This study assessed exposure to metal(loid)s among children and adolescents in Liuzhou City, Southwest China. We measured concentrations of urinary metal(loid)s and correlations among different elements. According to the World Health Organization recommendations, urine samples with creatinine values below 0.3 g/L or above 3 g/L were unrepresentative and they should not be included in the following statistical analysis in order to reduce the possibility of overestimation or underestimation in calculating concentrations. In our study population, 4 samples had creatinine levels above 3 g/L and 21.5% of samples showed levels below 0.3 g/L. We performed a sensitivity analysis by removing samples with creatinine levels without in the range of 0.3 g/L-3 g/L in order to compare the coefficients of the regression models. No significant changes were observed in the regression coefficients comparing to the whole analysis that requires the conclusions of the study to be changed.
The comparison of the concentrations of urinary metal(loid)s in different regions including Europe, the USA and Canada was presented in Table 3. The concentrations in our study fell within the range of values usually reported in other studies for many metals (Al, V, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Ag, Cs, Ba, Hg, U) or even lower (Mn). We observed that the average levels of urinary metal(loid) concentrations (Cr, Cd, Tl, Pb) in our study were slightly higher than those in other countries.
The GM of chromium found (1.53 µg/g creatinine) was close to the GM of 1.57 µg/g creatinine in China (Xu et al. 2020) but lower than the GM of 2.15 µg/g creatinine in southern Brazil (Rocha et al. 2016). The level was higher than those in Spanish children aged 5–17 years (0.43 µg/g creatinine) (Aguilera et al. 2010) and in French adults (0.33 µg/g creatinine) (Nisse et al. 2017). Animal studies showed that chromium could enter the brain through the olfactory pathway, which had a detrimental effect on experimental ethology and memory processes and retention (Singh &Chowdhuri 2017). Chromium in the environment mainly comes from the production of metal alloys, leathers, steel and so on (ATSDR &asp 2012). Automobile manufacturing and steel production are major industries in Liuzhou City, and previous studies have reported high levels of chromium in the Liujiang River flowing through Liuzhou City (Miao et al. 2020). Students’ chromium load level is high, which may be harmful to their health, affecting the normal growth and development of children.
Regarding cadmium levels, the GM of the student group (0.620 µg/g creatinine) was higher than those found in Spain (0.41 µg/g creatine) (Aguilera et al. 2010) and Japan (0.34 µg/g creatine) (Ilmiawati et al. 2015), but lower than that found in the industrial area of Spain (0.75 µg/g creatinine) (Molina-Villalba et al. 2015). Wastewater, waste gas and waste residue from industrial sites could increase the cadmium load in the local population, so the concentration of cadmium in the general population has received increasing attention worldwide.
The GM of urinary lead concentration measured in this study was 2.14 µg/g creatinine, higher than that in Spain (1.16 µg/g creatinine) (Roca et al. 2016) and similar to that in industrial areas of Spain (2.22 µg/g creatinine) (Molina-Villalba et al. 2015). In addition, the urinary lead level in our study was higher than that of adults reported in rural areas along the Yangtze River (1.51 µg/g creatinine) (Cui et al. 2017). Lead could hinder the absorption of trace elements in the human body and affect growth and development; long-term exposure to low doses of lead could also cause immune disorders and changes in host resistance (Fleisch et al. 2013). Lead is considered to represent pollution from vehicles, which often occurs in traffic areas (Zhou et al. 2016). Liuzhou is one of the five major motor cities in China. Although leaded gasoline has been phased out in China since 2001, other agents in unleaded gasoline could still introduce a certain amount of lead into the atmosphere, especially in areas where there are more gasoline-based motor vehicles and motorcycles on (Li et al. 2012). In addition, population growth and industrial expansion are potential sources of lead pollution that may contribute to the increased concentration of urinary lead.
Manganese is an element that is closely related to human health; either its excess or deficiency can lead to disease, and low levels of manganese can adversely affect neurodevelopmental outcomes (Riojas-Rodríguez et al. 2010). Our study showed that the GM of manganese (1.31 µg/g creatinine) was higher than that in Spain (0.48 µg/g creatinine) (Perez et al. 2018), but lower than those reported in Italy (7.52 µg/g creatinine) (Pino et al. 2012) and the industrial area of China (6.37 µg/g creatinine) (Zhang et al. 2017). Urinary concentrations in the range of 1 µg/L to 8 µg/L are considered to fall within the reference range; only 6 individual urine samples had manganese concentrations exceeding 8 µg/L, while more than 40% of individuals had urinary manganese concentrations below 1 µg/L (Godebo et al. 2019).
Urinary concentrations of cobalt, nickel and barium were comparable to those found in Mexican children aged 8–14 years (0.8 µg/L, 9.27 µg/L and 3.09 µg/L, respectively) (Lewis et al. 2018) and were slightly higher than the concentrations in Belgian adults (0.15 µg/g creatinine, 1.73 µg/g creatinine and 1.68 µg/g creatinine, respectively) (Hoet et al. 2013). Zinc and mercury concentrations were comparable to those of Spanish children aged 6–11 years (515 µg/g creatinine and 0.73 µg/g creatinine) (Molina-Villalba et al. 2015). The concentration of thallium was higher than that in several other countries (CDC 2015, Roca et al. 2016). Current studies of the health effects of Tl are limited, but they may be related to headache, anorexia, arm, thigh and abdominal pain (Peter &Viraraghavan 2005). The high levels of some elements may occur because Liuzhou City has a large number of factories. Industrial pollution is a major potential source of metal exposure and may also be a source of surrounding bioavailable metal(loid)s, making metal(loid)s more accessible to humans through water, food, and air particles.
According to the obtained Spearman matrix in Fig. S1, significant correlations were observed between almost all metal(loid)s, suggesting possible similar sources. The relationship between Mn and Cu (r = 0.47) and Hg and Se (r = 0.44) in our study were similar to those reported among Spanish children (Roca et al. 2016). Moderate to strong relationships were found between urinary Al and Cd (r = 0.38) and As and Se (r = 0.66). Lewis et al. also observed a moderate relationship between urinary Al and Cd (r = 0.38) in Mexican children (Lewis et al. 2018).
Regarding predictors of exposure to metal(loid)s, we found that age was an important predictor of element levels and that children exhibited higher metal(loid) exposure levels than adolescents, expect for Sr. Previous reports also found that older people present lower levels of metal(loid)s (Godebo et al. 2019, Rocha et al. 2016). Other studies also showed that children’s age was the one of the most important predictors among most metal(loid)s, possibly because younger children were more exposed to external substances through hand-to-mouth activity (Roca et al. 2016). It is also possible that the metabolism functions of elements in the lower age groups have not reached optimal levels, which increases the element levels in urine.
Taking sex into account, most urinary element levels were higher in girls, among which the differences in the concentrations of 13 elements were significant (P < 0.05). This finding may be explained by the fact that girls were more exposed to metal(loid)s in beauty products such as nail polish and hand cream. Continued use of these products increases the absorption of metal(loid)s, which is particularly harmful to children. In addition, high concentrations of lead and cadmium were found in white cosmetics such as creams and lotions (Orisakwe &Otaraku 2013). In relation to maternal education, children whose mothers reported higher education levels showed lower concentrations of Co, Sr, Cd and Pb, while a positive correlation was found in the levels of Cr and Se. This finding may be due to the association between family status and other variables potentially associated with exposure to metal(loid)s. The concentrations of Cr, Co and Ni in passive smokers were higher and of statistical significance (P < 0.05). Many metals found in cigarette smoke, tobacco, and cigarette paper, such as Cr, Ni, Cu, Cd, and Pb, are potential health threats(Bernhard et al. 2005). Our research also showed the passive-smoking group had higher levels of urinary cadmium, although no significant association with urinary cadmium levels and passive smoking was found. Smoking by family members is more likely to be inhaled by student groups in an indoor environment with poor ventilation, and attention should be given to passive smoking in children in this region.
Dietary habits may also affect the distribution of various metal(loid)s. Milk, vegetable and fish were important factors in assessing the source of metal(loid) exposure in our study. People who ate fish, the main dietary source of arsenic, showed higher levels of arsenic in their urine. Other studies observed that increasing arsenic levels were related to frequent consumption of fish (Bocca et al. 2016, Schulz et al. 2011). Studies showed a significant correlation between urinary arsenic concentrations and total protein intake, and fish are rich in protein (Kordas et al. 2016). People with higher dairy intake had lower urinary levels of V, Co, Ag, Cd and Tl, as in previous studies (Ashrap et al. 2020, Roca et al. 2016). Regarding Cd, these results may be related to the inverse ratio between the presence of large amounts of minerals (such as Ca) in dairy products and the absorption of Cd, as previously reported (Castaño et al. 2012). People who ate more vegetables had higher levels of Al, Cr, Mn, Zn, Rb, Cs, Ba and Tl in their urine. Vegetables are an important part of people's diet and contain some essential trace elements but may also contain heavy metal residues (Zeinali et al. 2019). People who ate more meat had higher levels of zinc in their urine. Due to lifestyle changes and increased demand for fast food based on meat, meat consumption has increased in recent years. Eating food containing metal residues may lead to neurological problems, headaches and liver dysfunction (Farmer et al. 2011). Although this work was a cross-sectional study, the participants' food intake and urine metal content remained in long-term equilibrium, thus indicating the potential role of food in metal(loid) exposure.
In terms of the risk of metals, 31.8% of the population had urinary cadmium levels between HBM-I and HBM-II levels, and only the HQ of cadmium was > 1 at the P95 value, indicating that exposure to this metal may pose a health risk to a small number of people. Similar tendencies were found in a previous biomonitoring study carried out in Italy (Bocca et al. 2016). Another study of wild fish in the Liujiang River also found that the potential health risks were mainly caused by the presence of Cr and Cd (Miao et al. 2020). The HI of multiple metals was near 1 at the P50 value and above 1 at the P95 values, indicating that the joint toxicity of multiple metals could not be ignored and that noncancer health consequences are likely to occur in children and adolescents exposed to metals in the assessed areas. However, this algorithm may be affected by many parameters. The mechanism of action and toxic effects of these metals in the human body may not be the same, and there may be antagonistic or synergistic effects among them. More data, including information on socioeconomic, personal care products, and environmental conditions, are essential to determine the real effect of metal exposure.