For the petrochemical-related metal exposure, the present study indicated that the study subjects lived in the areas nearby were with significantly higher urinary metal levels except only for Cu with the reverse result (Table 1), and there were limited research revealed the internal metal exposure dose in young children like the present study. In addition, our previous studies investigated the obviously elevated urinary metal levels in all of the residents with different age groups including elders, adults, and teenagers [6, 32]. And, we further conducted the distance-to-source analysis and it revealed that the increased concentrations of most urinary heavy metals in study subjects were associated with the decreased distance from the plant, with the only exception of Cu (data not shown). Among these metals, the Cr, Mn, Ni, and As showed the big different exposure levels to response the possible emission pollution by this petrochemical complex because these metals all were suggested be the key pollutants by petrochemical industry previously [37, 38]. On the other hand, there was only the Cu with non-significant finding in this study, and it also indicated the accuracy of the metal exposure representative for the study subjects in this areas because of the main source of Cu exposure are from natural sources (decaying vegetation, forest fires, and sea spray) and anthropogenic emission sources (nonferrous metal production, wood production, iron and steel production) not from petrochemical industry [39, 40]. In the past, the collected air samples in high exposure areas have found that the contents of many metals in PM10 were higher during the downwind season to provide the external metal exposure from the petrochemical complex [31]. Nevertheless, the findings in the present study implied the emission-related metal exposure existed even in the children with the kindergarten age, and it should pay more attention on the potential adverse health effects of children in this polluted area in the future.
In previous study conducted in this study area, it found that teenagers and elders who lived in the high exposure areas were with significantly higher levels of urinary oxidative stress markers [6]. However, the present study did not show any obvious differences in the oxidative stress marker levels of young children between high and low exposure groups (Table 1). One possible reason is due to the different exposure definitions of these two studies. The participants of previous study were selected from the extreme highest and lowest exposure status with about a two-fold difference in most of the urinary metal levels, but the participants in high and low exposure in the present study were only with relatively slight differences in urinary metal levels. Nevertheless, it showed the significant associations between urinary levels of metals and oxidative stress markers for all subjects in this study (Table 3). Among these four oxidative stress markers, we found 8-OHdG and 4-HNE-MA were more sensitive to the metal exposure, and the past studies indicated the consistent findings for the application of these two markers in the prediction of the oxidative stress caused by metal exposure [6, 41]. In addition, the levels of these two markers in young children in the present study, even with lower urinary metal levels, were obviously higher than those in the teenagers and elders in past study [6]. And, previous studies supported that the inverse age-oxidative stress relationship could due to the naturally low glutathione levels at children which means the ability to detoxify reactive oxygen is limited making the younger children more susceptible to oxidative stress [42]. On the other hand, the Hg and Sr showed the dominated contributions for most of the oxidative stress markers in this study (Fig. 2). These two metals were main petrochemical-related emission pollutants [37, 38], and several studies have confirmed their effects on the increasing of oxidative stress [43, 44, 45]. According to the above-mentioned results, young children might be with relatively higher oxidative stress when exposure to metal emission from the petrochemical industry, especially for Hg and Sr, and the subsequent effects of oxidative stress in these young children require further research to clarify.
The differences in dietary antioxidant intake varied slightly between the two groups in the present study, although the high exposure group having a higher average total antioxidant intake (Table 1). Currently, only a few studies have suggested dietary recommendations for vitamin C (the best-known antioxidant), but it has not been clearly defined and it was limited for adults [46, 47]. In fact, there was no well-established dietary recommendation for intake of antioxidants per week, especially for children. Limited studies have measured the intake of antioxidants in children populations. One Swedish study analyzed the associations between antioxidant intake and allergic disease on 8-year-old children by applying a food-frequency questionnaire. The result found that the intake of antioxidant, like β-carotene and magnesium in food, had inverse association with allergic disease such as rhinitis, atopic sensitization, and asthma [48]. However, most of the previous studies just estimated the single antioxidant not for the total intake of antioxidants in food. On the other hand, there was recently no clear definition on the amount of antioxidant intake enough to achieve the obvious antioxidant effects because of the difficulty to define the level of significant antioxidant protective effect. Therefore, it might be one important reason that we did not observe any significant association between antioxidant intake and oxidative stress (Table 3), even with no contribution for the oxidative stress levels when conducting the WQS model including the antioxidant intake levels (data not shown).
On the other hand, previous research has shown that foods with primarily higher antioxidant intake including fruits and vegetables (particularly strawberries, citrus, kiwi), soybeans, nuts, spices, herbs, yam, mackerel, and so on [35, 49]. Among them, soybeans, nuts, strawberries, and kiwi are the more common food in Taiwan. However, our study subjects ingested the antioxidant mainly from the berries, sugar tea, dark chocolate, guava, and sugarless tea, and this kind of dietary pattern might result in the lower antioxidant intake to against the oxidative stress in the body. In the present study, the average antioxidant intake were 43.18 and 34.20 mmol/per week for the high and low exposure groups, respectively. Previously, some studies provided the evidence for the Taiwanese with lower antioxidant when compared to other countries. One of the study found that the Vitamin E status in all age were relatively low in Taiwan when compared to Hungary, Eastern France, and Italy. Another study showed that the serum α-tocopherol (one of the Vitamin E) status in Taiwanese children (less than 6 years) was lower than that in France [50]. Meanwhile, the cooking methods might affected the antioxidant properties of food. Previous study showed that water-cooking treatments is a better way to preserved the antioxidant for vegetables when compared to the steaming and frying [51]. In addition, using the microwave to cook vegetables was reported the worse method to retain antioxidants compared to boiling and frying [52]. Unfortunately, these cooking methods described above are common in Taiwanese. For the reason that, how to change the dietary habits and cooking way to increase antioxidant intake is an important issue in the future, especially for children living in areas more susceptible to environmental pollution.
This study provided the first look at young children’s metal exposure associated with the oxidative stress near the petrochemical industry in Central Taiwan as well as their antioxidant dietary patterns. Traditional studies of exposure assessment usually conducted in combination with a greater view of the paths of exposure and possible adverse effects. Nevertheless, the present study based on the framework of simultaneous assessment of both positive and negative aspects of health effect to provide further insight into the exposure risk factors in this population of children. However, there were several limitations existed in the current study. First, this cross-sectional study might be difficult to represent the long-term exposure situation to observe the health effects for these young children. Nevertheless, the petrochemical complex in our study has started to operate from 1999 for more than 20 years, and it could be considered that the emitted pollutants from the complex were continuous and significant. In addition, the rural socioeconomic status of this study area resulted in most of the kindergarten children who grew up locally. Therefore, it can be expected that these young children were affected by a long-term exposure, and it should not have the directional bias on the exposure results in the present study. Second, this study did not consider other sources of potential oxidative stress besides heavy metal exposure. The polycyclic aromatic hydrocarbons and volatile organic compounds emitted from the petrochemical industry were considered to cause the oxidative stress [53]. The heavy metal pollutants were representative of those pollutants from the petrochemical industry in the present study, but it is necessary to clarify the oxidative effects of other exposure source on the young children in advanced studies. Third, it might be considered that the dietary of young children is changing and unstable. However, the children in our study are mostly over 4 years old that their chewing and swallowing ability is relatively developed and can have a diet similar to that of adults. [54]. Meanwhile, these children were all in kindergartens where have provide regular meals to the students, so the dietary pattern of the children in this study is relatively stable for a long time. Fourth, this study did not consider the other source of antioxidant except for food, and the nutritional supplements are one main source of antioxidant for human beings. Usually, the young children would not take those supplements unless they are with some health condition such as specific disease or malnutrition. Therefore, it would be reliable to estimate the mainly antioxidant intake from the dietary of young children in the present study.