Indoor Quality of Residential Homes and Schools of an Industrial Area in Asansol: Characterization, Bioaccessibility and Health Risk Assessment of Potentially Toxic Elements

Bioaccessibility of eight potentially toxic elements (PTEs), their human exposure and health risk assessments were determined in the indoor dust of residence and schools from the Asansol Industrial area, India. The PTEs concentrations were maximum during the winter both at houses and schools. The average PTEs concentrations throughout the year in Asansol were 3.16, 120, 156, 41708, 2354, 61.3, 115 and 345 mg.kg-1 for Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn respectively. X-ray powder diffraction indicated an abundance of quartz in the indoor dust. Principal component analysis (PCA) indicated multiple sources such as traffic, industries, and lithogenic sources for PTEs in indoor dust. Percentage of bioaccessibility was maximum for Cd (55.3% throughout the year). Total PTEs concentration and a bioaccessible fraction of PTEs both were used for health risk assessment, and non-carcinogenic health risk was <1 for total PTEs and the bioaccessible fraction of PTEs. Health risk of total PTEs’ (HItotal) for Mn was high for both children and adult (6.76E-01 and 1.3E-01, respectively). Monte Carlo simulation model indicated that all the cumulative probability of Hazard Quotient (HQ) for collectively eight metals was below 1.


INTRODUCTION
Indoor settled dust is an important environmental media, which needs serious attention due to increased levels of toxicants to which human are daily exposed. The primary sources of PTE's into the indoor environment are indoor activities and infiltration of the outdoor aerosols into indoors. Indoor activities like cooking, vacuum cleaning, smoking and resuspension of dust particles also contribute to indoor dust pollution (Ali et al. 2019). PTEs can enter into indoor through the suspended particles from the outdoor air (Rasmussen et al. 2018), high vehicular traffic (Rohra et al. 2018) and the dirt that adheres to footwear (Ali et al. 2019, Kelepertzis et al. 2019, Cheng et al. 2018. Various researches indicate that outdoor aerosols are a significant contributor to indoor pollutants, of which street dust can contribute to 20-95% to household dust (Rasmussen et al. 2001, Torres-Sanchez et al. 2017).
PTEs causes various health effects like decreased bone density, renal disorder, nervous system damage, disturb natural brain development etc. (Chen et al. 2015). Moreover, it interferes with the child's neural development which is highest during their growth period and is more risk-prone to PTEs (Muhamad-Darus et al. 2017). PTEs tend to accumulate into the tissue and results in biological magnification through time.
To accurately assess the ingested oral risk of PTEs exposure, it is essential to know the PTEs oral bioaccessibility. Although there are a lot of in vitro models developed to assess the bioavailability of PTEs among which SBET is a very simple process and also cost very low. Various studies were conducted to estimate the in-vitro bioaccessibility of PTEs in indoor settled dust (Bot et al. 2010, Turner 2011. However, in urban industrial areas like Vol. 20, No. 1, 2021 • Nature Environment and Pollution Technology Asansol, India, which has various anthropogenic activities which release various PTEs (Gope et al. 2017); however, the oral bioaccessibility of various PTEs in indoor dust has not been investigated.
The overarching aim of this study was to analyse (i) the seasonal variation of PTE's level in indoor settled dust of houses and schools (ii) mineralogical study (iii) sources identification of PTEs (iv) oral bioaccessibility, (v) health risk assessment, and (vi) Monte Carlo simulation.

Study Area
Asansol is a metropolitan city located in the western part of West Bengal, India, with medium industrial activities and a part of Chhotanagpur Plateau (Fig. 1). The population of this city was 5,64,491 as per the census of 2011 (Govt of India 2011). It is situated beside NH-2 'The Grant Trunk Road' one of the oldest and busiest roads in India. IISCO (Indian Iron and Steel Corporation), Eastern Coalfield, Burnpur Cement etc. are the primary industries located at Asansol industrial area. Very high traffic density was observed throughout 24 hrs. at Asansol, >7000 h in peak hour and >2000 h in the lean hour (Gope et al. 2017).

Sample Collection
Sites were selected randomly, and samples were collected from three residential areas, viz. Sarada Pally, Puranhat and Santinagar. Four houses from each residential area and four primary schools were selected for sample collection. Total 48 dust samples were collected throughout the study period, March 2014 to February 2015. A 0.5 square meter glass plate was kept on the top of the almirah and bunker. Settled dust was collected from the glass plate in a plastic zipper pack through a nylon brush and brought to the laboratory for analysis (Pitawala et al. 2013). Dust samples were collected in four months interval at schools and houses.
Ground floor was selected for sampling in case of houses, but in case of schools, the 1 st floor was used. Sarada Pally is very close to steel plant and NH. The houses are east facing and well ventilated with 2 windows and two doors in the room where the plate was kept. In Puranhat area, houses are old; these are housing complex of steel industries. Houses are not well ventilated with the presence of one door plus one window at one side and one window on the other side. Santinagar is a very congested area and rooms are very small (8×8 ft). Only one door and one window are present on one side. All the schools are well ventilated with windows on three sides and door present on one side. Fig. 1: The study area. Verandah is present in every school. Three schools (Naya Basti primary school, Allamnagar primary school and Rahamat Nagar primary school) are east facing and playground present in the east side just after verandah. Subhas Pally primary school is north facing, and they do not have any playground.

Sample Preparation for Pseudo Total PTEs Concentration
Dust samples of 0.2 g were weighed and transferred into a Teflon vessel. HNO 3 , HF and HCl were mixed with the dust samples in a ratio of 2:1:1 (v/v) and the mixture was digested for 35 minutes at 210°C temperature in a microwave digester (Jupiter-A, Sr. No. JP127A) and the digestion procedure was adopted from Hussain et al. (2015) with some modification as per our requirement. The digested solution was filtered using Whatman No. 42 filter paper and made up to 25 mL using 12N HCl. PTEs concentration was measured by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES, iCAP 6300, Thermo Fisher Scientific, UK). Quality control and quality assurance are provided in our earlier paper (Gope et al. 2018).

Sample Preparation for SBET
During SBET extraction, 0.1 g dust was weighed out from each sample and transferred into a test tube, and 10 mL 0.4 M Glycine (pH 1.5) was added to the sample, and the test tube was centrifuged at 30 rpm at 37°C for one hour. After digestion, the aliquot was filtered through a 0.45 µm cellulose acetate disk filter (Ruby et al. 1996, Oomen et al. 2002. Filtrates were stored at 4°C and analysed using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) for detection of PTEs.

Pseudo Total PTEs Concentration
weighed and transferred into a Teflon vessel. HNO3, HF and HCl were les in a ratio of 2:1:1 (v/v) and the mixture was digested for 35 minutes a microwave digester (Jupiter-A, Sr. No. JP127A) and the digestion m Hussain et al. (2015) with some modification as per our requirement. filtered using Whatman No. 42 filter paper and made up to 25 mL using ation was measured by Inductively Coupled Plasma-Optical Emission CAP 6300, Thermo Fisher Scientific, UK). Quality control and quality our earlier paper (Gope et al. 2018).

SBET
.1 g dust was weighed out from each sample and transferred into a test cine (pH 1.5) was added to the sample, and the test tube was centrifuged our. After digestion, the aliquot was filtered through a 0.45 µm cellulose al. 1996al. , Oomen et al. 2002. Filtrates were stored at 4°C and analysed d Plasma-Optical Emission Spectrometry (ICP-OES) for detection of lated using the following formula (Oomen et al. 2002

XRD Analysis
At room temperature, Rigaku Ultima IV with CuK a (k = 1.514 A˚, over the range, 2ϴ 4-84°) was used to study X-ray Powder diffraction patterns of dust samples. To identify the phase, the obtained diffraction designs were studied using X'Pert High Score software and ICDD database (Labar & Kundu 2018).

Human Exposure Risk Assessment
Ingestion, inhalation and dermal absorption are the three main exposure routes through which PTEs can enter into the body. The exposure risk assessment was calculated for children and adult. To assess the potential health risk, chronic daily intake (CDI) was calculated for each PTEs through individual exposure pathways.

= ∑
Hazard index (HI) was calculated for each PTE. HI is the = ∑

Statistical Analysis
Principal Component Analysis (PCA), and Tukey test of software version 21.
Monte Carlo simulation was performed by using Crystal

Total PTEs Content
The concentration of all PTEs in the indoor dust of hou season (Fig. 2). Studied eight PTEs had maximum conce Zn) during the winter followed by summer and monso Where, CDI = Chronic Daily Intake (mg. Monte Carlo simulation was performed by using Crystal

Total PTEs Content
The concentration of all PTEs in the indoor dust of hou season (Fig. 2
Hazard Quotient (HQ) was calculated for individual PTE in each exposure pathway. The calculated CDI values were subtracted with the corresponding reference dose (RfD) of PTEs. Reference doses were listed in Table 3 Where, CDI = Chronic Daily Intake (mg.  Table 3.

= ∑
Hazard index (HI) was calculated for each PTE. HI is the summati = ∑

Statistical Analysis
Principal Component Analysis (PCA), and Tukey test of PTEs wer software version 21.
Monte Carlo simulation was performed by using Crystal Ball (v11

Total PTEs Content
The concentration of all PTEs in the indoor dust of houses and s season (Fig. 2). Studied eight PTEs had maximum concentration (C Zn) during the winter followed by summer and monsoon ( Fig. 2 … (5) Hazard index (HI) was calculated for each PTE. HI is the summation of three HQ for each PTE.

Total PTEs Content
The concentration of all PTEs in the indoor dust of houses and sch season (Fig. 2). Studied eight PTEs had maximum concentration (Cd Zn) during the winter followed by summer and monsoon (Fig. 2).
windows and doors are kept open, leading to accumulation of dust in …(6)

Statistical Analysis
Principal Component Analysis (PCA), and Tukey test of PTEs were done by SPSS statistical software version 21.

Total PTEs Content
The concentration of all PTEs in the indoor dust of houses Vol. 20, No. 1, 2021 • Nature Environment and Pollution Technology and schools varied significantly by season (Fig. 2). Studied eight PTEs had maximum concentration (Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) during the winter followed by summer and monsoon (Fig. 2). In summer, for ventilation, windows and doors are kept open, leading to accumulation of dust in the indoor environment and due to windy (maximum 3.6km/hr) and dry condition (maximum 48°C). In monsoon due to heavy rain, pollutants could have washed-out from atmosphere leading to a low concentration of aerosols in the outdoor and ultimately leading to less PTEs concentration during monsoon (Kulshrestha et al. 2009). Low atmospheric temperature, low mixing height and temperature inversion during winter would have led to the movement of fine dust through the window and doorsill and get accumulated into the indoor environment from an outdoor source could be responsible for higher PTEs concentration in winter when compared to monsoon and summer (Khillare et al. 2004, Kulshrestha et al. 2009. A similar variation of PTEs concentration in street dust was also reported by Gope (2016) in Asansol.
A maximum Cd concentration of 4.94 ± 1.6 and 3.85 ± 1.9 mg.kg -1 was observed in houses and schools respectively during winter. The major source of Cd in indoor dust could be from other outdoor sources like coal combustion, petroleum combustion and fossil fuel burning (Li et al. 2016, Soleimani et al. 2018. The high concentration of Cd in indoor dust at Asansol is due to the influence of outdoor aerosols into the indoor environment, originating from industrial activities like ceramic industries, steel plant (Liang et al. 2017, Soleimani (Shen et al. 2017, fossil fuel burning, engine oil, and tires wear (Novo et al. 2017, Gope et al. 2018. During winter maximum average Cr concentration in Asansol was 169 ± 33 mg.kg -1 while the maximum average concentration at schools was 135 ± 2.5 mg.kg -1 . The high concentration of Cr in the indoor could be sourced to infiltration of outdoor aerosols into indoor which might be due to their release from the vehicles traffic (Muhamad-Darus et al. 2017, Soleimani et al. 2018, weathering of Cr-Ni plated automobile parts, yellow road paint (Iwegbue et al. 2017) and some indoor sources of Cr are mosquito coil and incense stick burning (Lin & Shen 2003, Cr coated household items (Iwegbue et al. 2017, Al-madanat et al. 2017, Muhamad-Darus et al. 2017).
Average Cu concentrations at schools were 164±,29 6.0±116 and 33±179 mg.kg -1 during summer, monsoon and winter respectively. Cu concentration was highest (196±42 mg.kg -1 ) during winter and was lowest during monsoon (117±11 mg.kg -1 ). Uses of Cu based biocides in home gardens and Cu containing wood preservatives (copper arsenate, copper sulphate, etc.) are the sources of indoor Cu contamination (Iwegbue et al. 2017). Vehicular emission is a very common source of Cu (Muhamad-Darus et al. 2017, Al-madanat et al. 2017) as well as coal combustion (Gope et al. 2018) which could be transported from outdoor to indoor environment. Besides, vehicular emission, industries at Asansol use coal as a fuel and which could increase Cu concentration in outdoor as well as indoor.
The average Fe concentration was highest during the winter (49538 ± 6004 mg.kg -1 ) followed by summer (44044 ± 6045 mg.kg -1 ) and monsoon (31544 ± 5697 mg.kg -1 ), while maximum Fe concentration at schools was 53863 ± 6656 mg.kg -1 during winter followed by summer (44523 ± 4576 mg.kg -1 ) and monsoon (27782 ± 4066 mg.kg -1 ). Fe concentration is high in lateritic soil (Goldberg 1989, Emeh et al. 2019, and this Fe containing dust enter into houses from the playground, roadside dust, and different barren places. Steel industries and various ferroalloy industries are also major sources of Fe pollution, which could enter into indoor through outdoor aerosol. During monsoon, heavy rain decreases the dust levels leading to decreased Fe concentration in indoor dust. The highest average Mn concentration was observed at houses during winter (2881 ± 499 mg.kg -1 ) at Asansol. Maximum average Mn concentration at schools was 3115±755 mg.kg -1 during winter. Major sources of Mn in household dust could be the use of household washing agents, and outdoor sources for automobile emissions (Iwegbue et al. 2017). Mn is the main raw material used in steel plant and is the main source of Mn contamination in an outdoor environment (Gope et al. 2018), and Mn contaminated aerosol enter into indoor from the outdoor environment.
Average highest Ni concentration at Asansol was 72.6 ± 7.93 mg.kg -1 during winter, while the highest average Ni concentration in schools was 65.5 ± 7.6 mg.kg -1 during winter. Some of the major indoor sources of Ni in the indoor environment are mosquito coil and incense burning (Lin & Shen 2005, Li et al. 2016. Ni is mainly released into the outdoor environment from vehicular emission, iron-steel plant, thermal power plant (mainly combustion of coal) (Muhamad-Darus et al. 2017, Li et al. 2016, Wan et al. 2016, Soleimani et al. 2018) which is entered into houses and schools through outdoor aerosol.
The average Pb concentrations were 117 ± 10.9, 75.4 ± 13.5 and 154 ± 27.9 mg.kg -1 during summer, monsoon and winter respectively, while in schools, the maximum Pb concentration was 136 ± 16 mg.kg -1 during winter. The major indoor sources of Pb are Pb-based paint, lead solder, lead pipes, cigarette smoke (Iwegbue et al. 2017). The sources of Pb in indoor could be traced the dust emitted from industries, which could penetrate the indoor environment like vehicular exhausts (Iwegbue et al. 2017, Nawazish et al. 2017, coal combustion (Pacyna et al. 2009, Sen et al. 016), fly ash from fly ash brick manufacturing plant (Gope et al. 2018), cement industries (Bermudez et al. 2010), etc.
The average Zn concentration was 363 ± 58.8, 263 ± 30.1 and 410 ± 94.7 mg.kg -1 respectively during summer, monsoon and winter in Asansol. Zn concentration was higher during winter than summer and monsoon. Maximum Zn concentration in schools was 345 ± 32 mg.kg -1 during winter. Zn can be sourced into indoor dust from rubber underlay, galvanized iron roofing, and carpets (Iwegbue et al. 2017). Depreciation of vulcanized rubber tires, decomposition of galvanized vehicular parts, and lubricating oils also release Zn in the outdoor environment which could be traced into the indoor environment by outdoor aerosols (Muhamad-Darus et al. 2017, Sulaiman et al. 2017. Moreover, brass and bronze, dry cell batteries, paints, rubber, ceramic are responsible for Zn contamination in indoor dust (Gope et al. 2018).
Comparison of total PTEs concentration in indoor dust reported from other countries along with this study is presented in Table 1.

Mineral Composition
X-ray diffraction study was performed to identify the various minerals present in the indoor dust samples (Fig. 3a, 3b). Quartz and magnetite was the most abundant mineral present in the analysed indoor dust. The identified quartz minerals Vol. 20, No. 1, 2021 • Nature Environment and Pollution Technology were preiswerkite, clintonite, fayalite with clay forming minerals such as calcite, gypsum, dolomite, microcline, muscovite, prasochrome, zincian. Pb containing mineral such as anglesite (PbSO 4 ) was observed which could be due to traffic and fly ash released from fossil fuel burning. Wulfenite (PbMoO 4 ), a secondary mineral, was detected in one sample, but the intensity was low. ZnMoO 8 is a zinc-containing mineral found in the Asansol industrial area. Magnetite and hematite were detected in both the samples. Siderite (FeCO 3 ) and pyrite (FeS 2 ), iron-containing minerals were found in the indoor dust samples. Iron-containing minerals are introduced in the indoor dust due to fossil-fuel burning, traffic emissions, steel plant and other industries (Ram et al. 2014, Lu et al. 2007). Presence of clay forming mineral-like dolomite [CaMg (CO 3 ) 2 ] could be due to cement industries present in the Asansol industrial area.

Source Identification
Principal component analysis (PCA): PCA was used to identify the PTEs sources in the indoor settled dust of Asansol   summer and winter. Fe, Ni, and Zn concentration were not significantly different during summer and winter, which might be due to the influences of some local sources of these PTEs. Cement industries at Asansol industrial area situated at the north-east region of the area and as in summer the wind at that area blow from the North-East direction, the Zn concentration in that area is not significantly different during winter with summer. During summer and winter, the dispersion of pollutants is different, which could be responsible for Fe, Zn, and Ni concentration not significant during these two seasons.

Bioaccessibility
Seasonal variation of PTEs bioaccessibility in indoor settled dust was observed and represented as the percentage of bioaccessibility (Fig. 4a). Cd and Zn bioaccessibility was higher during the summer, followed by winter. Bioaccessibility of Cr, Cu, Ni, and Pb was higher during the winter followed by the monsoon. Fe and Mn bioaccessibility was maximum during monsoon followed by summer. Seasonal variation of bioaccessibility was observed in the indoor settled dust at Asansol industrial area might be due to different soil parameters, and this theory was supported in a study reported from China . The bioaccessible fraction of Cd was higher when compared to other PTEs, and the bioaccessible fraction of Fe was lower in the present study.
The main controlling factors of bioaccessibility of PTEs are speciation of PTEs, pH, presence of organic matter and mineral composition of matter (Roussel et al. 2010, Hu et al. 2012, Pelfrene et al. 2012, 2013. Mineralogy is one of the important controlling factors of bioaccessibility (Ettler et al. 2019). PTEs might occur as exchangeable form, reducible form, oxidizable form and residual form, which plays an important role in Bioaccessibility. SBET estimate the amount of PTEs that is dissolved from ingested dust in the gastric juice and available for transport through the intestinal membrane (Cao et al. 2009, Hu et al. 2012. Low pH of gastric juice is responsible for the bioaccessibility of PTEs. The decreasing order of bioaccessible fraction was Cd > Zn > Cu > Pb > Ni > Cr> Mn> Fe during summer and winter, but during monsoon the order was Cu > Zn > Cd > Pb > Ni > Cr > Mn > Fe. In this study, the bioaccessibility fraction between schools and houses (Fig. 4b) was compared. Only Zn bioaccessibility was higher in schools than houses, while bioaccessibility of other 7 PTEs was higher in house dust. Mainly outdoor aerosol particulates vehicular exhausts are responsible for high Zn bioaccessibility in schools' dust.

Human Health Risk Assessment
The health risk of PTEs was calculated for children and adult in the indoor settled dust samples of Asansol industrial area (Table 3). Ingestion, inhalation, and dermal contacts are the three main pathways and were considered for health risk assessment. Hazard Quotient (HQ) and Hazard Index (HI) were separately calculated for each PTE. In this study, total PTEs concentration and SBET-extracted PTEs concentration were used separately for health risk assessment. During use of total PTEs (Cd, Cr, Cu, Mn, Ni, Pb and Zn) concentration for health risk assessment, ingestion was the main exposure route both in child and adults followed by dermal contact (Li et al. 2013) but in case of Fe, maximum risk observed for dermal contact. During use of SBET-extracted PTEs concentration for health risk assessment, the result was similar like total PTEs concentration. Here, ingestion was the main exposure followed by dermal contact for all observed PTEs except Fe. Decreasing order of non-carcinogenic health risk for total PTEs concentration was Mn > Fe > Cr > Pb > Cd > Cu > Ni > Zn and Fe > Mn > Cr > Pb > Zn > Cd > Cu > Ni > Zn in children and adults respectively, whereas risk for SBET-extracted PTEs concentration was Fe > Mn > Cr > Pb > Cu > Cd > Ni > Zn and Fe > Mn > Cr > Pb > Cd > Cu > Ni > Zn in children and adults respectively. In all cases, health risk due to Zn was lower when compared to other observed PTEs. The health risk assessment of the actual concentration of ingested and absorbed PTEs measured by SBET is more reliable and accurate (Oomen et al. 2002. Calculated health risk both for total PTEs concentration and SBET-extracted concentration of PTEs were < 1, indicating non-carcinogenic health risk was observed in the indoor settled dust of residence and schools at Asansol industrial area. Calculated health risk also indicated that children were more vulnerable group than adult (Gu et al. 2018, Gope et al. 2018. The HI values of Fe, Mn and Cr were close to 1, which indicates that these PTEs can cause health risk in future.  Monte Carlo simulation was performed only for ingested PTEs, and the distribution of the parameters are given in Table 4a and 4b. Cumulative hazard index of PTEs ingestion was calculated for the Monte Carlo simulation. From the sim-ulation, it is clear that 95% of the values of HQ (collectively for eight metals) was below 1. We can see that the cumulative HI was maximum during winter and was minimum during monsoon for both child and adult. From the cumulative  probabilistic simulation, it can be concluded that children are more susceptible than an adult during all three seasons. During the three seasons, the cumulative HQ of 8 metals indicated that risk was persistently higher for children than adult ( Fig. 5a and b). Calculated cumulative hazard index of all 8 PTEs are given in Table 5 were the distributions were considered as point distribution. It is found out that baseline case falls within 1. Predicted probability density functions of non-carcinogenic risk for summer, monsoon, and winter for adult and child are given in Fig. 6.

CONCLUSION
PTEs concentration in most of the cases is high in the Vol. 20, No. 1, 2021 • Nature Environment and Pollution Technology      house dust than school dust, which is highly influenced by the outdoor environment. During summer and winter, Fe and Mn concentrations were higher at schools. At schools, Cu concentration was higher during summer and Cr concentration during monsoon. It was observed that the major sources of PTE's in the indoor environment of schools and houses are from the outdoor environment, particularly from industries and vehicular exhaust. High Cd enrichment was noted at all four sampling sites of the Asansol industrial area. PCA indicated industrial emission, lithogenic and traffic-related materials as major sources. The health risk was <1 for total concentration and SBET-extracted PTEs concentration. Risk assessment with the average values for PTEs concentration indicated that Cr, Cu, Fe and Pb could cause health risk in future. Monte Carlo simulation depicted that chances of children getting affected by the PTEs are prominently higher than adults. Harmful effects of PTEs in different seasonal conditions vary differently, and the study concluded that in summer, monsoon, and winter scenario, monsoon, in general, is relatively less risky. Further studies are required encompassing a broader spectrum of the season and different social categories for a comprehensive understanding of risks associated with the PTEs. Studies enquiring further into identifying the particular sources, PTEs emission rate, chemical speciation and mobility of PTEs in indoor dust is required.

ACKNOWLEDGEMENT
The first author received fund from UGC Non-NET