Introduction

Heavy metals are prevalent in modern cities due to rapid urbanization and industrial development (Liu et al. 2016). The number of cities in China grew from 122 in 1978 to 655 in 2011, and the proportion of the population living in urban areas rose from 17.9% of the total population in 1978 to 51.3% in 2011 (China Statistics Press 2011, 2012). The dense levels of habitation, centralized vehicle emissions, increasing amounts of industrial wastes, domestic emissions, and the weathering of building and pavement surfaces has led to continuous and increased emissions of heavy metals. Once released, these emissions can spread throughout a city and eventually deposit into surface soils (Peng et al. 2012). The accumulation of heavy metals in urban soils is of increasing concern due to the potential health risks and detrimental effects on soil ecosystems via long-term exposure, even at very low concentrations.

Cities are the most densely populated areas in China. The intake of heavy metals via soil-crop systems in agricultural areas has been widely studied, with oral ingestion considered to be the predominant pathway for human exposure to heavy metals in soil. Although urban soils are not used for farming, pollutants in urban soils can be easily transferred into humans through non-dietary ingestion, inhalation, or dermal contact. During the past two decades, heavy metal pollution in urban areas has become a serious concern due to rapid urbanization and industrialization. A large-scale survey of urban park soils compared to rural soils in Hong Kong revealed that the mean concentrations of Cu and Zn in urban soils were at least four and two times higher than those of rural soils, respectively, while the mean Pb concentration of urban soils was an order of magnitude higher than that of rural soils (Wong et al. 2006). Exposure to metal-contaminated soil and dust through skin contact and hand-to-mouth contact can adversely affect human health, particularly through unintentional uptake by children in playgrounds and city streets (Saeedi et al. 2012). Lead is a non-essential element for humans, and excessive intake can damage the nervous, skeletal, circulatory, enzymatic, endocrine, and immune systems (Dean et al. 2017). Moreover, chronic exposure to Cd can have adverse effects such as lung cancer, pulmonary adenocarcinomas, prostatic proliferative lesions, bone fractures, kidney dysfunction, and hypertension, while the chronic effects of As exposure include dermal lesions, peripheral neuropathy, skin cancer, and peripheral vascular disease (Żukowska and Biziuk 2008, Lin et al. 2016). Therefore, recognizing the spatial distribution and human health risk levels of heavy metals in urban soils in China is vitally important.

Many studies of heavy metal contamination in urban soils have been conducted in several Chinese cities during the past decade, which have showed heavy metal pollution levels in quite few Chinese cities. However, quantitative data on heavy metal concentrations, their contamination levels, and particularly their effects on human health have not been systematically gathered, accessed, and compared. A comprehensive nationwide assessment of heavy metal pollution in urban areas in China is urgently needed. Therefore, the purpose of this study was to assess, on a national scale, the pollution levels and health risks of heavy metals in the soils of urban areas in China. The objectives of the study were (1) to evaluate heavy metal contamination levels in urban soils in China, (2) to identify which heavy metals in urban soils are of most concern, and (3) to assess the health risks posed by heavy metals in urban soils in China.

Methods

Data collection

A number of studies related to heavy metal pollution in soils from urban areas in China between 2006 and 2016 were systematically reviewed. Eight heavy metals were considered: Pb, Cd, Cr, Cu, Zn, Ni, As, and Hg, all of which are defined as priority heavy metals by the USEPA. Literature sources were obtained from Web of Science, Google Scholar, Science Direct, and Springer using the search terms “urban soil,” “heavy metal,” “health,” “risk,” and “China.” Data from 32 cities were gathered and analyzed. The urban areas investigated were located in 23 provinces throughout China and included megacities, typical industrial cities, and small cities from across the country (Fig. 1). Table S1 lists the details recorded from the literature, including sampling sites, number of samples, sampling time, soil depth, and references.

Fig. 1
figure 1

A schematic map of China reporting the cities reviewed in this study

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Statistical analysis

Statistical analyses were conducted using Microsoft Excel (Microsoft Inc., Redmond, WA, USA) and Origin 9.0 (OriginLab Corporation, Northampton, MA, USA). Heavy metal pollution levels were assessed using enrichment factors (EFs) and the geoaccumulation index (I geo). According to Sutherland (2000), EF values are calculated by the following equation:

$$ \mathrm{EF}={C}_{\mathrm{i}}/{C}_{\mathrm{B}} $$

where C i is the ith heavy metal concentration (mg kg−1) in the soils, and C B is the background values (mg kg−1) of ith heavy metal in the soils, six contamination categories are generally recognized on the basis of the enrichment factors: EF ≤ 1 indicates no pollution.

  • 1 < EF < 2 indicates slight pollution.

  • 2 ≤ EF < 5 indicates moderate pollution.

  • 5 ≤ EF < 20 indicates significant pollution.

  • 20 ≤ EF < 40 indicates strong pollution.

  • EF ≥ 40 indicates extremely strong pollution.

The I geo is a geochemical criterion introduced by Müller (1969), which can be used to evaluate soil contamination by comparing the differences between current and preindustrial concentrations. Unlike other methods of pollution assessment, the I geo takes the natural diagenesis process into account, making the assessments more practical. The I geo is calculated using the following equation:

$$ {I}_{\mathrm{geo}}={\log}_2\left({C}_{\mathrm{n}}/1.5\times {B}_{\mathrm{n}}\right) $$

where C n is the measured concentration of the heavy metals in soil (mg kg−1), B n is the geochemical background value of the corresponding heavy metals (mg kg−1), and the coefficient 1.5 is used due to potential variations in the baseline data (Solgi et al. 2012). According to Müller (1969), the I geo consists of seven classes. The corresponding relationships between I geo and the pollution level are as follows: unpolluted (I geo ≤ 0), unpolluted to moderately polluted (0 ≤ I geo ≤ 1), moderately polluted (1 ≤ I geo  ≤ 2), moderately to heavily polluted (2 ≤ I geo ≤ 3), heavily polluted (3 ≤ I geo ≤ 4), heavily to extremely polluted (4 ≤ I geo ≤ 5), and extremely polluted (I geo ≥ 5).

The mean, maximum, minimum, and standard deviations of the heavy metal concentrations reported in the various urban areas were collated. Additionally, the percentages of the urban areas where soil concentrations complied with soil standards were calculated.

Human health risk assessment

Exposure assessment

Human health risk assessment is the process of estimating the nature and probability of adverse health effects in humans who may be exposed to chemicals in contaminated environmental media. In general, individuals are exposed to hazardous contaminants through three pathways: oral ingestion, inhalation, and dermal contact, which can be estimated according to the Exposure Factors Handbook (USEPA 1997). However, for heavy metals in soil, oral ingestion, and dermal absorption are considered the main exposure pathways (Fryer et al. 2006; Qu et al. 2012). For these two pathways, the average daily intake (ADI) of chemicals in soils is calculated using the following equations:

$$ {\displaystyle \begin{array}{c}{\mathrm{ADI}}_{\mathrm{oral}}=\frac{C\times {\mathrm{IR}}_{\mathrm{ing}}\times \mathrm{EF}\times \mathrm{ED}}{\mathrm{BW}\times \mathrm{AT}}\times {10}^{-6}\\ {}{\mathrm{ADI}}_{\mathrm{dermal}}=\frac{C\times \mathrm{SA}\times \mathrm{SA}\mathrm{F}\times \mathrm{ABS}\times \mathrm{EF}\times \mathrm{ED}}{\mathrm{BW}\times \mathrm{AT}}\times {10}^{-6}\\ {}{\mathrm{ADI}}_{\mathrm{inh}}=\frac{C\times {\mathrm{IR}}_{\mathrm{inh}}\times \mathrm{EF}\times \mathrm{ED}}{\mathrm{PEF}\times \mathrm{BW}\times \mathrm{AT}}\end{array}} $$

where C is the concentration of the heavy metals in soil (mg kg−1); IRing is the ingestion rate (mg day−1), 100 mg kg−1 for adults and 200 mg kg−1 for children (USEPA 2002); EF is the exposure frequency, 350 days year−1 (USEPA 2002); ED is the exposure duration, 30 years for adults and 6 years for children(USEPA 2002); SA is the exposed skin area, 5700 cm2 for adults and 2800 cm2 for children (USEPA 2002); SAF is the skin adherence factor, 0.07 mg cm−2 for adults and 0.2 mg cm−2 for children (USEPA 2002); ABS is the dermal absorption factor, 0.001 for adults and children (USEPA 2002); PEF is the particle emission factor that relates the concentration of a pollutant in the soil to the concentration of a respirable particle in the air due to fugitive dust emissions from contaminated soils, 1.36 × 109 m3 kg−1 (USEPA 2002); BW is the body weight, 70 kg for adults and 20 kg for children (USEPA 2002); and AT is the averaging time: for non-carcinogens, ED × 365 days and for carcinogens (As, Cr, and Ni), 70 (lifetime) × 365 days (USEPA 2002).

Non-carcinogenic risk assessment

Non-carcinogenic hazards are typically characterized by the hazard quotient (HQ), which is calculated as the ratio of the average daily dose and a reference dose (RfD). The equation is as follows:

$$ \mathrm{HQ}=\frac{\mathrm{ADI}}{\mathrm{RfD}} $$

where RfD is the reference dose of the ith heavy metal (mg kg−1 day−1) as listed in Table 1. This is the maximum allowable level of a heavy metal that has no harmful effects on human health. In this study, the RfDing (RfDing is the reference dose via non-diary ingestion for heavy metals, mg kg−1 day−1) value for both adults and children in soil was taken into consideration. Because there are no reference doses for evaluating dermal absorption exposure to chemicals, the USEPA (2002) provides a method to assess dermal risk, i.e., multiplying the soil oral reference dose by a gastrointestinal absorption factor.

Table 1 Summary of reference dose (RfD) and cancer slope factor (SF) of heavy metals through oral, dermal, and inhalation pathways
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To assess the overall non-carcinogenic effects posed by multiple chemicals, the sum of the HQ values of all chemicals is expressed as a hazard index (HI) as follows:

$$ \mathrm{HI}=\sum {\mathrm{HQ}}_{\mathrm{i}}=\sum \frac{{\mathrm{ADI}}_{\mathrm{i}}}{{\mathrm{RfD}}_{\mathrm{i}}} $$

If the HI value is less than 1, the exposed individual is unlikely to experience adverse health effects. In contrast, if the HI value exceeds 1, there is a chance that a non-carcinogenic health effect may occur, with a probability that tends to increase as the HI increases (USEPA 1989).

Carcinogenic risk assessment

Carcinogenic risks are estimated by calculating the incremental probability of an individual developing cancer over a lifetime as a result of exposure to the potential carcinogen. The slope factor (SF) converts the estimated daily intake of a toxin averaged over a lifetime of exposure directly to the incremental risk of an individual developing cancer (USEPA 2011):

$$ \mathrm{CR}=\mathrm{ADI}\times \mathrm{SF} $$

where SF is the carcinogenicity slope factor (mg kg−1 day−1) as listed in Table 1. If multiple carcinogenic contaminants are present, the cancer risk from all chemicals and routes are summed. Risks lying between 1 × 10−4 and 1 × 10−6 are generally considered acceptable (USEPA 1989), while a value exceeding 1 × 10−4 represents a lifetime carcinogenic risk to the human body. The RfD and SF through non-dietary ingestion, dermal contact, and inhalation of the eight heavy metals are also illustrated in Table 1.

Results and discussion

Heavy metal concentrations in urban soils

Table 2 presents the statistical characteristics of the heavy metal concentrations in urban soils from all of the cities investigated in China, together with the background concentrations (BK) in the same areas. Among the cities, the contamination levels of the heavy metals varied over a large range. However, in general, there were relatively high levels of heavy metals in the urban soils of all cities. The median concentrations of Pb, Cd, Cr, Cu, Zn, Ni, As, and Hg were 50.13, 0.37, 73.00, 40.77, 155.33, 31.14, 12.05, and 0.17 mg kg−1, and the ranges were 18.20–25,380.58, 0.11–6.90, 22.40–228.89, 19.20–112.14, 52.00–1964.12, 11.10–107.42, 8.49–39.88, and 0.02–0.93 mg kg−1, respectively. The percentages of the urban areas exceeding the Chinese Environmental Quality standards for soils (CEPA 1995) for Pb, Cd, Cr, Zn, Ni, and Hg were 6.50, 51.90, 7.70, 36.70, 21.70, and 35.70%, respectively. In general, Cu and As levels did not exceed the Chinese standard values. To contextualize the heavy metal concentrations in urban soil, the background values in the soils of different cities are also listed in Table 2. The concentrations of heavy metals in most urban areas exceeded their background values. With the exceptions of Lhasa City, Xining City, and Urumqi City, there were relatively serious levels of Cd, Hg, Pb, and Zn contamination in urban areas compared to their background values (Table 2).

Table 2 Statistical summaries of heavy metal concentrations (mg kg−1) in urban topsoil in Chinese cities
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High heavy metal concentrations were mainly found in the soils of industrial cities and rapidly developing cities, such as Baoji, Xi’an, Shenyang, Anshan, Taiyuan, Qingdao, and Dongguan City. Industrial drainage, waste, and exhaust gases containing heavy metals are discharged into the environment of these cities, which has resulted in elevated concentrations of heavy metals in urban soils over an extended period of time (Luo et al. 2012). Considering Dongguan City as an example, the agricultural acreage in the city was 30,816 ha in 2004, and average concentrations of Cd, Cu, Ni, and Zn in agricultural soils were 0.12, 20.30, 21.60, and 53.50 mg kg−1, respectively (Wu et al. 2015). By 2012, the area of arable land in Dongguan had decreased to 24,800 ha, which represented a decline of 19.5% compared with 2004. Over the same time period, the levels of Cd, Cu, Ni, and Zn in soils from urbanized areas of Dongguan increased by 28.60, 33.00, 16.20, and 55.90%, respectively. Therefore, in urban areas, industrial emissions may be an important factor leading to the accumulation of heavy metals. According to a study of Peng et al. (2013) on how urbanization affects the soil concentrations of heavy metals, the exposure of surface soils to urban air was the primary factor that determined the contents of heavy metals in urban soils. Up to 59.0% of the anthropogenic heavy metals in urban soils resulted from citywide atmospheric depositions. Around 15.3% derived from other input loads including indoor volatilization, irrigation, fertilization, and waste discharge, which significantly correlated to local road density and population density.

Heavy metal pollution levels in urban soils

The contamination levels of heavy metals in urban soils were assessed using the EF. The EF can be used to evaluate the degree of anthropogenic influence on soil contamination by heavy metals and to differentiate between metals originating mainly from human activities and those from natural sources (Pan et al. 2016). Table 3 lists the EF values for the eight heavy metals in each investigated city, and Table S2 lists the percentage area of different heavy metal contamination levels in the urban soils of the same cities. Heavy metals were widespread in the urban soils, with a large range of EF values, confirming the variability of urban soil properties and heavy metals pollution in China. Nearly all the EFs for the eight heavy metals in urban soils in the cities investigated were higher than 1 (Table 3). This indicates that the urban soils in these cities were contaminated by metals on different levels, deriving from anthropogenic sources. Among the cities investigated, 6.45, 30.77, 9.68, 13.33, 39.13, 27.77, and 29.41% of the urban areas can be classified as not polluted by Pb, Cr, Cu, Zn, Ni, As, and Hg, respectively; 32.26, 29.63, 53.85, 35.48, 33.33, 43.48, 64.71, and 14.29% of the urban areas was slightly polluted by Pb, Cd, Cr, Cu, Zn, Ni, As, and Hg, respectively; 41.10, 25.93, 15.38, 48.39, 43.33, 13.040, 11.76, and 42.86% of the urban areas was moderately polluted by Pb, Cd, Cr, Cu, Zn, Ni, As and Hg, respectively; 16.13, 33.33, 6.45, 6.67, 4.35, and 35.71% of the urban areas was significantly polluted by Pb, Cd, Cu, Zn, As, and Hg, respectively; 7.41, 3.33 and 7.14% of the urban areas was strongly polluted by Cd, Zn, and Hg, respectively; and 3.23 and 3.70% of the urban areas was extremely strongly polluted by Pb and Cd, respectively (data summarized in Table S2). Extremely strong pollution by Pb was found in Baoji City, and moderate pollution by Pb and extremely strong pollution by Cd were found in Changsha City, respectively. In Baoji City, the samples were collected from the center of an industrial area, where a battery manufacturing plant was located, with Pb being the main primary raw material in battery production.

Table 3 EFs of heavy metals in urban soils in the investigated cities in China
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In more than half of the cities investigated, urban soils were slightly or moderately polluted with regard to most heavy metals. The most severe pollution was related to Cd. Approximately one-third of the cities investigated had significant pollution levels for Cd, indicating that it is the element of most concern in the urban soils of China. The most significant pollution of urban soils by Cd was found in developed cities (Hangzhou and Lishui City) and industrial cities (Shenyang and Changsha City), and these results are consistent with those of a study conducted by Chen et al. (2015), who also reported that Cd is the element responsible for the most serious contamination of urban soils in China. However, for those cities contaminated by Cd in urban soils, the sources always varied for different cities. For example, in the western part of the Shenyang Tiexi Industrial District, hotspots of high Cd concentration were identified. The notorious Shenyang Smelting Plant was located in this area for 64 years until it was closed by the government because of heavy pollution in 2000 (Sun et al. 2010). These findings suggest that the large-scale smelting operations greatly contributed to the urban soil contamination in the region. In another study of Xi’an, Cd showed a contamination hotspot located between the second and third ring roads in the western section of the downtown area. The results pointed out that Cd was mainly derived from industrial sources, traffic, and urban wastes in this area of the city (Chen et al. 2012). In contrast to the patterns observed in Shenyang and Xi’an, a high Cd concentration was found across the whole city of Changsha along the Xiangjiang River, upstream of which operated a large-scale mining for Pb, Zn, and Cd metals (soil Cd mean concentration, 10.34 mg kg−1; maximum value, 219.9 mg kg−1) (Zhang et al. 2012). This observation suggested that excess soil Cd was mainly derived from mining activities. Thus, the increased anthropogenic inputs due to the rapid economic development (especially since the late 1970s) and the establishment of industrial operations, as well as the rapid urban expansion in China, have substantially increased discharges of Cd.

In nearly half of the cities investigated, there were moderate pollution levels for Pb, Cu, Zn, and Hg. In the survey conducted by Chen et al. (2015), Pb and Hg were the elements that made the second and third largest contributions to the overall heavy metal pollution of urban soils in China. Because of its significant potential ecological toxicity, Hg is of particular concern (Hakanson 1980). Several studies have reported that fossil fuel emissions in China are the largest anthropogenic Hg source (Jiang et al. 2006), which makes China the largest Hg emitter in the world (Pacyna et al. 2010). These Hg emissions from coal combustion may be deposited and accumulated in topsoil and enter urban areas through atmospheric depositions.

As can be seen from the list in Table 4, the I geo values of the eight heavy metals in the urban soils in China varied widely, ranging from uncontaminated to extremely contaminated levels. The median I geo values for Cd and Hg all fell into the category of “moderately to heavily polluted.” In particular, the 55th percentile I geo values for Cd and the 65th percentile I geo values for Hg were higher than 1, suggesting that more than half of the urban soils were moderately to heavily contaminated by Cd and Hg. From Table 4, it can be also noticed that the I geo values for Cd in 37% of samples and for Hg in 26% of samples were above the moderately to heavily contaminated level. The I geo values for Pb, Cu, and Zn all fell into the “unpolluted” to “moderately polluted” categories. For Cr and Ni, the soils in the cities investigated can be classified as “unpolluted.” It has been reported that agricultural soils in China are also heavily contaminated by Cd and Hg; however, the degree of pollution is lower than in urban soils (Wei and Yang 2010). Table 5 shows the median values of pollution index and pollution level of the eight heavy metals in urban soils in China. The pollution levels assessed by the EFs and I geo present small differences, for Cd, Ni, and As. EFs suggest slight pollution, while those of I geo suggest “no pollution.” This is mainly due to the fact that I geo takes the natural diagenesis process into account, making the assessments more practical.

Table 4 Geoaccumulation indexes of heavy metals in urban soils in the investigated cities in China
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Table 5 Median values of pollution index and pollution level of eight heavy metals in urban soils in the investigated cities in China
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With regard to the geographical location of the cities investigated, based on the EF factors and I geo, the pollution levels in eastern China were much higher than in western regions, especially in the southeastern coastal cities. The region located to the east of the Aihui-Tengchong line has a high population density, accounting for 90% of the population, with just 40% of the country’ land area. The eastern cities have undergone rapid urbanization and industrialization in the last decades, since China implemented an open-door policy in 1978. Industrial drainage, wastes, and exhaust gases containing heavy metals are discharged into the environment in these areas (Luo et al. 2012). On the basis of Tables 3 and 4, the pollution level of heavy metals in the urban soils of different types of cities followed the order: industrial based cities > more developed cities > metropoles > underdeveloped cities. The heavy metal accumulation in urban soils in metropoles such as Beijing, Shanghai, and Shenzhen City were lower than in more developed cities, especially in recent years. Although cities such as metropolitan Beijing City, Shenzhen City, and Guangzhou City have experienced extremely fast rates of development and construction within the past 30 years, in the last 5 years, local governments have implemented comprehensive strategies to prevent and supervise soil pollution by heavy metals, which have effectively controlled urban soil contamination.

Non-carcinogenic risk assessment

Geostatistical techniques facilitate the identification of pollution sources and the spatial distribution of pollutants. They are ideal for evaluating interactions between heavy metal emissions and the receiving environment based on spatial information regarding emission sources, processes governing pollutant distribution, and population density (Martínez-Murillo et al. 2017). Figure 2 presents the non-carcinogenic risk via non-dietary ingestion, dermal contact, and inhalation due to heavy metals exposure in urban soils. Due to the high concentrations in soils or low RfD values, Pb appeared to pose significantly higher non-carcinogenic risks to urban residents than the other four heavy metals. For example, the total HQ of Pb for both adults and children accounted for 93.3% of the entire HI value. In contrast, the total HQ for the other four heavy metals accounted for only 6.7% of the entire HI value. The average HQs of heavy metals for the three populations decreased in the order of Pb > Cu > Hg > Zn > Cd.

Fig. 2
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Non-cancer risks values due to Cd, Cu, Pb, Zn, and Hg and cancer risks values due to As, Ni, and Cr in urban soil in Chinese cities

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According to USEPA (2002), if HI < 1, an exposed individual is unlikely to experience an obvious adverse health effect. In contrast, if HI > 1, there is a probability that a non-carcinogenic effect will occur. It can be concluded that the heavy metals in urban soils across China generally have low potential for non-carcinogenic risks to the public, except in some individual cities, such as Baoji City, where the non-carcinogenic risk HI values of Pb were 25.20 and 174.00 for adults and children, respectively. This value is significantly higher than the acceptable health risk for the general population. The HI values of Pb for children due to urban soil contamination in Xi’an City, Dongguan City, Qingdao City, Shenyang City, and Taiyuan City were 1.58, 1.10, 1.70, 0.80, and 0.90, respectively, which were close to, or slightly higher than, the acceptable risk levels, suggesting that in these areas, children may be exposed to potential non-carcinogenic risks via exposure to Pb in urban soil. The non-carcinogenic risks of heavy metal exposure for children are higher than for adults due to their physiological characteristics. For example, children are always more susceptible to a given dose of toxin and likely inadvertently ingest significant quantities of soil due to their finger sucking behavior, which is generally regarded as one of the key exposure pathways for soil metals in children (Zhao et al. 2013).

For children, the HI values of the five heavy metals via non-dietary ingestion were hundreds of magnitudes higher than those via dermal contact and inhalation, and generally followed the order of non-dietary ingestion > dermal contact > inhalation. A similar trend was observed for adults, suggesting that non-dietary ingestion was the predominant exposure pathway for both adults and children, which is consistent with the results of previous studies (Ning et al. 2015; Wei et al. 2015).

Carcinogenic risks assessment

The carcinogenic risks were only estimated for Cr, Ni, and As due to lack of a carcinogenic SF for Pb, Cd, Cu, Zn, and Hg. Figure 2 also presented the carcinogenic risk via non-dietary ingestion, dermal contact, and inhalation due to heavy metal exposure in urban soils. The average carcinogenic risk values of Cr, Ni, and As were 2.86 E−05, 4.06 E−05, and 1.32 E−05 for adults and 3.75 E−05, 5.50 E−05, and 1.84 E−05 for children, respectively. The majority of the carcinogenic risk was around 1.00 E−05, with a few percent of the urban soils having high risk levels in excess of 1.00 E−04, e.g., the cancer risk of Cr (1.050 E−04) and Ni (1.61 E−04) in Taiyuan City for children, and the cancer risk of Ni (1.18 E−04) in Taiyuan City for adults and (1.25 E−04) in Qingdao for children. Risks surpassing 1.00 E−04 are viewed as unacceptable, whereas risks below 1.00 E−06 are not considered to have significant health effects. Risks lying in the range of 1.00 E−06–1.00 E−04 are generally regarded as tolerable. Thus, the risks posed by Ni and Cr in urban soils in Taiyuan and Qingdao City exceeded the acceptable level, indicating that residents in the two cities, particularly children, faced serious potential carcinogenic risks. Among the cities investigated, the cancer risk level decreased in the order Ni > Cr > As for both adults and children. As in the trend for non-carcinogenic risk, the carcinogenic risk level via non-dietary ingestion was significantly higher than the risk level via dermal contact and inhalation, which followed the order of non-dietary ingestion > dermal contact > inhalation for both adults and children. The average carcinogenic risk values among the cities investigated via non-dietary ingestion were 10.03 and 306.18 times greater than the risks via dermal contact and inhalation for adults and 14.95 and 1483.95 times greater for children, respectively.

When heavy metals enter the human body through any of the three main routes (inhalation from air, soil/dust ingestion, and skin contact), a body burden will ensue. Heavy metals are known to accumulate in five human tissues: placenta, umbilical cord blood, blood and serum, hair, and urine. The subsequent human body burden of heavy metals can have an impact on human health and can cause various diseases, e.g., cancers, mental health and neurodevelopment disorders, thyroid dysfunction, and general physical health deterioration resulting from DNA damage and effects on gene expression (Song and Li 2014). Children are particularly sensitive to heavy metals because they experience additional routes of exposure (breastfeeding, placental exposure), undertake high-risk behavior (hand-to-mouth activities in early years, higher risk-taking activity in adolescence), and undergo changes in physiology (higher comparative uptakes and lower toxin elimination rates) (Ma et al. 2016). Excessive heavy metal exposure is known to lead to adverse health effects. According to Han et al. (2007), children’s blood Pb levels in an exposure group were significantly higher than in a control group, and the mean IQ (Intelligence Quotient) of children aged from 3 to 4 years in the exposure area was significantly lower than that of children of the same age in the control area (10.24 vs. 12.92, p < 0.05). Zheng et al. (2012) evaluated the damage caused by Cr, Ni, and Mn exposure on lung function in 144 school children (aged 8 to 13 years) and found that there was a decreased forced vital capacity (FVC) of lung function in boys aged 8–9 years (1859 vs. 2121 ml, p = 0.003).

Although the health risks in most of the cities investigated were within an acceptable range, this was likely due to the young age of the soil, and soil-related risks will increase over time as traffic levels and the population increase. Therefore, environmental protection agencies should pay careful attention to the health risks induced by heavy metals in urban soils, and more work is needed to assess the risks to the residents, especially children, of industrial based cities and more developed inner cities. Heavy metals in urban soils mainly originated from vehicle emissions, the chemical industry, coal combustion, municipal solid waste, the sedimentation of dust, and suspended substances in the atmosphere (Gu et al. 2016). This diverse range of sources makes it difficult to control soil heavy metal contamination, which is a significant environmental pollution issue that has not yet been resolved in China. Several soil pollution incidents have occurred in recent decades and the Chinese government has undertaken measures to improve soil quality. The Chinese State Council issued a plan called “Action on Soil Pollution Prevention and Control” (Soil Ten Regulations) on May 31, 2016 (http://www.gov.cn/xinwen/2016-05/31/content_5078445.htm), which represents a milestone for the control of soil contamination by government. The Soil Ten Regulations required environmental agencies and government to monitor soil heavy metals, and strengthen the supervision and enforcement of pollution control from sources such as industrial and mining enterprises. With the proposed pollution control policies and strategies, soil heavy metal pollution issues in China can be solved effectively; however, several decades may be required for successful solutions to be achieved.

Moreover, cancer risks due to Cr, As, and Ni of urban soil in some cities presented higher than the acceptable level, especially for children, suggesting that in these cities, some prevention measures must be taken, e.g., in the industrial areas with high heavy metal concentration and carcinogenic risk, children’s activities should be reduced. Furthermore, quantitative estimation of source for heavy metal in urban soil also should be conducted, which is an effectively way to reduce heavy metal contamination and human health risks.

Uncertainty analysis

There are tens of metropolis and hundreds of medium-sized cities and small-sized cities in China; therefore, the reviewed cities in this study may not fully represent China’s overall urban soil heavy metal pollution situation. Furthermore, when obtaining heavy metal concentration data, some discrepancies may occur due to variations among different studies, which may impact the consistency of the obtained data. In addition, quantitative human risk assessments also have several inherent uncertainties. First, the exposure parameters are described in the USEPA exposure handbook, and this document may not be suitable to the people of China. There are no published exposure guidelines for non-dietary ingestion, dermal contact, and inhalation for both adults and children in China. Second, the bioaccessibility of trace elements in the human gastrointestinal tract were not taken into account in our study. Instead, we used the common approach of conservatively assuming that 100% of the ingested trace elements were absorbed by the human body. However, several studies have reported the bioaccessibility of trace elements in soil to be typically below 50%, although they can range between 10% (for Cr) and 92% (for Cd) (Cao et al. 2016). However, despite the lack of a completely accurate risk assessment, this study assessed the potential health effects induced by various heavy metals in urban soils based on a well-defined investigation on non-dietary ingestion, dermal contact, and inhalation exposure pathways in China.

Finally, a number of other emerging metal contaminants (e.g. platinum, rhodium, and palladium) should be also considered in future environmental studies since nowadays they are very limited. Most of these studies focused on detection methods, and on the chemical forms of these pollutants in the environment. Anyway, studies on these new contaminants are growing rapidly and should be considered in future surveys on soil heavy metal pollution.

Conclusions

Data regarding heavy metals in soils from 32 Chinese cities were collected for this study via a systematic literature review of English-language databases. Despite its limitations, this study described the overall pollution levels and health risks posed by eight heavy metals (Pb, Cd, Cr, Cu, Zn, Ni, As, and Hg) in urban soils in China. Based on the results of pollution and health risk assessments, it was apparent that the concentrations of heavy metals in urban soils were higher than their background values, with the EFs and I geo values reflecting moderate pollution levels and a strong possibility of being influenced by anthropogenic activity for Cd, Hg, and Pb. More than half of the cities investigated had slight and moderate pollution levels for most heavy metals in urban soils. The pollution levels in eastern cities were much higher than those in western cities. Compared with farmland soil, heavy metal pollution was much higher in urban soils. Heavy metals in the urban soils investigated generally posed low non-carcinogenic and carcinogenic risks to the public; however, the carcinogenic risk to children in some cities (e.g., Baoji City and Taiyuan City) should be given careful attention. The health risks posed by heavy metals via non-dietary ingestion was higher than that posed by dermal contact and inhalation for both adults and children.