb synthesis in children who live in e-waste dismantling areas than in those who live in non-e-waste dismantling areas. Other toxins released from e-waste may also contribute to the inhibition of Hb synthesis and may even lead to anaemia in local children. Further investigations are needed to provide evidence for the development of relevant protective measures.