Mining activity was a chief source of metals entering into the environment. In the process of mining exploitation and ore concentrating, mine tailing and waste waters were created, and dust was emitted. These industrial endeavours brought tremendous pollutants to the surrounding environment. Since 1970, there had been 35 reported major mine tailing dam failures around the world resulting in significant soil and river pollution and the loss of more than 500 lives . In 2000 alone, there were a total of five reported accidents (in China, Romania, Sweden, and USA) .
Environmental pollution brought by Dabaoshan mine
The Dabaoshan Mining in Guangdong province, China, has been operated for 50 years. The mine tailing dam of this mine collapsed due to heavy rain in 1970. Highly poisonous heavy metals spilled into the Hengshihe River. During this incident, a wide strip of farmland on both sides of the Hengshihe River was covered with a black sludge layer. Although the toxic sludge and a major portion of the contaminated soil surface were mechanically removed, most of the contaminated farmlands and waters are continuously polluted with the spills from the mine. Unfortunately, a part of these contaminated farmlands and waters are still used by local residents today.
This cross-sectional study was carried out before the rainy season (September to October) in Hengshihe River area in 2006. The spilled mine tailing and the concentrations of heavy metal in the studied sites may be much less than what would be measured in a rainy season. Even though, compared with the government standards in China, the heavy-metals pollution in this area were actually over the thresholds. For example, the PH value of the water in the Hengshihe River across Shangba Village was highly acidic, with a value of 4.92. Furthermore, the soils in the vast downstream area, as a long-term sink for potentially toxic elements, were increasingly polluted by more and more depositing heavy metals, including lead, cadmium, copper and zinc. In Shangba and Xiaozhen villages where the Hengshihe River runs through, the crops such as rice were polluted by uptaking heavy metals through their roots from the contaminated soils. The local residents in turn absorbed heavy metals by eating the rice grown in this region. Compared with the government standard, the concentrations of heavy metals in the rice grown in these villages were higher than the critical maximum levels of heavy metals in rice, established to protect the residents' health .
Children's behavioural development
The high concentration of heavy metals in water, soil and crops, is imposing a strong threat on the health of the local residents. Continuous uptake of heavy metals has cumulative effects on the residents, especially for children, since there is not an efficient way for elimination of the heavy-metals residue in child bodies, and the detrimental impact, either physiological or mental, can become apparent only after several yeas of exposure [19, 20].
The emotional and behavioural problems survey showed that the unadjusted levels of behaviour problems differed across the study sites, with Shangba Village being at the highest level. According to fact that heavy metal pollution in Shangba is the most severe, which may explain in part of the highest prevalence of behaviour problems in this village? In our study, one of the influential factors on the eight subscale scores of the emotional and behavioural problems is the parents' education, indicating that the parents' ignorance of the detriments brought by heavy metals may increase the environmental exposure to their children, thus leading to the higher occurrence of heavy-metals-induced psychological problems in their offspring.
Relationship between heavy metal concentration and behavioural development
The present study suggests that there is a significant relation between measured lead concentration and scores on the Child Behaviour Checklist. Measured hair lead is significantly associated with scores on the eight specific behaviour syndrome scales for school children aged 7–16 years.
Although the cross-sectional design of the present study cannot produce a causal association between lead and children's behavioural problems, it does provide evidence for the hypothesis that lead exposure has long term effects on the prevalence of behavioural and emotional problems. There are a number of arguments in favour of this hypothesis. First, the neurotoxic effects of exposure to high levels of lead are well known and documented [21–25], and case reports have suggested a causal link between lead exposure and psychiatric status [26–28]. Second, animal experiments that have examined the effect of lead exposure have found adverse effects on both early mother-infant interaction and social play, and have reported increased aggression and hyperactivity in exposed offspring .
Consistent with the support for the impact of lead on behaviour problems, the magnitude of lead effect is very large. After adjusting for confounders (sex, age, family incoming, farther education and mother education), measured heavy metals, especially lead, account for about 10% to 15% of the variance in the subscale scores. Slight disturbances in early behaviour, coupled with the known adverse effect of lead on intelligence [30, 31], may compound over time, contributing to more substantial difficulties by adult age. It is important to strengthen the management of environmental pollution conditions in the Hengshihe area, and to improve the children's health through lowering their body lead concentration.
This study was hindered by some unavoidable limitations. Of particular importance is the limited information about potential confounders. Due to the limited financial support and human resource, our study aimed to survey the pollution condition and children' health status during our study period, and then estimated their preliminary relationship in the Dabaoshan area. More comprehensive information about potential confounders would be obtained by further study.