Baseline Soil Dioxin Levels from Sites Where Municipal Solid Waste Incineration Construction Is Planned throughout China: Characteristics, Sources and Risk Assessment

Abstract

The determination of baseline dioxins levels in soils is fundamental for the quantitative assessment of the net environmental and health impacts of municipal solid waste (MSW) incineration plants, which remains unexplored. Therefore, this study develops a Chinese baseline soil dioxins database (covering 918 soil samples from 292 pre-construction MSW incineration plants nationwide during 2016–2020) to thoroughly explore the baseline contamination characteristics and health risks of dioxins in soils. The empirical results show that (1) for concentration levels, the baseline international toxic equivalency (I-TEQ) concentrations vary from 0.0015 to 32 ng I-TEQ/kg, which are close to or even lower than those in most existing studies and show significant heterogeneity across provinces; (2) for dioxins homolog, highly chlorinated dioxins (i.e., PCDDs) are the dominant contributor to the total dioxins in soils (contributing 54.30% of the total I-TEQ concentrations), implying that dioxins in baseline soils often have an atmospheric fingerprint; and (3) for health risks, the carcinogenic risk and non-carcinogenic risk of dioxins in soils are mostly below acceptable levels, close to or even lower than the results of previous studies. Despite these results being considered as quite preliminary, they have certain implications for local authorities. Future studies can expand the size of the database and the generalizability of the results, and if necessary, establish a long-term dynamic monitoring of dioxins in soils for systematically evaluating the net impact of MSW incineration on environment and human health.

1. Introduction

Currently, the fast-increasing amount of municipal solid waste (MSW) provides a broad market for China’s emerging MSW incineration industry, although they may cause environmental issues [1]. Since 2004, China’s MSW production has surpassed that of the United States by 190 million tons, and it now ranks first in the world [1,2]. It has continued to grow rapidly at an annual growth rate of 2.63% until 2020 [3] and is predicted to grow to 480 million tons by 2030 [4]. However, the MSW incineration rate is rather low in China (e.g., only 44.70% in 2018) [5], which is far below that in developed countries or regions; for instance, Monaco (85%), Japan (80.20%) and the British Virgin Islands (80.30%) [6]. Therefore, China’s MSW incineration industry has much scope for growth, and it is expected that the MSW incineration rate will exceed 65% of the total MSW treatment capacity by 2025 and even approach 100% in developed regions (such as Zhejiang, Fujian and Chongqing) by 2030 [7,8,9,10]. However, MSW incineration is questionable due to the high emissions of dioxins (known as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), together PCDD/Fs) [11,12,13], which are assessed as a Group 1 carcinogen for humans by the International Agency for Research on Cancer [14].

Soil is an important environmental medium where dioxins from different emission sources gradually accumulate during long-term transportation, thereby affecting human health through the food chain, inhalation and dermal contact [15,16]. Furthermore, considering the immobility and stability in soils, dioxins in soils have been widely examined to detect the environmental impacts and health risks from waste incineration [17,18]. Notably, dioxins in surface soil near MSW incineration plants mainly originate from baseline soil contamination (i.e., the dioxins level in the surrounding soils before the construction of MSW incineration plants) and the deposition of MSW incineration [11,19,20]. On one hand, dioxins released through the MSW incineration can be deposited in soil by wet and dry deposition, contaminating the surrounding soil environment and causing health risks to humans [21,22,23]. On the other hand, compared with the dioxins released by MSW incineration, the baseline soil concentration is the principal source of human exposure to soil dioxins [20,24,25,26]. Therefore, it is essential to analyze the baseline level and health risk from dioxins in soils around MSW incinerators, which is also valuable for future assessments of the eventual environmental and health impacts of MSW incineration.

To date, existing studies on baseline levels of dioxins contamination in soil have mostly focused on hazardous waste (HW) incineration [27,28] and medical waste (MW) incineration [29,30]. In contrast, only one study has been performed on the baseline levels of dioxins contamination in soil around MSW incineration plants [20]; data for both the baseline levels of contamination and health risk from dioxins in soils around MSW incineration plants are still lacking. Furthermore, existing studies focusing only on the dioxins in soils around the established MSW incinerators [11,15,18,31], have confirmed that disregarding the baseline levels of dioxins in soil may lead to an overestimation of contamination and health risks in soils resulting from the construction of waste incineration plants [32,33,34]. This study is conducted to systematically study the baseline levels of contamination and health risk for dioxins in soils around pre-construction MSW incineration plants in China.

Overall, this study is the first attempt to comprehensively explore the baseline levels of contamination and health risk from dioxins in soil before the construction of Chinese MSW incineration plants nationwide. First, the baseline concentrations of dioxins in soils before the establishment of MSW incineration plants in China (a total of 292 MSW incineration plants) are systematically explored. Second, the contamination characteristics (including congener, homologue profiles and source apportionment) of dioxins in soils are investigated via statistical analyses. Third, the baseline levels of health risks (considering carcinogenic risk (CR) and non-carcinogenic risk (NCR)) for dioxins in soils under different exposure pathways are thoroughly assessed.

This study makes contributions to the existing literature from the following two perspectives:

(1)

This study will develop a Chinese baseline soil dioxins database, presenting and analyzing the actual measurements of dioxin concentrations in soils determined before the construction of 292 Chinese MSW incineration plants nationwide during 2016–2020.

(2)

Using the database, this study will be the first attempt to comprehensively explore the baseline contamination characteristics and health risks from dioxins in soils surrounding pre-construction MSW incineration plants in China.

2. Materials and Methods

2.1. Data Preparation

This study focused on 292 pre-construction MSW incineration plants in 30 provincial-level administrative regions of China (excluding Tibet, Hong Kong, Macao and Taiwan) (Figure 1). The actual measurement data of dioxin concentrations in soils, covering a total of 918 soil samples collected during 2016–2020, were obtained from the official websites of several provincial ecology departments and MSW incineration companies (see Table S1 in the Supplementary Materials for details). These soil samples were collected, processed and analyzed by the laboratories with the China Inspection Body and Laboratory Mandatory Approval (CMA), according to the Chinese standards of HJ/T 166-2004 and HJ 77.4-2008 [35,36].

First, the soil samples were collected from the topsoil (within a depth of 20 cm), following the Chinese standards of HJ/T 166-2004 [35]. Second, the dioxins in soil samples were subjected to strict pretreatment, purification, instrumental analysis and data processing, according to the Chinese standard of HJ 77.4-2008 [36] (see Figure S1 in the Supplementary Materials for details). In particular, for data processing, the international toxic equivalent factor (I-TEF) for dioxins was used to calculate the international toxic equivalency (I-TEQ), and concentrations below the detection limit was calculated as half of the detection limit [36]. Third, for quality control and quality assurance, recoveries of the sampling standards, blank experiments and parallel samples were conducted. Accordingly, detailed information on TEF, method detection limits (MDLs) and recoveries of dioxins were listed in Table S2 in the Supplementary Materials.

In addition, other statistical data, including total population, population density, GDP, per capita GDP, the added value of the secondary industry, industrialization degree (defined as the ratio of the added value of the secondary industry to GDP), urbanization level (the proportion of the urban population) and the proportion of environmental protection expenditures (the proportion of local fiscal expenditure on environmental protection in the general budget of local finance), were collected from the China Statistical Yearbook [37], as listed in Table S3 in the Supplementary Materials.

2.2. Health Risk Assessment

Health risks (including CR and NCR) were calculated based on the USEPA standard model [38]. According to existing research [11,15], the chronic daily intakes (CDIs) of soil pollutants through three main exposure pathways, namely dust ingestion (CDI_ing), air inhalation (CDI_inh) and dermal contact (CDI_der), were estimated. The intake methodology was utilized in this study to calculate the CDIs of dioxins in soils for different age groups (i.e., children, adolescents and adults) [11,15,39,40,41]:

$$CD{I}_{ing}=\frac{C_s\times IngR\times ED\times EF}{AT\times BW}\times {10}^{-6}$$

(1)

where, CDI_ing is the CDI related to dust ingestion (mg·(kg∙day)⁻¹); C_s denotes the toxic equivalent concentration of dioxins in soils (ng I-TEQ·kg⁻¹); IngR means the rate of soil ingestion (mg·day⁻¹); ED and EF are the duration (year) and frequency (day·year⁻¹) of exposure, respectively; and AT and BW indicate the average exposure time (day) and body weight (kg), respectively. In particular, for CR, the AT values for the three age groups are 25,550 days, while for NCR, the AT values of dioxins are 4015, 2190 and 19,345 days for the three age groups, respectively.

$$CD{I}_{inh}=\frac{C_s\times InhR\times EF\times ED}{PEF\times AT\times BW}$$

(2)

where, CDI_inh is the CDI related to air inhalation (mg·(kg∙day)⁻¹); InhR expresses the rate of air inhalation (m³·day⁻¹); and PEF is the particulate emission factor (m³·kg⁻¹).

$$CD{I}_{der}=\frac{C_s\times ABS\times AF\times EF\times ED\times SA}{AT\times BW}\times {10}^{-6}$$

(3)

where, CDI_der is the CDI related to dermal contact (mg·(kg∙day)⁻¹); ABS describes the dermal absorption factor (unitless); AF represents the relative skin adherence factor for soil (mg·(cm∙day)⁻¹); and SA denotes the surface area of the skin that contacts soil (m²). The values and parameters for the health risk assessment are listed in Table S4 in the Supplementary Materials.

The CR of dioxins in soils for different age groups was assessed using Equation (4) [11,42]:

$$C{R}_{soil}={\displaystyle \sum _{i=1}^3(CD{I}_i\times S{F}_i)}$$

(4)

where, SF denotes the carcinogenicity slope factor and represents the three exposure pathways (i.e., 1.30 × 10⁵ for the ingestion exposure pathway, 1.50 × 10⁵ for the inhalation exposure pathway and 1.30 × 10⁵ for the dermal exposure pathway) (mg kg⁻¹ day⁻¹)⁻¹ [15,43].

The NCR of dioxins in topsoil was assessed via the Hazard Index (HI) [11,44,45].

$$H{I}_{soil}={\displaystyle \sum _{i=1}^3(\frac{CD{I}_i}{Rf{D}_i})}$$

(5)

where, RfD denotes the reference dose concentration for different exposure pathways (7.00 × 10⁻¹⁰, mg kg⁻¹ day⁻¹) [15,43].

2.3. Statistical Analysis

In this study, dioxin concentrations were expressed on the basis of dry weight. The detailed analysis steps are as follows. First, all concentration data were normalized before homologue profile analysis. Second, the Pearson correlation coefficient was introduced to evaluate the correlation between economic indicators and dioxin concentrations in soils. Third, statistical significance among different groups was calculated using variance (ANOVA) or the non-parametric Mann–Whitney test, where a result of p < 0.05 was considered statistically significant. Finally, principal component analysis (PCA) and hierarchical cluster analysis (HCA) were employed to identify the underlying emission sources of dioxins in soils. All statistical analyses were performed in RStudio, v.4.1.3.

3. Results and Discussion

3.1. Baseline Concentration Level of Dioxins

The baseline I-TEQ concentrations of dioxins in soil samples, taken before the construction of 292 MSW incineration plants in China, are displayed in Figure 2. Overall, the dioxin concentrations vary across spatial distributions in China. At the national level, the dioxin concentrations range from 0.0015 to 32 ng I-TEQ/kg, with average and median values of 1.83 and 0.89 ng I-TEQ/kg, respectively (Figure 2a). At the regional level, the dioxin concentrations decrease in the order of eastern region (3.06 ng I-TEQ/kg) > central and southern regions (1.80 ng I-TEQ/kg) > southwestern region (1.62 ng I-TEQ/kg) > northern region (1.21 ng I-TEQ/kg) > northeastern region (0.95 ng I-TEQ/kg) > northwestern region (0.81 ng I-TEQ/kg) (Figure 2b); by averaging the I-TEQ concentrations of all sample points in certain region). The results show a significant heterogeneity in the spatial distribution pattern for dioxin concentrations in soils (ANOVA: F = 10.31, p < 0.001). The reason for this might lie in the higher industrialization degree and intensive dioxins emission sources (e.g., cement production, MSW incineration and steel industry) in the eastern region, which results in the increase in dioxin concentrations in soils through atmospheric deposition [46,47,48].

At the provincial level, the distribution of I-TEQ concentrations across provinces also shows a high degree of spatial heterogeneity (ANOVA: F = 4.04, p < 0.001), which is mainly attributed to the influence of the provincial economic development level, population density and industrialization degree (Figure 2c); by averaging the I-TEQ concentrations of all sample points in certain province) [47,49,50,51]. For example, Inner Mongolia, Qinghai and Hainan, with lower levels of economic development (accounting for 1.71%, 0.30% and 0.55% of the national GDP in 2020, respectively), population density (1850.42, 2929.52 and 2444.40 people/km², respectively) and added value of secondary industry (6868.00, 1143.50 and 1055.30 billion yuan, respectively), have the lowest average dioxin concentrations: 0.25, 0.30 and 0.51 ng I-TEQ/kg, respectively. In comparison, Fujian, Jiangxi and Hunan have the highest dioxin concentrations in China, with averages of 6.41, 3.12 and 3.09 ng I-TEQ/kg, respectively. The reason for this might be the higher total economic output (4.34%, 2.54% and 4.13%, respectively), population density (3545.37, 4426.17 and 3676.79 people/km², respectively) and added value of secondary industry (20,328.80, 11,084.80 and 15,937.70 billion yuan, respectively).

To further investigate the driving factors of spatial heterogeneity, correlation analysis between dioxin concentrations in soils and various economic indicators across provinces is performed. In particular, the dioxins concentration is significantly negatively correlated with the proportion of environmental protection expenditure (p < 0.05), whereas it is significantly positively associated with GDP, per capita GDP, industrialization and urbanization level (p < 0.01). These findings demonstrate that the enhancement of pollution control measures can inhibit the increase in dioxin concentrations in soils, while the improvement of economic development, urbanization and industrialization promotes the increase in dioxin concentrations. This implies that with rapid economic development, investment in environmental protection should be increased to reduce soil pollution.

3.2. Comparative Analysis of Dioxin Concentrations

To better understand the relative extent of dioxins contamination in soils, the baseline levels of dioxins in this study are compared with the results for different regions obtained in previous studies (Figure 3). Compared with the other studies on background dioxins levels, the baseline I-TEQ concentrations of soil dioxins in this study are significantly greater in the same areas, such as Qinghai-Tibet Plateau [52,53], Wolong Mountain [54] and Qinghai Lake [55]. Compared with other studies on baseline levels (Figure 3b), the baseline I-TEQ concentrations of dioxins under this research are comparable to those in the existing research on newly built HW incineration plants (0.12–17.2 ng I-TEQ/kg) [27,28], MW incineration plants (0.46–2.63 ng I-TEQ/kg) [29] and MSW incineration plants (0.32–11.40 ng I-TEQ/kg) [20].

Compared with the literature on operated MSW incineration plants (Figure 3c), the I-TEQ concentrations of dioxins near the studied pre-construction MSW incineration plants are mostly close to or lower than those around the operated MSW incineration plants in other regions, such as Sichuan [11], Shanghai [56], Zhejiang [57] and East region [15] in China and Sant Adrià del Besòs in Spain [58] and Rimini in Italy [59]. Notably, in some cases, the I-TEQ concentrations of dioxins under this study are greater than those surrounding the operated MSW incineration plants of Tianjin in China [60], Spain [18,25,61] and Rimini in Italy [59].

Compared with studies on soils for other land uses (Figure 3d), the I-TEQ concentrations of dioxins in the studied baseline soils are similar to or below those in soils for waste disposal [17,62], parks [63], agricultural land [31,64,65,66,67], and industrial land [68,69,70]. Above all, the baseline levels of dioxins in this research are close to or below the results of most existing research; however, it should be noted that there is still ample room to decrease soil dioxins pollution in China, compared with developed countries (e.g., Spain and Italy).

Furthermore, the maximum acceptable levels of different land use listed in the international soil guidelines and regulations [71] are utilized for comparison with the I-TEQ concentrations in this study. The comparison results show that (1) the I-TEQ concentrations of all samplings are below the US and China guideline values (i.e., 1000 and 40–400 ng I-TEQ/kg) [38,72]; (2) the I-TEQ concentrations of approximately 91.29% of soil samples in this study are lower than the strictest Canadian guideline value (i.e., 4 ng I-TEQ/kg) for agricultural, residential/parkland, commercial and industrial uses [73]; and (3) approximately 97.18% and 93.68% of soil samples have lower contamination levels than the guideline values of Dutch, Swedish and German for agricultural land use (i.e., 10 and 5 ng I-TEQ/kg) [74,75,76]. These findings imply that in most cases, the baseline levels of dioxins in soils surrounding the pre-construction MSW incineration plants are relatively low, but this will also affect the accuracy of soil contamination assessment after MSW incineration.

3.3. Multivariate Analysis

The congeners data of dioxins in only 113 soil samples were available in the official websites. Figure 4 shows the congeners, homolog and correlation analysis of dioxins in these soil samples determined before the construction of MSW incineration plants in China. As illustrated in Figure 4a, highly chlorinated dioxins (i.e., PCDDs) of all samples account for 54.30% of the total TEQ concentrations, compared to 45.70% for low-chlorinated dioxins (i.e., PCDFs), which is consistent with previous studies on baseline soils [20,77,78]. In particular, the PCDDs are mainly formed by condensation of chlorophenols, while the PCDFs are formed by further chlorination of non- or low-chlorinated precursors [79]. Among the PCDD congeners, 1,2,3,7,8-PeCDD is the primary compound, contributing 16.86% of the total I-TEQ concentrations of dioxins, followed by 2,3,7,8-TeCDD (15.74%). Among the PCDF congeners, 2,3,4,7,8-PeCDF is the predominant component, representing 19.45% of the total I-TEQ concentrations of dioxins.

Notably, the differences in formation mechanisms between PCDDs and PCDFs remain unclear since the chemical reactions and formation stability during high-temperature processes are more complex than theoretical explanations [80]. Therefore, the values of the ∑PCDFs/∑PCDDs (abbreviated as F/D) ratio are utilized to help identify the characteristics of dioxins in environmental media [15]. Among 113 soil samples, PCDD homolog are dominant in 72 soil samples (accounting for 63.72% of the total samples), with an F/D ratio of 0.04 to 0.95, indicating that dioxins in baseline soils often have an atmospheric fingerprint (with higher levels of chlorinated PCDDs than chlorinated PCDFs) [20,77,78]. For the other 41 soil samples, the F/D ratios are higher than 1, implying that dioxins in these baseline soils had been contaminated by anthropogenic activities, such as MSW incineration [11,20].

Next, the PCA and HCA methods are utilized to further explore the homolog profiles and potential sources of dioxins in soils, as shown in Figure 4b, Figure S2 and Figure S3 in the Supplementary Materials. According to the principle, for an eigenvalue greater than 1, the four principal components (i.e., PCs with a variance contribution rate of 81.73%) can be extracted via the PCA method, among which the first two PCs declare 68.5% of the total variance (Figure 4b). In detail, PC1 accounts for 59.5% of the total variance and primarily correlates with 1,2,3,7,8-PeCDF, 2,3,4,6,7,8-HxCDF, OCDF, 1,2,3,4,6,7,8-HpCDF and 1,2,3,4,7,8,9-HpCDF, while the PC2 accounts for 9.0% of total variance and positively correlates with 1,2,3,7,8-PeCDD, 2,3,7,8-TeCDD and 1,2,3,4,7,8-HxCDD.

Then, the HCA method is used to characterize the distribution of samples with similar characteristics of dioxin congeners and homolog. In Figure 4c,d, the PCA and HCA results indicate that the characteristics of dioxin congeners and homolog in the four groups of samples are significantly different. First, soil samples in Group 1 are characterized by 2,3,4,7,8-PeCDF, 2,3,7,8-TeCDD and 1,2,3,7,8-PeCDD, with the largest proportion of I-TEQ levels (accounting for 20.01%, 16.91% and 15.98%, respectively), of which the dioxin profiles are very similar to those in the background soils of southeast China and Cambodia [69,81]. Second, the average proportions of 1,2,3,7,8-PeCDD and 2,3,7,8-TeCDD in Group 2 are relatively higher, accounting for 33.84% and 17.56% of the total I-TEQ concentrations, respectively. This profile is similar to that in soils of industrial parks from Taiwan, which may come from industrial waste incineration and the recycling of aluminum and electric arc furnaces [82]. Third, the TEQ concentrations of PCDFs homolog in Group 3 are higher than those of PCDDs homolog, which may have been polluted by waste incineration or other thermal sources [20]. Finally, soil samples in Group 4 are characterized by the highest I-TEQ concentrations of 1,2,3,6,7,8-HxCDD (72.28%), which may be related to pulp and paper production and tall-oil distillation [83].

3.4. Multivariate Analysis

3.4.1. Carcinogenic Risk

The carcinogenic risks from dioxins in soils for different age groups via different exposure routes are estimated, as illustrated in Figure 5 and Figure S4a in the Supplementary Materials. In general, carcinogenic risks above 1.00 × 10⁻⁴ are considered unacceptable; carcinogenic risks below 1.00 × 10⁻⁶ are considered to cause no obvious health effects; and carcinogenic risk between 1.00 × 10⁻⁶ and 1.00 × 10⁻⁴ is generally considered to be a tolerable level [84]. Regarding the spatial distribution, 70.00% of the studied provinces have higher CR values than the lower limit of the acceptable level (CR < 1.00 × 10⁻⁶), but all provinces have CR values below the upper limit of the acceptable level (CR < 1.00 × 10⁻⁴), implying that a potential carcinogenic risk occurred in some provinces. For different age groups, the CR values for children, adolescents and adults in all provinces range from 3.66 × 10⁻⁸ to 5.54 × 10⁻⁶, 1.12 × 10⁻⁸ to 1.69 × 10⁻⁶ and 4.28 × 10⁻⁸ to 6.47 × 10⁻⁶, respectively. These CR values for different age groups are all within the acceptable level (1.00 × 10⁻⁶ < CR < 1.00 × 10⁻⁴), suggesting that no obvious carcinogenic risk is developed. In comparison, adults are susceptible to dioxins in soils, and their CR values are much higher than those of children and adolescents (1.17 and 3.82 times, respectively). The reason for this might be that the ED and InhR values of adults are much higher than those of the other two age groups [15].

Regarding diverse exposure routes, ingestion is the main pathway of exposure to dioxins in soils, which is in line with the existing literature [11,15]. The CR values for ingestion, dermal and inhalation exposure routes in all provinces ranged from 5.88 × 10⁻⁸ to 8.90 × 10⁻⁶, 3.17 × 10⁻⁸ to 4.80 × 10⁻⁶ and 5.37 × 10⁻¹¹ to 8.13 × 10⁻⁹, respectively. In detail, 56.67% and 33.33% of provinces for ingestion and dermal exposures have higher CR values than the lower limit of the acceptable level, respectively, while the CR values for inhalation exposure in almost all provinces are below the acceptable level. These findings suggest that there is spatial heterogeneity in the CR values, especially in the regions with CR values higher than 1.00 × 10⁻⁶, which are mainly concentrated in relatively developed provinces, indicating that residents living in developed regions may have potential carcinogenic risks due to by dioxins exposure. Therefore, the carcinogenic risk from dioxins in these regions deserves special attention.

3.4.2. Non-Carcinogenic Risk

Figure 6 and Figure S4b show the provincial non-carcinogenic risks (i.e., HI values) posed by dioxins for different age groups associated with the three exposure routes. For the spatial distribution, the HI values of all provinces for different age groups and exposure routes are lower than the upper limit of non-carcinogenic risks (HI < 1), indicating no obvious non-carcinogenic risk. In the three age groups, the HI values of children, adolescents and adults in all provinces range from 2.53 × 10⁻⁵ to 3.84 × 10⁻³, 1.43 × 10⁻³ to 2.17 × 10⁻¹ and 6.21 × 10⁻⁴ to 9.40 × 10⁻², respectively, which are significantly below the threshold of non-carcinogenic risk (i.e., HI < 1), implying that no significant non-carcinogenic risks occurred. For the three exposure pathways, the HI values for inhalation, dermal and ingestion exposure pathways in all provinces range from 1.23 × 10⁻⁶ to 1.86 × 10⁻⁴, 8.08 × 10⁻⁴ to 1.22 × 10⁻¹ and 1.27 × 10⁻³ to 1.92 × 10⁻¹, respectively, indicating that ingestion exposure is also the dominant exposure pathway for non-carcinogenic risks from dioxins in soils. Compared with the CR values, the distribution of HI values is significantly different among the three age groups. This is mainly due to the differences in the average exposure times across age groups when calculating the carcinogenic and non-carcinogenic risks.

3.4.3. Comparative Analysis of Health Risks

For a deeper understanding of the baseline health risk levels of dioxins in soils, Figure 7 presents the results of this study compared with the existing literature. The comparison results display that the baseline level of health risks in this study is close to or even significantly lower than those of the studies on soils surrounding the operated MSW incineration plants in Sichuan [11], East region [15] and Shanghai [85] in China, Spain [61,86] and Korea [87], but higher than that of the research focusing on Hainan (i.e., a relatively polluted region) in China [88]. Overall, the baseline health risks (involving the CR and NCR) from dioxins in soils near the pre-construction MSW incineration plants of this study are relatively low and below the acceptable level, suggesting no significant adverse effects on human health.

4. Conclusions

This study is the first attempt to develop a national baseline soil dioxins database to comprehensively explore the contamination characteristics and associated health risks of soil dioxins determined before the construction of Chinese MSW incineration plants. From the empirical study, some interesting findings can be drawn: (1) For concentration levels, the baseline levels of dioxin concentrations in this research are close to or below those in most existing studies and show significant heterogeneity across provinces (highly correlated with economic development). (2) For dioxin homolog, the highly chlorinated dioxins (i.e., PCDDs) are the dominant compounds in the total dioxins in soils (accounting for 54.30% of total I-TEQ concentrations), implying that dioxins in baseline soils tend to have an atmospheric fingerprint. (3) For health risks, the CR and NCR aroused by dioxins in soils for different age groups via three different exposure routes are almost all within acceptable levels and are close to or lower than the results of previous studies.

Although these findings are rather preliminary, only focusing on the pre-construction MSW incineration plants, our findings have certain implications for health practitioners, policy makers and general public. First, medical institutions can prevent and control the exposure of susceptible groups to dioxins by regular inspection and publicity of common dioxins diseases. Second, the government should increase investment in environmental protection and implement strict emissions standards to reduce soil pollution and potential health risks. Third, public can report environmental violations via official complaint platform, to urge the treatment of environmental problems. Future studies can expand the size of the database and the generalizability of the results. Furthermore, long-term dynamic monitoring of soil dioxins is necessary, which can help to investigate the net impact of MSW incineration on environment and human health.