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Source, Characterization of Indoor Dust PAHs and the Health Risk on Chinese Children

Polycyclic aromatic hydrocarbons (PAHs) in indoor dust are one of the common exposure sources for children worldwide. The aim of this study is to explore PAHs pollution status in indoor dust and estimate health risk on Chinese children with big data. Weighted average concentration was used to analyze source and characterization of PAHs in indoor dust based on peer-reviewed literature. According to specific inclusion criteria, 17 studies were included finally to analyze weighted average concentration. The national average concentration of Σ16PAHs was approximately 25.696 µg/g. The highest concentration of Σ16PAHs was in Shanxi (2111.667 µg/g), and the lowest was in Hong Kong (1.505 µg/g). The concentrations in Shanxi and Guangdong were higher than national level and the over standard rate was 18.18%. The concentrations of individual PAHs varied greatly across the country, and Flu in Shanxi was the highest (189.400 µg/g). The sources of PAHs varied in different regions and combustion processes played a leading role. PAHs exposure through ingestion and dermal contact was more carcinogenic than inhalation. The incremental lifetime cancer risk model indicated that children lived in Shanxi were found in the highest health risk coupled with the highest BaPE concentration (54.074 µg/g). Although PAHs concentrations of indoor dust showed a downward trend from 2005 to 2018, indoor environmental sanitation should be improved with multidisciplinary efforts. Health standard should be possibly established to minimize children exposure to PAHs in indoor dust in China.

few studies on indoor PAHs pollution and risk analysis. The objectives of this study are: (1) to determine the concentrations and profiles of PAHs in indoor dust in China; (2) to distinguish the sources of PAHs in indoor dust in China; (3) to assess health risk on Chinese children via inhalation, dust ingestion and dermal contact using the incremental lifetime cancer risk (ILCR) standard model established by US EPA [19] ; (4) to enrich the research of indoor PAHs in China and to provide basic information for indoor environment and health management of residents.

Data Sources
Papers were identified through searching PubMed, Science Direct, Web of Science, China National Knowledge Infrastructure and WanFang online electronic database, using relevant terms for PAHs and indoor dust. The Boolean operators "AND" and "OR" were used to combine topic areas, such as [(indoor) OR (house) OR (residential) OR (residence)] AND [(China) OR (Chinese)] AND (dust) AND (PAHs). A total of 2533 articles were retrieved from January 1975 to December 2020. The flow chart of qualified literature screening process is shown in fig.1. According to the relevant requirements of this study, articles including raw numeric values of PAHs in indoor dust in China were retained, and review articles were excluded.

Inclusion Criteria and Exclusion Criteria
Inclusion criteria: (1) Research on PAHs concentrations in indoor dust in China; (2) raw numeric of PAHs concentrations in indoor dust can be extracted; (3) the unit of PAHs concentrations can be converted into μg/g.
Exclusion criteria: (1) Research on indoor dust concentration based on simulated model; (2) review articles; (3) research on air detection of airborne dust; (4) research on forced intervention measures during the experiment.

Data Standardization
The types and sample sizes of PAHs involved in published studies are different. Direct calculation of the average concentration will lead to deviation. In order to make the results more accurate, the weighted average concentration is used in this study. The specific formula is shown in (1): C n represents the raw concentration of each sample in the original literature; N n represents the sample size in the original literature; C t represents the weighted average concentration of sample.

PAHs Health Risk Assessment
ILCRs model was used to evaluate the latent cancer risk of exposure to indoor dust [23] . Humans expose to indoor dust PAHs via dermal contact, ingestion and inhalation. The ILCR (no unit) is calculated on the basis of three contact routes [23] and the following standard models from the US EPA are used: The parameters in the child model for the risk assessment are shown in table S1. TEQ BaP represents the summation of PAH concentrations calculated according to the toxic equivalent factor (TEF) and the toxic equivalent of BaP [24] . TEQ BaP is calculated by formula (5): Where TEQ BaP is total toxic equivalent concentration of BaP (μg/g); C i is the mass concentration of the initial PAHs (μg/g); TEF i is its toxic equivalency factor [24] (Nap, Acy, Ace, Fl, Phe, Flu and Pyr are 0.001. Ant, Chr and BghiP are 0.01. BaA, BbF, BkF and IcdP are 0.1. BaP is 1. DahA is 5) [22] .
BaP is the first and strongest discovered environmental carcinogenic PAHs, and other HWM PAHs (BaA, BbF, BkF, IcdP and DahA) also have carcinogenic potential. The BaP equivalent (BaPE) is a useful index to assess the potential toxicity of PAHs in sediments, which can be calculated according to the following equation (6) [25,26] .

Statistical Analysis
Excel2010 was used for data entry, data sorting and chart drawing, and SPSS21.0 was used for statistical analysis. Non-parametric Kruskal-Wallis H test was used for analyzing the difference in the level of risk among the three exposure routes. Spearman correlation analysis was used for analyzing the relationship between BaPE and ILCR Ingestion , ILCR Inhalation , ILCR Dermal .

Characteristics of PAHs Concentrations in 11 Provinces
The concentration of different PAHs varied widely across the country ( fig. 2). The concentration of Flu in Shanxi province was the highest (189.400 μg/g) and the concentrations of Ace, Acy and Fl were relatively A n h u i H e il o n g ji a n g H u n a n J ia n g s u G u a n g d o n g S h a n x i  Sam-year means sampling year. Sam-size means sampling size. ND means under the detection limit. LN&I-Mongolia represents Liaoning and Inner Mongolia.
low. Fig. 3 shows the analysis of the weighted average concentration of indoor dust ∑ 16 PAHs. The highest residential ∑ 16 PAHs concentration was in Shanxi (2111.667 μg/g), and the lowest was in Hong Kong (1.505 μg/g). The national average concentration of ∑ 16 PAHs was 25.696 μg/g in this study. Only the concentration in Shanxi and Guangdong was higher than the national level, and over standard rate was 18.18%. The composition profiling of PAHs was different in 11 provinces/region ( fig. 4). There were mainly 4 rings PAHs (83.13% and 59.22%) in Guangdong and Shanxi, respectively, 3 rings PAHs (63.53%) in Hong Kong, 3 and 4 rings PAHs (77.97%, 67.21% and 60.92%) in Liaoning, Jiangsu and Inner Mongolia, respectively.

Concentration of Indoor Dust PAHs
PAHs concentration in Shanxi province from one study was significantly higher than that in other regions. Therefore, the data of Shanxi province (2005)

Source Analysis of PAHs
HMW PAHs are mainly originated from hightemperature combustion process and LMW PAHs are chiefly originated from low or moderate temperature combustion [42] . Table S2 shows the principle of PAHs source classification. In general, combustion induces the production of relatively higher concentrations of (Continued from the last page) H e i l o n g j i a n g H u n a n J i a n g s u G u a n g d o n g S h a n x i Fig. 3 Comparison of ∑ 16 PAHs concentrations in 11 provinces/ region with national concentrations ∑COMB; therefore, a large proportion of ∑COMB is a characteristic of PAHs that originated from combustion processes [43] . Table S3 and 7E), PAHs may originate from petroleum combustion.

Health Risk Assessment of PAHs Exposure
Human exposure to PAHs may occur via ingestion, inhalation, and dermal contact [44] . The most sensitive subpopulation is young children because of their handto-mouth activity, whereby contaminated dust can be readily ingested [45] . The ILCR formula with default parameters was used to identify the different exposure pathways for children to evaluate the health risk. Table 2 presents the calculated ILCR values. Previous studies indicated that an ILCR value of 10 −6 generally represented an acceptable level, an ILCR value from 10 −6 to 10 −4 indicated a potential human carcinogenic risk, and an ILCR value higher than 10 −4 indicated a serious carcinogenic risk [46,47] . The calculated results indicated that PAHs exposure posed a potential human carcinogenic risk in Shanxi, Guangdong and Anhui. The ILCR value was the highest in Shanxi province and the lowest in Hunan province. And the cancer risk levels via ingestion, dermal contact and inhalation were 5.301×10 -8 vs. 2.572×10 -4 , 6.608×10 -8 vs. 3.207×10 -4 , and 1.028×10 -12 vs. 4.988×10 -9 , respectively. Since the level of risk among the different exposure pathways was non-normal distribution, the non-parametric Kruskal-Wallis H test was performed to compare the difference. The results indicated that the health risk level was different among different exposure routes. All these results indicated that ingestion and dermal contact exposure was more carcinogenic than inhalation [22] .
The concentration of BaPE ranged from 0.030 μg/g to 54.074 μg/g. Spearman correlation analysis revealed there was a positive correlation between BaPE     and ILCR Ingestion , ILCR Inhalation , ILCR Dermal . Therefore, there was the highest health risk level in Shanxi where BaPE concentration was also at the highest level ( fig.  8). There were low BaPE concentration and low health risk level in Hunan.

DISCUSSION
This article reviewed the literature on PAHs contamination in indoor dust in 11 provinces across China. PAHs concentrations varied greatly across the country, and they were the highest in Shanxi and the lowest in Hong Kong. The concentrations in Shanxi and Guangdong were higher than national level in this study. The source of PAH varies in different regions, but almost originates from combustion processes.
PAHs exposure through ingestion and dermal contact were more carcinogenic than inhalation. Shanxi had highest BaPE concentration and health risk level.
In China, few papers evaluated the PAHs contamination in dust, especially in indoor dust [48] . While in the other countries, a large number of papers were related to indoor dust PAHs, little is known about the impact of indoor pollution sources on personal PAHs exposure [49] . In recent years, the number of published articles related to indoor dust of PAHs increased exponentially. However, in this paper, 17 included studies were published from 2007 to 2020, and there were no data records before 2007, indicating that indoor dust PAHs draw more attention just in recent years.
Sixteen PAHs have been prioritized for control by the US EPA because of their carcinogenic and mutagenic properties [15] . These 16 PAHs were detected and analyzed in many studies. The concentrations of Ace, Acy and Fl were relatively low in 11 provinces/region and Flu in Shanxi was the highest. The concentration of PAHs varied greatly across the country, with the lowest average concentration in Hong Kong and the highest in Shanxi. Shanxi is the leading province in coal production in China, and coal is commonly used indoors for cooking and heating in this region [27] . The major products of thermal decomposition of coal include PAHs as well as related nitrogen-and sulfur-containing PAHs [50,51] .
The national average concentration of ∑ 16 PAHs was approximately 25.696 μg/g in this study. It was higher than those of settled house dust (SHD) samples collected from homes in urban areas of San Diego County, CA, USA (0.163-4.390 μg/g; median, 0.990 μg/g) [14] but was lower than those in Ohio, USA (11.1-513 μg/g; median, 47.4 μg/g) [9] , and Texas, USA (1.12-341 μg/g; median, 28.8 μg/g) [10] . The PAHs concentrations of indoor dust tended to decrease with time approximately. It may be because people's awareness of environmental protection has been raised, and the national governance of indoor and outdoor environment has been done. However, due to the lack of consecutive surveillance data of PAHs, the actual changing trend of PAHs can't be observed. PAHs diagnostic ratios have been used to identify different sources that contribute PAHs to environmental samples [52] . PAHs in most samples mainly originate from combustion processes. Our results indicated that petroleum combustion was the origin of PAHs in Hong Kong, Inner Mongolia, and Guizhou. PAHs may originate from combustion of coal, grasses, and wood in Henan, Anhui, Shanxi, Guangdong and Heilongjiang. The PAHs source of indoor dust in other provinces may originate from complex sources. Vehicle restriction and switching clean energy can effectively reduce indoor PAHs pollution levels. However, this method is not an exact method of distinguishing sources and only provides a qualitative estimation [53] . If we want to identify the sources of PAHs in indoor dust more clearly, we need more data to analyze or use other analytical methods such as Principal Component Analysis. Gustafson et al [54] concluded that the concentration of PAHs of households using wood burning for heating in winter in Sweden was significantly higher. Ansari et al [55] found that indoor PAHs mainly originated from cooking fuel in rural India such as wood. Abbasi et al [56] showed that indoor PAHs may come from petroleum in developing and oil-rich countries such as Iran. So the source of PAHs in indoor dust is related to the local industrial structure and the lifestyle of residents.
BaP is the first environmental carcinogen to be discovered with highly carcinogenic PAHs. The BaPE value is often used as an index to evaluate the toxicity of PAHs. BaPE concentration was the highest in Shanxi which is the leading province in coal production in China and provides as much as one-third of China's coal [27] . High temperature combustion process (such as vehicular exhaust, mining processing activities) [35] and living habits (such as cooking, heating, smoking) [57] may make contributions to the high BaPE concentration.
At present, there is not a perfect standard for health risk assessment of PAHs in indoor dust in China. The results of exposure assessment of PAHs in house dust to children is quantitative and therefore is much more comparable than health risk of different endpoints. The ILCR model is widely used now [23] . The ILCR is taken as an ensign to identify the age-specific potential cancer risks in the study of human exposure to environmental PAH pollution sources [54,58] . Results indicate that dermal contact and ingestion exposure are more carcinogenic than inhalation [22] . Dermal contact is the most dominant exposure route, which is easily to be ignored [34] . Due to behavioral characteristics such as hand-to-mouth activities and food handling, non-dietary inhalation of contaminants is potentially the main route of exposure for children. Dermal contact and ingestion exposures are also important route for children and are associated with behaviors such as crawling on the floor and contact with dirt and grass [59] . In addition, early development of organ, nervous, and immune systems may enhance the carcinogens sensitivity in children [56] . However, the results calculated based on ILCRs model still have certain limitations. Exposure by ingestion, inhalation, or dermal contact is according to the size of the dust particles. Li et al [39] found that inhalation was the main exposure route for LWM PAHs, and ingestion was the major route for HWM PAHs. Most current research is unaware of this problem. So, the accuracy of the assessment results will be affected to some extent [37] . Therefore, relevant standards and models for health risk assessment of PAHs in indoor dust need to be improved to be more suitable for the actual conditions of indoor environment in China, and provide basic information for residents' protection.
There are several limitations in this paper. Although we searched literature of nearly 45 years, raw numeric concentrations of PAHs in indoor dust in other provinces were lack to estimate national concentration of PAHs in indoor dust. In addition, different instruments and methods were chosen to analyze dust samples in the 17 studies, resulting in different detection limits of PAHs. PAHs were detected with different methods including gas chromatography-mass spectrometry [28,33,35,[37][38][39] , high performance liquid chromatography [27,29,30,34] , gas chromatograph equipped with a flame ionization detector [31] . Moreover, the sampling requirements or sampling sites were not consistent in these papers. The influence of these confounding factors existed and would bring some bias on the results in this study to some extent.
In conclusion, this study analyzed public literature in the last 45 years to explore PAHs pollution characterization in indoor dust nationwide. PAHs are widely found in indoor dust. There are differences in PAHs concentrations (they were the highest in Shanxi province) and sources in indoor dust at different provinces. Although there is a downward trend of PAHs over time and the indoor environment condition has been greatly improved, its negative impact can't be ignored. Especially children as sensitive subpopulation, efforts should be conducted to prevent and control their dermal contact and ingestion exposures. There are rare articles similar to this study. Therefore, in our opinion, the study on PAHs in indoor dust from Chinese households should be further strengthened.

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