Science of the Total Environment 479–480 (2014) 117–124

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Contamination and risk assessment (based on bioaccessibility viaingestion and inhalation) of metal(loid)s in outdoor and indoor particlesfrom urban centers of Guangzhou, China

Minjuan Huang a,b, Wei Wang a, Chuen Yu Chan b, Kwai Chung Cheung a, Yu Bong Man a,c,Xuemei Wang b, Ming Hung Wong a,c,⁎a Croucher Institute for Environmental Sciences and Department of Biology, Hong Kong Baptist University, Kowloon Tong, Kowloon, Hong Kong, Chinab Faculty of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, Hong Kong, Chinac Department of Science and Environmental Studies, The Hong Kong Institute of Education, Tai Po, Hong Kong, China

H I G H L I G H T S

• The average content of Zn was the highest in road dust.• The average content of Pb was the highest in household AC filter dust.• Cr, Ni, Hg and Pb Contamination were significantly elevated in residential houses.• Mobilization of metal(loid)s was significantly correlated from stomach to intestine.• As was observed as the most risky element via ingestion and inhalation of particles.

⁎ Corresponding author at: Croucher Institute forDepartment of Biology, Hong Kong Baptist University, KKong, China. Tel.: +852 2948 8706.

E-mail address: minghwong@ied.edu.hk (M.H. Wong)

0048-9697/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.scitotenv.2014.01.115

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 September 2013Received in revised form 27 January 2014Accepted 28 January 2014Available online 19 February 2014

Keywords:Metal(loid)sOral bioaccessibilityRespiratory bioaccessibilityDaily intakeRisk assessment

Road dust, household air-conditioning (AC) filter dust and PM2.5 were collected to investigate the contaminationofmetal(loid)s (Cr,Mn,Ni, Cu, Zn, As, Cd, Sn, Sb, Hg and Pb) in outdoor and indoor urban environments ofGuang-zhou. Zinc was found to be the most abundant element in road dust and household PM2.5, while the concentra-tion of Pb was the highest in AC filter dust. Enrichment factor (EF) was used to assess the influence of humanactivity on the contamination of these metal(loid)s. Ingestion and inhalation were the two exposure pathwaysapplied for risk assessment. Physiologically based extraction test (PBET) was used to estimate the oralbioaccessibilities of metal(loid)s in road dust and AC filter dust. Respiratory bioaccessible fraction ofmetal(loid)s via household PM2.5 was extracted with lung simulating solution. Household AC filter dust wasmore hazardous to human health than road dust, especially to children. Arsenic was found to be the mostrisky element based on the risk assessment.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

As the sink and source of pollution, urban deposits (road dusts, gullysediments and tunnel ceiling dust) and urban soils are good indicatorsof heavymetal accumulation in the urban surface environment. In addi-tion to outdoor dust, metal(loid)s contained in indoor dust and airborneparticles should be taken into consideration as well when evaluatingurban metal(loid) contamination. The total contents of metal(loid)s inthe street dust and household floor dust have been studied extensivelyaround the world (Al-Khashman, 2004; Chattopadhyay et al., 2003;Hassan, 2012; Li et al., 2001; Shi et al., 2011). Recently, research has

Environmental Sciences andowloon Tong, Kowloon, Hong

.

ghts reserved.

extended to the investigation of re-suspended dust on building surfaces(roofs and window sills) (Kong et al., 2011) and indoor air-conditioner(AC) filter dust (Huang et al., 2012; Kang et al., 2010, 2011; Wang et al.,2013). AC filter dust is referred to as the particles that settle on the filterof air conditioner through air current effect. The AC filter dust is hypoth-esized to be less than 100 μminparticle size, which can be re-suspendedfrom settled dust. The re-suspended particles will adhere onto the sur-face of food, skin, toys and furniture and be ingested by humans, espe-cially by children (Butte and Heinzow, 2001). However, the fineportion of particles may pass through the AC filters rather than settleon it. Hence, the particles settling on the AC filter represent the coarseportion of re-suspended dust particles more than the fine portion.

Dust plays an important role on human health due to the complexchemistry and the possibility of re-emission. In particular, the indoorparticles can be muchmore hazardous to human health, as people usu-ally spend more than 70% of their time indoors (Maertens et al., 2004).

118 M. Huang et al. / Science of the Total Environment 479–480 (2014) 117–124

Long-term exposure to particles contaminated with metal(loid)s maypose human health risks, such as renal dysfunction (Staessen et al.,1994), osteoporosis, incidence of bone fractures (Staessen et al., 1999)and lung cancer (Verougstraete et al., 2003).

Health risk assessment of exposure to total contents of metal(loid)sin soil and urban dust has been attempted in many different cities, suchas Luanda, Angola (Ferreira-Baptista and De Miguel, 2005), Beijing,China (Khan et al., 2008), Hulodao, China (Zheng et al., 2010) andShanghai, China (Shi et al., 2011). In general, these studies identifiedoral ingestion as themost critical exposure route to coarse dust particlesfor humans, compared with inhalation and dermal contact. The coarseparticles can be directly swallowed or finally reach the gastro-intestinal tract after a short stay in tracheal and bronchial regionsthrough inhalation (Butte and Heinzow, 2001). However, few of thestudies mentioned above employed bioaccessibility to risk assessment.

Physiologically based extraction test (PBET), an in-vitro gastrointesti-nal method, has been extensively used to estimate the oral bioaccessibi-lity of metal(loid)s in the stomach and intestinal tract (Turner andHefzi, 2010; Turner and Ip, 2007). The oral bioaccessible fraction of met-al(loid)s, referred to as the amount that is released during gastro-intestinal digestion thereby available for absorption, could be employedto estimate the daily intakes and allow risks to be characterized (Turnerand Ip, 2007). Different from coarse particles, more than 80% of the parti-cles smaller than 2.5 μmcan reach the pulmonary alveoli after inhalation,where they can be deposited and stay for months to years (Falta et al.,2008). Accordingly, the lung serum stimulant should be selected to ex-tract the potential respiratory bioaccessible fraction of metal(loid)scontained in PM2.5 (Hodgson et al., 2002; Voutsa and Samara, 2002).

The Pearl River Delta (PRD) region is one of the largest metropolitanregions in south China, where the mega cities and a number of recentlyestablished urban centers are located with concentrated stationary andmobile pollution sources (power plants, factories and traffic emissions)(Shao et al., 2006; Zhang et al., 2008). Guangzhou (22″26′–23″56′N,112″57′–114″03′E) is one of the most densely populated city in thePRD region, with a population of over 10 million and an area of7545 km2 (Zheng et al., 2011). The contaminations of Cu, Zn, Cd andPb in the urban deposits of Guangzhou are evident (Duzgoren-Aydinet al., 2006). However, there is a lack of information of other trace ele-ments contained in urban dust from Guangzhou, such as As, Cr, Sn andSb, as these elements may pose potential health risks to residents. Inhouses, air conditioners are widely used during the summer, and there-fore AC filter dust would provide abundant information about the in-door atmosphere and environment. Furthermore, the indoor airborneparticles, such as PM2.5 levels, in urban sites of Guangzhou have beenfound to frequently exceed the US National Ambient Air Quality Stan-dard (NAAQS) (35 μg/m3) (Huang et al., 2012; Li et al., 2005). Therefore,the contamination of metal(loid)s in such a high level of indoor PM2.5

from Guangzhou has become a public health concern (Wang et al.,2006a, 2006b).

With the background mentioned above, the present study aims togive a holistic assessment of contamination and risk of metal(loid)scontained in urban dust and airborne particles from the Guangzhouurban area. More specifically, the objectives include: (1) investigatingcontamination and distribution of metal(loid)s (Cr, Mn, Ni, Cu, Zn, As,Cd, Sn, Sb, Hg and Pb) in road dust, household AC filter dust and PM2.5

collected from the Guangzhou urban area; (2) evaluating the bioacces-sibility of thesemetal(loid)s via ingestion and inhalation; and (3) apply-ing the bioaccessibilities to estimate chronic daily intakes (DIs) ofmetal(loid)s via ingestion and inhalation, and characterize their risks.

2. Materials and methodology

2.1. Sampling and sample preparation

A total of thirty road dust samples, seven household PM2.5 samplesand ten household AC filter dust samples were collected in the

Guangzhou urban area from July to the end of August, 2010 (Fig. S1).The road dust sampleswere collected fromdifferent locations, includingscenic parks (n=3), educational sites (n=6), residential sites (n=9),heavy traffic sites (n=6), commercial sites (n=3) and peri-urban dis-trict (n = 3). Ten out of thirty road dust samples were collected specif-ically near the sampling houses for AC filter dust. The household ACfilter dust was collected using a 3M™ membrane. In brief, the 3M™membrane, which was fixed on the inside of the AC filter during sam-pling, was used to collect the household re-suspended dust particlespassing through the AC filter along with the indoor air current(Fig. S2). In order to collect enough dust samples, the sampling periodlasted for 6–8 weeks for each house. The AC operation time was atleast 10 h per day during the sampling period. The 3M™ membraneswere desiccated and weighed before and after sampling respectivelyas the PM2.5 membranes. Fig. S3 shows the illustrative diagram of splitair-conditioner applied in the present study, inwhich the indoor systemand outdoor system are separated. In order to compare the coarse par-ticles with the fine ones in household environment, household PM2.5

samples were additionally collected from seven out of all the samplinghouses. The information of each sampled house is listed in Table S1.The sampling program, weighing methods for PM2.5 membrane andACfilter dust, and samplingquality controls had been described in detailin our previous paper (Huang et al., 2012), in which household AC filterdust was mentioned as household total suspended particulate matter(TSP) with the diameters less than 100 μm. Hence, the road dust sam-ples were sieved to 100 μm ahead of analysis so as to compare withthe AC filter dust samples.

2.2. Extraction and determination

About 0.2 g road dust, a strip of 1″ × 8″ from the 8″ × 10″ 3M™ filtercollecting dust from air conditioners (US EPA, 1999) and a half of PM2.5

membrane were respectively microwave digested with 65% HNO3 (USEPA, 1994) for the extraction of total contents of metal(loid)s. The di-gestion solutions were then centrifuged, filtrated with 5CWhatman fil-ter paper and 0.45 μm syringe filter, and diluted withMilli-Q water. Thediluted solution was determined using ICP-MS (Perkin Elmer Elan9000) for all of metal(loid)s, except for Hg, which was determinedusing atomic spectrometry (FIMS 100 Perkin Elmer).

Physiologically based extraction test (PBET)was employed to evalu-ate the oral bioaccessibility of metal(loid)s, simulating the chemicalconditions of the gastro-intestinal tract. The procedure adopted in thisstudy was based on previous studies with slight modification (Rubyet al., 1996; Turner et al., 2009). In brief, gastric solution was preparedas in Table S2 immediately before use. About 0.25 g of road dust and astrip of 1″ × 8″ from the 8″ × 10″ AC filter were respectively added to50 mL polyethylene tubes with 40 mL gastric solution and end to endshaken at 37 °C. After 1 h, 8mLof themixtures (stomach phase)was ab-stracted, centrifuged (37 °C, 2000 rpm, for 10 min) and filtrated with a5C Whatman filter paper and a 0.45 μm syringe filter successively. Theremaining contents in the reaction tubes were added with 70.0 mg ofbile salts and 20.0 mg of pancreatin and adjusted to pH 7.0 with a fewaliquots of saturated NaHCO3, then digestion was continued. Next wasthe extraction of two other 8 mL mixtures after 2 h and 4 h (intestinalphase 1 and intestinal phase 2), and the rest of procedure was carriedout as mentioned above. The digested solutions were stored at 4 °C be-fore analysis with ICP-MS (Perkin Elmer Elan 9000) for themetal(loid)s.Hg was determined using atomic spectrometry (FIMS 100 PerkinElmer).

The lung simulating serum was also prepared immediately beforeuse (Table S2) (Hodgson et al., 2002; Voutsa and Samara, 2002). Theother half of PM2.5membranewas extractedwith 10mL serumby shak-ing at 37 °C. After filtrations with 5CWhatman filter paper and 0.45 μmsyringe filter, the extracted solutionswere stored at 4 °C before analysis.Except for Hg, whichwas determined using atomic spectrometry (FIMS100 Perkin Elmer), other metal(loid)s were determined with ICP-MS

Table1

Descriptive

statistics

ofco

ncen

trations

ofmetal(loid)

sin

road

dust

(n=

30),ho

useh

oldACfilter

dust

(n=

10)an

dho

useh

oldPM

2.5(n

=7)

(mg/kg

).

CrMn

Ni

CuZn

As

CdSn

SbHg

Pb

Road

dust

mg/kg

Mea

n17

6.22

539.76

41.38

192.36

1777

.18

20.05

2.14

8.44

14.83

0.22

387.53

Med

ian

125.61

535.89

43.46

166.8

801.07

16.52

1.43

4.10

5.78

0.22

199.07

RSD(%

)83

.38

37.83

5566

.96

138.2

51.15

93.26

99.48

91.85

104.27

145.82

Hou

seho

ldACfilter

dust

mg/kg

Mea

n18

8.03

321.16

94.86

68.37

344.17

17.08

0.43

4.29

7.28

0.56

699.05

Med

ian

134.48

257.52

48.81

26.68

152.19

58.26

0.33

3.33

4.65

0.42

643.29

RSD(%

)14

9.69

59.93

218.27

40.99

42.96

103.39

30.07

104.63

125.95

254.55

351.16

Hou

seho

ldPM

2.5mg/kg

(ng/m

3)

Mea

n97

0.42

(76.68

)51

2.6(50.09

)79

.93(8.09)

98.03(9.28)

2068

.31(188

.04)

231.55

(17.58

)0.89

(0.07)

34.67(3.59)

29.78(2.79)

1.11

(0.11)

917.62

(78.04

)Med

ian

486.95

(85.41

)46

3.17

(54.17

)60

.32(7.61)

45.77(8.22)

2283

.75(165

.90)

78.16(12.78

)0.51

(0.04)

40.43(3.05)

25.4

(2.35)

0.94

(0.07)

601.05

(52.87

)RS

D(%

)11

4.99

(54.43

)60

.46(28.96

)71

.76(49.12

)10

1.31

(70.67

)58

.2(39.33

)11

2.77

(66.68

)11

8.36

(90.89

)49

.16(24.13

)69

.88(39.65

)73

.87(87.77

)86

.09(109

.46)

119M. Huang et al. / Science of the Total Environment 479–480 (2014) 117–124

(Perkin Elmer Elan 9000). The entire processes of PBET and extraction ofPM2.5 with lung simulating serumwere performed in a sealed and darkcondition.

Standard addition method was used for each sample to correct theinterference of 40Ar35Cl and 40Ar12C on 75As and 52Cr. Standard refer-ence material NIST 2584 (indoor dust, NIST, USA), filter blanks andmethod blank were processed and analyzed in parallel with the sam-ples. In addition, standard spikes (50 μg/L) were also performed in trip-licate respectively for PBET and lung simulating extraction, to controlthe loss of solutions during shaking and transfer. The mean recoveryrates of NIST 2584 and standard spike are presented in Table S3. Andthe limits of detection for metal(loid)s are also summarized in Table S4.

2.3. Calculation of enrichment factor (EF)

Enrichment factor was applied to differentiate the anthropogenicsources fromnatural process, aswell as to assess the degree of influenceof human activity, which was calculated as Eq. (1) (Yongming et al.,2006). In this study, Mn was used as the reference element. The back-ground values of elements of soil in Guangdong, China (CNEMC, 1990)were applied as reference concentrations.

EF ¼ Cn sampleð Þ=Cres sampleð ÞBn baselineð Þ=Bres baselineð Þ ð1Þ

where Cn(sample): concentration of the determined element in thesamples; Cres(sample): concentration of the reference element (Mn)in the samples; Bn(baseline): background value of the determined ele-ment; and Bres(baseline): background value of the reference element(Mn).

2.4. Statistical analysis

The statistical analyses in the present study, including non-parametric analysis and linear regression analysis, were performedusing SPSS 16.0.

3. Results and discussion

3.1. Total concentrations of metal(loid)s

The descriptive statistics of total concentration of metal(loid)s (Cr,Mn, Ni, Cu, Zn, As, Cd, Sn, Sb, Hg and Pb) in road dust and household par-ticles (ACfilter dust and PM2.5) from theGuangzhouurban area are sum-marized in Table 1. The total concentrations of metal(loid)s in allsamples, excluding Mn, exhibited considerable variation reflected bytheir high relative standard deviation (RSD) (Table 1), demonstratingthe inherent heterogeneity of the urban particles. The large variation interms of road dustmight be attributed to the various land uses, includingscenic parks, educational sites, residential sites, heavy traffic sites, com-mercial sites and peri-urban district. Table 2 shows the descriptive sta-tistics of metal(loid)s contained in road dust with different land uses.Most of the metal(loid)s, except for Cr and Sn, were at the lowest levelsin peri-urban districts, where is the transition zone of urban and south-ern rural districts of Guangzhou with less anthropogenic emission. Thisdemonstrates the importance of anthropogenic sources (such as thetraffic effect and industrial emissions) contributing to outdoor contami-nation of metal(loid)s in the Guangzhou urban area.

On the other hand, the substantial variations for household particles(AC filter dust and PM2.5) are mainly due to the heterogeneities and di-versities of decorationmaterials, furnishings and human activities (suchas cooking, religious habits andmosquito coil usage) in samplinghouses.Table S1 summarizes the information of each sampling house in detail.Although the numbers of household AC filter dust and PM2.5 are at thelow side, it can basically reflect the general household environment in

Table 2The average concentrations (mg/kg) of metal(loid)s contained in road dust with different land uses.

Cr Mn Ni Cu Zn As Cd Sn Sb Hg Pb

Scenic parks, n = 3 Mean 242.85 562.18 38.23 167.71 1350.54 14.07 0.74 7.09 17.39 0.21 166.09RSDa 57.4 19.7 39.7 38.2 50.9 26.3 25.7 78.6 84.5 41.5 39.3

Commercial districts, n = 3 Mean 106.59 414.98 30.92 313.28 552.03 12.50 1.84 8.03 5.52 0.63 154.63RSDa 80.0 24.9 26.5 57.4 63.2 44.8 51.5 70.7 73.7 91.3 63.4

Residential districts, n = 9 Mean 183.97 581.87 38.54 142.10 3512.23 26.80 2.04 5.82 29.25 0.22 229.02RSDa 65.4 25.2 70.5 39.3 69.3 27.0 57.3 73.0 89.4 41.8 20.5

Traffic pivotal districts, n = 6 Mean 239.07 526.84 51.81 174.03 1530.21 19.46 2.69 10.06 14.72 0.25 259.65RSDa 74.8 30.8 42.5 32.2 78.0 54.3 72.7 77.5 85.7 31.3 71.5

Educational districts, n = 6 Mean 59.16 611.86 47.41 142.89 1374.53 16.22 2.57 2.78 5.52 0.15 258.43RSDa 65.1 62.8 29.3 72.1 71.6 20.2 68.8 82.9 78.0 49.1 82.7

Peri-urban districts, n = 3 Mean 94.51 381.13 10.49 90.45 252.00 13.11 0.46 6.19 2.83 0.04 133.06RSDa 65.2 9.1 46.4 57.5 46.4 23.7 13.5 27.5 56.2 23.0 31.0

a RSD: relative standard deviation (%).

Fig. 1.Metal(loid) composition in dust particles.

120 M. Huang et al. / Science of the Total Environment 479–480 (2014) 117–124

the Guangzhou urban area, based on the diversities of sampling housesemployed in the present study.

Zinc was found to be the most abundant element in road dust andhousehold PM2.5, while the concentration of Pbwas the highest in ACfil-ter dust. The Hg and Cd were at the lowest levels amongst all elements(Table 1). Themore severe contamination of Zn than other metal(loid)sin road dust in the present study agreed with the results of previousstudies in Guangzhou (Duzgoren-Aydin et al., 2006; Zhong et al.,2012). Zn in outdoor urban environment may be released fromgalvanized-steel road equipments, such as crash barriers, road signsand lamp posts, as they corrode (Yuen et al., 2012). The rubber under-lay, galvanized-iron roofing and carpet are considered as importantsources of Zn in indoor environment (Kim and Fergusson, 1993). Theconcentrations of Hg determined in this study (Table 1)were consistentwith those reported by Huang et al. (2012), who discussed in detailabout Hg distribution with different land uses and different particles.Therefore, this paper only focused on the enrichment factors, bioacces-sibility and its application to risk assessment in terms of Hg.

The metal(loid) composition in AC filter dust was largely differentfrom that of road dust (Fig. 1), especially Ni, Mn, Cu, Zn, As, Cd and Pb,of which the concentrations were significantly different between thesetwo dust particles (p b 0.05, Table S5). This indicates that there mightbe extra household contamination sources of metal(loid)s in additionto penetration and infiltration from outdoor sources. A more detaileddiscussion on the household sources will be given in Section 3.2. Ontheother hand, the concentrations ofmetal(loid)swere generally higherin PM2.5, compared with AC filter dust in the household environment(Table 1), especially Cr, Zn, As, Sn and Sb with statistical significance(p b 0.05, Table S5). This was consistent with numerous previous stud-ies that metal concentrations increase with decreasing particle size ofdust and airborne particles (Cao et al., 2012; Kong et al., 2011), whichcan be explained by the larger surface area per unit of mass of the fineparticles (Luo et al., 2011).

3.2. Enrichment factors

The major crustal and conservative element (such as Al, Fe, Me, Mn,Sc and Ti) is often used as the reference element in the calculation ofEFs (Yongming et al., 2006). In this study,Mn is considered as the conser-vative element, because its RSDs in dust and airborne particles weremuch smaller than those of othermetal(loid)s (Table 1). The contamina-tion of metal(loid) was classified into five categories, the metal(loid)swith EFs more than 5 indicate that they are significantly enrichedand contaminated (Table S6) (Yongming et al., 2006). Based on the cal-culated EFs of each metal(loid), Cu, Zn, Cd, Sb and Pb in road dust, Zn,Cd, Sb, Hg and Pb in AC filter dust, and Cr, Zn, As, Cd, Sb, Hg, and Pb inPM2.5, with the EFs larger than 5, are shown to be significantly contami-nated (Table 3). The higher EFs of Cr, Ni, Hg and Pb (Table 3) and theirhigher concentrations in household AC filter dust relative to road dust

(Table 1) indicate extra complicated indoor pollution sources for thesemetal(loid)s. TheHg contamination in housesmight be related to indoorlatex paints, breakage of thermometer and fluorescent bulbs, and burn-ing of religious items with mercury additive (Huang et al., 2012). Lead-based paint could be considered as a major exposure source of Pb inhouses, as it is very commonly used in existing residences in China,such as walls, toys and furniture (Lin et al., 2009). It has been notedthat about 56% of paints purchased from China (Clark et al., 2006) and50% of samples of housing paints from Guangzhou (Lin et al., 2009) con-tain Pb equal to or larger than the Chinese regulation limits (600 ppm)(GAQSC, 2001a, 2001b). Burning of mosquito coils and Chinese religiousitems could release a large amount of Ni and Cr (Lin and Shen, 2003,2005). The higher enrichment of Cr and Ni in houses was in line withthe usage of mosquito coils and burning of Chinese religious items inthe sampling houses (Table S1).

Table 3Average enrichment factors (EFs) in different particulate matter (The EFs larger than 5 were marked in bold).

Cr Mn Ni Cu Zn As Cd Sn Sb Hg Pb

Background values (mg/kg) 50.50 279.00 14.40 17.00 47.00 8.90 0.06 5.80 0.54 0.08 36.00Road dust 1.8 1 1.49 5.85 19.55 1.16 19.8 0.72 14.2 1.56 5.56AC filter dust 3.23 1 2.94 3.49 6.36 1.67 6.74 0.64 11.72 6.69 16.87PM2.5 10.45 1 3.02 3.14 23.95 14.16 8.63 3.25 30.01 7.02 13.88

121M. Huang et al. / Science of the Total Environment 479–480 (2014) 117–124

3.3. Oral bioaccessibilities of metal(loid)s in road dust and AC filter dust

Oral bioaccessibility of a metal(loid) in this study was calculated asthe percentages of the soluble concentrations sequentially in stomachphase (SP), and two intestinal phases (IP1 and IP2) respectively, relativeto its total content. As no significant distinctions were observed be-tween IP1 and IP2 (p N 0.05), average percentage was employed asthe bioaccessibility in intestinal phase (IP). Compared with IP, mostmetal(loid)s were found to be more bioaccessible in SP (Fig. 2), espe-cially Mn, Cu, Zn, Ni, Cd and Pb in road dust, as well as Cu and Zn inAC filter dust (p b 0.05) (Table S7). Ni, Cd, Zn, Pb and Cu could bindwithmalate ions and bile salts to different degrees in intestine to reducetheir bioaccessibilities, especially Cu, which could constitute a greaterand more stable complexation (Feroci et al., 1995; Turner and Ip,2007). The cationic metals, such as Cu, Zn and Pb, have been generally

Fig. 2. Bioaccessibility of metal(loid)s from road dust (RD) and hou

reported by PBET studies that their bioaccessibilities reduce from stom-ach to intestine, through precipitation of insoluble species and the re-adsorption of cations to preexistent or altered sites on the solid surfacein the higher pH and carbonate-rich environment of the intestine (Rubyet al., 1996; Turner and Ip, 2007). Therefore, the bioaccessibilities ofthese metals in this study underwent significant reduction from stom-ach (pH = 2.5) to intestine (pH = 7.0).

In addition, the bioaccessibility of Cr was also observed to be highlysensitive to pH (Turner and Radford, 2010). However, no significant dis-tinction between SP and IP was found in the present study (Table S7),which might be attributed to the existence of negatively charged poly-atomic forms in intestine, like CrO4

2−/HCrO4−. These oxyanionic forms

can be released easily in a high pH environment and are favorable in acarbonate-rich intestinal environment. Carbonate acts as the complet-ing ligandwith the oxyanionic forms to other complexants and particles

sehold AC filter dust (AC) in SP (black bars) and IP (gray bars).

122 M. Huang et al. / Science of the Total Environment 479–480 (2014) 117–124

to increase the mobilization of Cr (Villalobos et al., 2001). Similarly, nosignificant differences of As and Sb bioaccessibilities were found be-tween two digestion phases for both road dust and AC filter dust inthe present study (Table S7), also presumably due to the existence oftheir oxyanionic forms in higher pH environment (Denys et al., 2009;Rieuwerts et al., 2006).

The bioaccessibility of As relatively reduced from stomach to intestinein road dust but slightly increased in AC filter dust, in contrast to Sb(Fig. 2). As reported by other studies, there seems to be no specific trendsfor the mobilization of the two elements from stomach to intestine. Forexample, enhanced accessibility of As in intestine relative to stomach isobserved in the soil samples from a former mining district in England(Rieuwerts et al., 2006). However, As in Guangzhou urban soil exhibitsdecreased mobilization in intestine (Lu et al., 2011). Denys et al. (2009)reported that accessibility of Sb in saliva-gastric phase is lower for gardensoil and mine waste material, but higher for background soil.

The mobilization for each metal(loid) in road dust and AC filter dustin IP relative to SP was quantified using linear model regression in theconfidence interval of 95%. The regression results shown in Fig. S4 illus-trated that a bioaccessible fraction ofmetal(loid)s in IPwas significantlydependent on that in SP (p b 0.05), and suggested a common sequenceof reaction mechanisms from stomach to intestine taking place regard-less of different samples and types of particles, which was in line withthe study of Turner and Ip (2007).

3.4. Inhalation bioaccessibilities of metal(loid)s in PM2.5

The highest bioaccessible percentage was noted for Zn (57.32%),followed by Mn (41.76%), Cu (35.06%), As (29.88%), Sb (25.91%), Sn(23.21%), Ni (22.09), Hg (20.66%), Pb (19.36%), Cr (16.99%) and Cd(6.77%). The relatively higher bioaccessibilities of Zn and Cu could bedue to the existence of cysteine in the synthetic solution (Table S2), pro-viding thiol groups that are strongly coordinated to Zn and Cu. The lowaccessibility of Cd noted in this study was in accordance with the resultof a previous study (Voutsa and Samara, 2002), which stated that thesolubility of Cd in lung is dependent on its chemical status. For example,cadmium oxide and cadmium chloride can be easily dissolved in lung,but not for cadmium sulfide. Therefore, speciation ofmetal(loid)s is rec-ommended for further study of their bioaccessibilities.

3.5. Risk assessment based on the bioaccessibilities of metal(loid)s

The risk assessment of a metal(loid) depends on its daily intake (DI),reference dose (RFDo) or reference concentration (RFC) and slope factor(SF). The calculations of oral daily intakes (DIs) via household AC filterdust and road dust (US EPA, 1989) were based on the resident model(US EPA, 2012a) and outdoor model (Hu et al., 2011; Zheng et al.,2010) respectively (Table 4), while the respiratory DIs via household

Table 4Parameters in models for daily intakes (DIs).

Parameters Descriptions

95% UCL 95% upper confidence limit for the mean of concentrationsBA (%) Bioaccessible percentagesIR (mg/day) Dust ingestion rateIRF (mg-year/kg-day) Dust age adjusted ingestion rate for carcinogenic riskEF (days/year) Exposure frequencyED (years) Exposure durationATnc (days) Averaging time for non-carcinogenic risksATc (days) Averaging time for carcinogenic risksBW (kg) Body weightET (hours/day) Exposure time per day

a Cited from US EPA (2012a).b Cited from Hu et al. (2011) and Zheng et al. (2010).

PM2.5 (US EPA, 2009) was derived from the resident air model (US EPA,2012a) (Table 4). The residential models (Eqs. (2) and (4)) were usedto estimate the DIs via household AC dust, while the outdoor models(Eqs. (2) and (5)) were employed to calculate the DIs via road dust interms of non-carcinogenic risks and carcinogenic risks. The residentialambient air models (Eqs. (3) and (6)) were employed to estimate the in-halation intakes via respiratory particles (PM2.5). The parameters used inEqs. (2)–(6) are summarized in Table 4. The metal(loid)s selected forquantitative risk assessment, except for Pb, were those which presentthe toxicity value profiles in the Regional Screening Level GenericTable from the US EPA (US EPA, 2012b). The RFDo of Pb via ingestionwas cited from the study of Baars et al. (2001). The toxicity value ofCrVI, much more toxic than CrIII, was used to estimate the worst scenarioof Cr. The toxicity value of inorganic Hg salts, the major portion in theparticle-phase Hg (Fitzgerald and Lamborg, 2003), was applied for therisk assessment of Hg. The toxicity values of eachmetal(loid) for differentexposure routes are shown in Table S8.

Ingestion exposure via road dust and household AC filter dust fornon-carcinogenic risk:

DI mg=kg−dayð Þ ¼ 95% UCL � BA� IR � EF� EDATnc � BW

� 10−6: ð2Þ

Inhalation exposure via household PM2.5 for non-carcinogenic risk:

DI mg=m3� �

¼ 95% UCL� BA�EF� ED� ET� 1day

24hATnc

: ð3Þ

Ingestion exposure via household ACfilter dust for carcinogenic risk:

DI mg=kg‐dayð Þ−95% UCL � BA� IRF� EFATc

� 10−6: ð4Þ

Ingestion exposure via road dust for carcinogenic risk:

DI mg=kg‐dayð Þ ¼ 95% UCL � BA� IR� EF� EDATc � BW

� 10−6: ð5Þ

Inhalation exposure via household PM2.5 for carcinogenic risk:

DI mg=m3� �

¼ 95% UCL� BA�EF� ED� ET

1 day24 h

ATc� 10−6

: ð6Þ

The DIs for non-carcinogenic risks calculated from Eqs. (2)–(3) weresubsequently divided by the RFDos or RFCs to yield the hazard quotients(HQs) for all of metal(loid)s and exposure pathways. With the hypoth-esis that the adverse effect is proportional to the sum of HQs, hazard

Values

Residential modela Residential air modela Outdoor modelb

Adults Children Adults Children Adults

100 200 100114350 350 350 350 18030 6 30 6 2430 ∗ 365 6 ∗ 365 30 ∗ 365 6 ∗ 365 24 ∗ 36570 ∗ 365 70 ∗ 365 70 ∗ 36570 15 70

24 24

Table5

Risk

assessmen

tofm

etal(loid)

sviadiffe

rent

urba

npa

rticles(H

Qsan

dCR

slarger

than

thethresh

oldva

lues

weremarke

din

bold).

CrMn

Ni

CuZn

As

CdSn

SbHg

PbSu

m

Road

dust

(outdo

orworke

rsoilmod

el)

HQs

Adu

lt1.1×

10−

032.3×

10−

036.7×

10−05

5.1×

10−04

2.6×

10−

031.5×

10−02

3.0×

10−04

7.4×

10−

075.0×

10−

032.1×

10−04

3.2×

10−03

3.0×

10−

02

CR6.2×

10−

072.4×10

−06

3.0×10

−06

Hou

seho

ldACfilter

dust

(residen

tial

soilmod

el)

HQs

Adu

lt3.7×

10−

033.5×

10−

036.5×

10−04

2.4×

10−03

2.4×

10−

036.2×

10−02

2.1×

10−04

2.8×

10−

061.4×

10−

026.4×

10−04

3.5×

10−02

1.2×

10−

01

Child

ren

3.4×

10−

023.3×

10−

026.1×

10−03

2.3×

10−02

2.2×

10−

025.8×

10−01

1.9×

10−03

2.6×

10−

051.3×

10−

015.9×

10−03

3.2×

10−01

1.2×10

+00

CR6.3×10

−06

3.2×10

−05

3.8×10

−05

Hou

seho

ldPM

2.5(residen

tial

ambien

tairmod

el)

HQs

Adu

lt1.8×

10−

014.8×

10−

014.5×

10−02

4.6×

10−03

4.9×

10−01

5.1×

10−04

1.5×

10−03

1.2×10

+00

Child

ren

1.8×

10−

014.8×

10−

014.5×

10−02

4.6×

10−03

4.9×

10−01

5.1×

10−04

1.5×

10−03

1.2×10

+00

CR1.9×10

−05

2.5×10

−06

1.3×10

−05

7.8×

10−09

3.5×10

−05

123M. Huang et al. / Science of the Total Environment 479–480 (2014) 117–124

index (HI) is produced by summing up the HQ of each metal(loid) foreach pathway. HI larger than 1 indicates a significantly adverse non-carcinogenic effect.

Whether an element causes carcinogenic risk depends on the expo-sure route and their specific toxicity. CrVI and As are considered ascarcinogenic risks when ingested and/or inhaled; Mn and Cd areconsidered as carcinogenic risks only when inhaled. For carcinogenicmetal(loid)s, the obtained doses from Eqs. (4)–(6) were multiplied bythe corresponding SFs to obtain the levels of cancer risks (CRs). CRequal to 1 × 10−6 is considered as the most tolerable regulated risk.

HI via ingestion of the whole fraction of metal(loid)s in householdAC filter dust for adults was 0.12, larger than that of road dust (0.030),indicating that household AC filter dust is more hazardous to humanhealth than road dust. HI via ACfilter dust for children (HI=1.2 N 1) ex-hibited to be ten times higher than that for adults (HI = 0.12), indicat-ing that children suffer much higher risks than adults in a householdenvironment, which could be explained by children's hand to mouthand crawling behaviors (Roberts et al., 2009).Moreover, theHI via inha-lation of household PM2.5 for both adults and children was 1.2, largerthan 1 (Table 5). All of total CRs of the whole bioaccessible fraction ofmetal(loid)s via road dust, household AC filter dust and PM2.5 exceeded1.0 × 10−06 (Table 5), indicating the potential carcinogenic risks. In thepresent study, an HI= 1.2would be so close to the threshold value thatit indicates the need of refinement in the risk assessment. The uncer-tainty on the risk assessment might arise from the limited number ofhousehold samples and the assumed values of exposure parameters inthe US EPA models, which are not authentically applicable in Guang-zhou. However, the present study just identifies the relative risksamong different metal(loid)s. Arsenic and Cr were observed as thetwo metal(loid)s of the most concern (Table 5).

Arsenic was found to be the most risky element in terms of non-carcinogenic risk, of which the HQs accounted for about 50% of theHIs caused by exposure to the whole fraction of elements contained inroad dust, AC filter dust and PM2.5 (Table 5). Additionally, the carcino-genic risk of As was also the highest for both road dust and AC filterdust, but that was not in the case for PM2.5. The carcinogenic risk of Cr(1.9 × 10−05) in PM2.5 was slightly higher than that of As (1.3 × 10−05)(Table 5). However, the result of the carcinogenic risk assessment ofCr was based on the assumption that all of the Cr contained in dustand airborne particles were in the form of the more toxic hexavalentchromium (CrVI), to estimate the worst scenario of Cr. Furthermore, itwas speculated that most of the Cr in PM2.5 was in the form of less sol-uble trivalent chromium (CrIII), according to the low percentage ofwater-soluble Cr (60.23 mg/kg) (data yet to be published) relative tototal Cr (970.42 mg/kg) (Table 1). Moreover, the endogenous reducingagents within the upper gastrointestinal tract and blood serve to pre-vent systemic uptake of CrVI in the human body, by reducing CrVI toCrIII (FNB, 2001). Hence, the health risks of Cr were overestimated inthe present study.

4. Conclusion

The present study investigated the contamination of metal(loid)s inoutdoor and indoor urban environments of Guangzhou and their risksbased on oral and respiratory bioaccessibilities. The inherent heteroge-neity of the Guangzhou urban area was reflected by the considerablevariations of total concentrations of metal(loid)s. Extra indoor sourceswere observed to lead to the elevated levels and higher enrichment ofCr, Ni, Hg and Pb. Household AC filter dust seems to bemore hazardousto human health than road dust, especially to children. Although therewere some uncertainties that need to be refined in the risk assessment,As was observed as the most risky element via ingestion and inhalationcompared with other metals. Hence, it is recommended to study the Asspeciation in different particles and their bioaccessibilities, and refinethe parameters applicable to the Guangzhou urban population for thehealth risk assessment models.

124 M. Huang et al. / Science of the Total Environment 479–480 (2014) 117–124

Acknowledgments

Financial support from the Special Equipment Grant (SEG HKBU09),the Research Grant Council of the University Grants Committee ofHong Kong (HKBU260209) and the National Natural Science Founda-tion of China (NSFC)/Research Grants Council (RGC) (N_HKBU210/11)is gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.scitotenv.2014.01.115.

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