Mining and associated industries can cause heavy metal contamination in the environment and communities nearby even after the facilities have closed or work has ceased. Exposure to various heavy metals from the mining industry can occur via inhalation, ingestion of windblown soil dust, and ingestion of contaminated drinking water (Miranda et al., 2016[12]).

In Thailand, heavy metal contamination, particularly arsenic, had been reported in 1987 at the Ron Pibul district of the Nakorn Srithammarat Province in the southern region of Thailand. It caused more than 1,000 people to be contaminated with arsenic and clinical symptoms including skin keratosis, a melonosis with “raindrop in the dust” appearance, skin cancer, weakness, mild fever, anemia, numbness, and dysfunction of upper and lower extremities were observed. The mining industry finally closed in 1994 (Vitayavirasak et al., 2005[20]).

It is assumed that exposure to arsenic occurred through drinking contaminated water from mines and using contaminated ground water (Williams et al., 1996[25]). The previous review showed that widespread contamination of ground water by arsenic has been reported in many countries such as Bangladesh, West Bengal, China, Taiwan, Thailand, Ghana, Argentina, Chile, Mexico, Hungary, Canada, the United Kingdom, and areas of the United States (Kapaj et al., 2006[9]). In Thailand, there have been reports of arsenic contamination from surface drainage and leachate from mine waste causing the distribution of arsenic over an area of about 12 square kilometers (BOE, 2010[2]).

Exposure to heavy metals such as arsenic in children is a public health concern because they are one of the most susceptible groups in the population with bodies that are still developing (Hines et al., 2010[6]). In addition, children are more likely to play outdoors or on the floor and inadvertently can ingest dust that adheres to hands or other objects. For example, in the community near a former copper smelter in Montana, Hwang and colleagues found a significant correlation between arsenic concentrations in residential soils and urinary arsenic in children less than 72 months old, with the highest correlation for soils in bare yards (Hwang et al., 1997[8]).

Arsenic exposure at an early age may also lead to diseases later in life, particularly long-term lung function deficits and shortness of breath (Dauphine et al., 2011[4]). Nowadays, arsenic is a known carcinogen which has also been associated with various cancers including bladder, liver, lung, prostate, and skin cancer (Zhou and Xi, 2018[26]). Arsenic has also been associated with increased susceptibility to respiratory infections and cardiovascular disease. Interestingly, fetuses are extremely vulnerable to the effects of heavy metals, particularly arsenic (Lantz et al., 2009[10]; Vahter, 2009[19]; Ahsan et al., 2006[1]; Navas-Acien et al., 2005[14]).

A study in a Thai cohort has shown that arsenic exposure in utero increased the expression of genes involved in various responsive mechanisms such as apoptosis, stress responses, and inflammation (Fry et al., 2007[5]). Furthermore, DNA damage found in newborns indicated increased levels of urinary 8-nitroguanine, which is significantly correlated with an increased expression of inflammatory genes (COX2, EGR1, and SOCS3) in cord blood (Phookphan et al., 2017[15]).

A follow-up study in these prenatally arsenic exposed children showed an increase in oxidative and DNA damage, represented by increased levels of 8-Hydroxy-2'-deoxyguanosine and 8-nitroguanine, suggesting a risk of mutagenesis (Hinhumpatch et al., 2013[7]; Phookphan et al., 2017[15]). However, there are no firm conclusions as to whether the intake of low concentration arsenic contaminated drinking water adversely affects children's brains. An epidemiological study indicated that central nervous system (CNS) impairments such as cognitive or intellectual deficits were associated with arsenic exposure in children (Wasserman et al., 2007[23]; Nahar et al., 2014[13]; Tolins et al., 2014[18]). However, a study in West Bengal showed no association between long-term arsenic exposure in water and intellectual functions in children (von Ehrenstein et al., 2007[21]).

Over the past 10 years, arsenic contamination has been reported again in northern Thailand. The report on the health impact of the population within a 10 kilometer radius of a mine tailing in Phichit, Phitsanulok, and Phetchabun Provinces of Thailand by the Ministry of Public Health; Thailand, Forensic Science Institute Thammasat University and the Research Center to Promote Safety and the Prevent Injuries in Children Ramathibodi Hospital showed that more than 30 % of children (aged 8-13 years) had arsenic contamination higher than the normal range as indicated by the Agency for Toxic Substances and Disease Registry (ATSDR). After that, the mining was temporarily halted in 2016.

Therefore, this study aimed to follow arsenic contamination after the mining industry had stopped operation for 3 years and to assess the health situation such as the nutrition status and intelligence quotient of children. Four approaches were used:

1) assess the nutrition status

2) assess an intelligence quotient

3) assess the learning disability

4) assess an inorganic arsenic in urine

The most recent report provided by the ATSDR has suggested that toxic exposures to arsenic may result in memory loss and emotional instability in humans. Epidemiological studies from the past decade have provided a wealth of information supporting a strong correlation between arsenic exposure and neurological and cognitive dysfunction in children and adults. The clinical symptoms and pathology depend on concentration, timing, and duration of exposure; particularly, chronic arsenic exposure can induce various pathologies (Wang et al., 2007[22]; Roy et al., 2011[17]; Manju et al., 2017[11]).

A current study of arsenic contamination in Thailand found that 4.5 % of school-age children had inorganic arsenic levels in their urine, higher than the normal range (ATSDR). This data showed that there is still inorganic arsenic contamination above the normal range in children living near an inactive mining site, even though the incidence is low.

The cause of contamination can be due to exposure, inhalation, and consumption. However, this study did not analyze any inorganic arsenic contamination in the environment, such as in the soil, in natural waters, or in food and drinking water. Therefore, the source of contamination cannot be identified in this study.

Several studies have indicated that arsenic is contaminating the food chain, which can affect health and lead to chronic illnesses in children (Fry et al., 2007[5]; Rosado et al., 2007[16]; Dauphine et al., 2011[4]; Tolins et al., 2014[18]; Phookphan et al., 2017[15]; Manju et al., 2017[11]). We examined nutritional status and clinical symptoms of children. The results showed that 25.1 % of children were obese. Male children were twice as likely to be overweight and obese than female children.

In addition, we did not find any clinical symptoms from arsenic contamination among all children in this study. However, more studies such as those on stress response, inflammation, DNA damage, and apoptosis are needed in the future.

This study found that the mean of inorganic arsenic in urine was 17.69±11.81 µg As/L (below normal range) and level of inorganic arsenic in urine did not have any relationship with the IQ scores of children. Our study differed from several studies such as a 2007 study from Mexico, which found a significant relationship between urinary arsenic concentrations greater than 50 μg As/L and poor scores on intelligence tests of children aged 6-8 years (Rosado et al., 2007[16]). A 2007 study from China found that low exposure to arsenic (<10 μgAs/L urinary arsenic) resulted in no measurable effects on IQ. However, moderate and high exposure to arsenic (46 μg As/L and 73 μg As/L) resulted in decreases of 5 and 10 IQ points, respectively (Wang et al., 2007[22]).

A cohort study of children from Mexico (6-7 years old with 55 μg As/L urinary arsenic) showed poorer scores in arsenic exposed children on measures of language and vocabulary and a modest association with hyperactive behavior using the ADHD index (Roy et al., 2011[17]).

As mentioned above, a dose-dependent relationship existed between arsenic levels and poor performance scores on intelligence measures. Interestingly, several studies found that children who had a urinary arsenic level of more than 50 μg As/L will have a high risk of poorer intellectual performance (Wang et al., 2007[22]; Roy et al., 2011[17]; Manju et al., 2017[11]).

In addition, we found 1 LD child (3.6 %) who had inorganic arsenic levels above the normal range. However, more research is needed for LD groups because research on arsenic contamination of children with LD has rarely been done in Thailand.

Hence, the next phase of research should involve more studies being done, especially in Thailand. This would be useful in determining if the current cutoff (35 μg AS/L) will be sufficient to block arsenic-induced neurotoxicity, and if not, what can be done, either legislatively or through medical therapy to alleviate arsenic-induced neurotoxicity.

Therefore, the monitoring of heavy metal contamination in Thailand and any health effects in vulnerable children who live near the mining industry during normal operation or after it being temporarily suspended will prevent and mitigate health impacts that may occur in the future.