Introduction

Small children and infants are particularly vulnerable to pollutants because of their rapid growth and cell differentiation, immaturity of metabolic pathways and their development of vital organ systems (Moshammer et al. 2006; Sunyer 2008). Preschools are an important microenvironment associated with children’s exposure to indoor pollutants due to the large portion of time they spend at such establishments on weekdays (Lupoli et al. 2009; Yang et al. 2009). Indoor air quality in preschools has received wide interest in recent studies due to the possible impact of air pollutants on preschool children (Wichmann et al. 2010; Kang et al. 2011). The air quality in an environment is usually associated with children’s respiratory illnesses (Dockery et al. 1989; Von Mutius et al. 1995; Pekkanen et al. 1997; Qian et al. 2000; Jalaludin et al. 2004). Furthermore, several studies also indicated the influence of indoor pollutants on the academic performance and mental stability of the children in question (Mendell and Heath 2005; Mohai et al. 2011).

Indoor dust is one locus of indoor contaminants containing toxic materials, particularly heavy metals, in quantities that may be potentially harmful to human health (Landrigan et al. 2004; Roberts and Ott 2007; Hochstetler et al. 2011; Aucott and Caldarelli 2012). Indoor dust has been found to contain a quantity of heavy metals which may potentially affect the health of young children (Akhter and Madany 1993; Al-Rajhi et al. 1996; Kim et al. 1999; Meyer et al. 1999; Butte and Heinzow 2002; Latif et al. 2009). A high intake of heavy metals by children has been associated with mood swings, poor impulse control and aggressive behaviour along with a poor attention span, depression and apathy, disturbed sleep patterns and impaired memory and intellectual performance (Holford and Colson 2008).

It is known that the sources of heavy metals in indoor dust are varied and depend on the condition and location of a building, the activities occurring in the indoor environment, as well as outdoor sources (Abdul-Wahab and Yaghi 2004; Jaradat et al. 2004; Abdul-Wahab 2006; Latif et al. 2009; Pekey et al. 2012). A study by Kumpiene et al. (2011) has suggested that industry and city traffic, top soil and building materials, especially during renovation, are among the sources of heavy metals in preschools. Heavy metals may be transported via ventilation systems and by the children’s movement from outside to inside a classroom which both in turn contribute to the presence of heavy metals in the classroom in the form of dust brought in from outdoors. Several studies have indicated heavy traffic as a source of heavy metals in road dust, which subsequently becomes another source of heavy metals in the indoor environment (Akhter and Madany 1993; Tiittanen et al. 1999; Adachi and Tainosho 2004; Al-Khashman 2007; Gaudry et al. 2008). In addition, it is possible that contaminated soils or dust ingested, either directly or indirectly as a result of hand-to-mouth activity, represent a significant pathway of lead intake during early childhood (Al-Rajhi et al. 1996).

The objective of this study was to determine the concentrations of selected heavy metals (Pb, Zn, Cd, Fe and Cr) in the dust inside classrooms to which children in preschool could be exposed. Furthermore, this study will investigate the correlation of classroom dust with dust collected from the interior walls and surface soils surrounding the preschool areas. This study will also correlate the composition of heavy metals in all types of dust samples collected with the heavy metals on children’s palms so as to determine the degree that preschool children are exposed to heavy metals.

Methodology

Sampling sites

Sampling was conducted between September and December 2009 at ten selected preschools in Bandar Baru Bangi and Kajang, Selangor, Malaysia (Table 1 and Fig. 1). Bandar Baru Bangi and Kajang are located in the southern part of Kuala Lumpur, which has approximately 343,000 residents. These areas are dominated by residential areas with heavy traffic particularly during rush hours when people are either going to or from work. Additionally, these two areas also have highways, busy roads and small industrial areas which may in turn contribute to the level of air pollutants in the surrounding areas.

Table 1 Description of the sampling stations
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Fig. 1
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Map showing the locations of study area and ten sampling sites in this study

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The present study was designed to document the quantity of heavy metals that preschool children, between the ages of 5–6, were exposed to from indoor dust in their classrooms. Generally speaking, preschools are home to 20 to 40 preschool children and the school’s indoor environment contains furniture and toys for them to use. All school children’s shoes are kept outside the classroom and the classrooms are cleaned daily after the children go home. In the mornings, these children usually take part in outdoor activities.

Dust sampling

Dust samples from the indoor environment were collected, using a soft paintbrush, from items of furniture (desks, chairs, windows and fans) around the classrooms and from the surface soil in the outdoor environment. At each sampling site, the samples (n = 9) from the indoor environment were taken from both areas which were generally occupied by the children and those where they routinely played. If the children played outdoors in the garden or playground in the close vicinity of the preschool, the soil was sampled using a ring (2 cm deep) and soil surface of 0.04 m2 (0.02 m × 0.02 m). These dust samples were then transferred into individual resealable plastic bags. In the laboratory, samples were gently filtered using a <63 μm size sieve prior to sample digestion. Dust was also collected by wiping 0.09 m2 (0.03 m × 0.03 m) of the surface of the indoor walls and the children’s palms with dried and pre-weighed Kimwipes. The dust from the children’s palms was collected after the children had finished their first activities at around 10:00 a.m. in the morning.

Sample digestion and heavy metal analysis

Each indoor dust sample (1 g) was dissolved in a mixture of nitric and perchloric acid (v/v, 16:4) on a hot plate for 1 h. For the Kimwipes’ dust samples, the Kimwipes were cut into small pieces before being dissolved on a hot plate using a mixture of nitric and perchloric acid (v/v, 16:4). Each sample solution was filtered using Whatman cellulose acetate filter papers (with a 0.2-μm pore size and 47 mm diameter) and filtration apparatus linked to a vacuum pump. The filtered solution was then diluted to 100 ml in a volumetric flask with deionised water and kept in a polyethylene bottle at 4 °C until analysis. Reagent blanks for the samples of dust from the interior walls were similarly prepared using unused Kimwipes. Heavy metal concentrations in solutions were determined using inductively coupled plasma mass spectrometry (ICP-MS, PerkinElmer ELAN 9000). Linear calibration graphs (absorbance versus concentration) for each heavy metal tested were prepared using a commercial standard solution (1,000 ppm multi-element ICP-MS calibration standard 3, PerkinElmer). The instrumental detection limits per mass sample (g) were determined as 0.05 μgg−1 for Pb, 1.0 μgg−1 for Zn, 0.01 μgg−1 for Cd, 20 μgg−1 for Fe and 0.5 μgg−1 for Cr. The measurement of heavy metals in indoor dust and soil were measured in units of μgg−1 (mass of heavy metal (μg)/mass of sample (g)) while the concentration of heavy metals on indoor walls and children’s palm were measured in μgm−2 (mass of heavy metal (μg)/surface area (m−2)).

Enrichment factor

Enrichment factor (EF) is calculated to determine the source of heavy metals in samples based on the primary metal element found in earth crust. Values lower than 10 indicate that the element investigated has a significant crustal source (soil), while EF values higher than 10 are ascribed to elements of anthropogenic origin (Biegalski et al. 1998). EF value between 10 < EF < 100 can be considered as moderately enriched whilst the value >100 can be deemed as highly enriched (Wang et al. 2006).

The crustal enrichment factors of elements in indoor dust in this study were calculated using the average concentration of heavy metals and indoor dust applying the formula used by Brauer et al. (2002), Al-Momani (2007) and Hassan (2012). The enrichment factor (EF) for a generic element X in comparison with a crustal reference element Y is defined as:

$$ {\mathrm{EF}}_X=\frac{{\left(X/Y\right)}_{\mathrm{indoor}\ \mathrm{dust}}}{{\left(X/Y\right)}_{\mathrm{earth}\ \mathrm{crust}}} $$

in which (X/Y)indoor dust is the concentration ratio calculated starting from X concentration and Y concentration measured in the indoor dust sample, and (X/Y)earth crust is the concentration ratio in the crust (Taylor et al. 1983). In this study, Fe has been used as an indicator for the main source of earth crust composition, Y. The enrichment factor for heavy metals recorded on the wall and children’s palms was also calculated by using the same formula given above but replacing the concentration of heavy metals and Fe concentration in indoor dust (X/Y)indoor dust with the concentration of heavy metals and Fe recorded on the wall (X/Y)indoor wall and children’s palms (X/Y)children’s palms respectively.

Quality control

Quality control was practiced throughout the analysis to avoid any interference and minimize the risk of error. Powderless gloves were used while performing all stages of the experiment, most notably when handling the Kimwipes and indoor dust. Furthermore, all analyses of samples involving Kimwipes were conducted in a laminar flow chamber. All the polyethylene bottles and glassware utilized for heavy metal analysis were pre-cleaned by being soaked in nitric acid overnight before being rinsed with deionised water. Additionally, any instruments involved in this analysis were calibrated before use.

Statistical analysis

All statistical calculations were performed using the Student Version of the Statistical Package for Social Sciences (SPSS Version 17.0) for Windows. Correlation coefficients and correlation significance among measured heavy metals were analysed using the one-way variance test with a 95 % level of confidence after the data was found to be in normal distribution (Young et al. 2011).

Results

Heavy metals in indoor dust

The heavy metal concentrations (Pb, Zn, Cd, Fe and Cr) in the indoor dust samples collected from the selected preschools are presented in Table 2. Fe was recorded as having the highest concentration among the heavy metals tested, with an average concentration of 4,801 ± 1,873 μgg−1, followed by Pb (253.5 ± 83.2 μgg−1), Zn (144.9 ± 73.4 μgg−1), Cr (11.91 ± 6.75 μgg−1) and Cd (0.23 ± 0.10 μgg−1). Fe concentrations across the samples ranged from 1,439 ± 2,810 μgg−1 (S5) to 7,969 ± 1,216 μgg−1 (S7). There were significant differences (p < 0.05) between Fe concentrations in indoor dust among the sampling stations. The level of Fe found in indoor dust was expected due to the abundance of Fe as one the major components of the earth’s crust (Wang et al. 2006; Sultan and Shazili 2009; Hunt et al. 2011). At the same time, the differences in Fe concentrations in indoor dust sampled at different stations were also estimated because of the influence of soil dust entering the indoor environment through the movement of students and also the influence of the wind blowing from outdoors.

Table 2 The average of heavy metals concentration in indoor dust (n = 9) and its range (in parenthesis)
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The concentrations of Zn ranged from 25.0 ± 13.3 μgg−1 (S10) to 256.8 ± 30.0 μgg−1 (S3) and there were significant differences in Zn concentrations between stations (p < 0.05). The highest concentration of Zn recorded was at S3, a preschool located in the city centre area, while the lowest concentration of Zn recorded was at S10, which is located in a residential area. Pb concentrations ranged from 57.0 ± 83.7 μgg−1 (S10) to 328.6 ± 27.6 μgg−1 (S3). There were significant differences (p < 0.05) between Pb concentrations in the indoor dust collected from different stations. S3 (the sampling site located in the city centre) recorded the highest concentration of Pb.

Cd and Cr were found at lower concentrations than the other metals investigated. The Cd concentration ranged from 0.07 ± 0.03 μgg−1 (S10) to 0.38 ± 0.14 μgg−1 (S9) and there were significant differences (p < 0.05) between Cd concentrations among the sampling stations. S10, which is located in a residential area a distance from any main road, recorded a very low Cd concentration compared to those recorded at other stations. However, the results for the other stations were nearly the same as the highest concentration recorded at S9. The Cr concentration in the indoor dust samples ranged from 4.79 ± 1.75 μgg−1 (S10) to 24.29 ± 13.96 μgg−1. Like Cd, the lowest concentration of Cr was found in the preschool located at S10.

The calculation of the enrichment factor (EF) through using the ratio of selected heavy metals and Fe in indoor dust and the ratio of the selected heavy metals in earth crust material compared to Fe, as recorded by (Taylor et al. 1983), are shown in Table 3 and Fig. 2a. Overall results show that Pb has the highest EF value (83.6 ± 52.5) followed by Cd (15.3 ± 11.7), Zn (12.5 ± 7.6) and Cr (1.9 ± 1.0). These results indicate a higher enrichment of Pb in indoor dust compared to the other heavy metals determined. Even though the amount of Pb in petrol is no longer an issue, the accumulation Pb in road dust resulting from an accumulation of past emissions may influence the quantity of Pb in the road dust around the schools in question. With an absence of any major industry in or around the sampling sites, the levels of Cd could originate from lubricating oils and/or old tyres (Abechi et al. 2010). Zn was found to be one of the major components of road dust as a result of the accumulation of Zn emitted from tyres, motor oil and the usage of motor vehicle brakes (Li et al. 2001; Almeida et al. 2006; Duong and Lee 2011; Han et al. 2011). Road dust is also expected to have contributed to the level of Cd in the indoor dust collected from the designated preschools.

Table 3 The average Enrichment Factors (EF) of heavy metals in all samples (n = 9)
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Fig. 2
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Value of enrichment factor (EF) of heavy metals concentration on indoor dust, dust on surface soil, interior wall and children’s palm, including the uncertainty for each element

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Overall concentrations of the heavy metals recorded in the preschools are higher compared to those in the surface soils collected around each preschool compound, that is, except for Fe and Cr (Table 4). Fe is the major element in the surface soil and hence has the ability to influence the level of Fe in indoor dust. In this study, the level of Cr in the surface soil collected from several school compounds showed a higher concentration compared to that in indoor dust. The calculation of the EF of heavy metals in the surface soil also showed that the average value of the EF for each heavy metal is between 0.8 and 5.2, indicating that those heavy metals in the surface soil are not significantly enriched by anthropogenic sources (Table 3 and Fig. 2b).

Table 4 Heavy metals concentration in surface soils (n = 9) and its range (in parenthesis)
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A comparison of our findings with those of a previous study undertaken by (Mohd Tahir et al. 2007) at a nursery/child care centre in the small Terengganu town of Dungun on the east coast of the Malaysia Peninsular (Table 5), shows the concentrations of all heavy metals but Zn in the indoor dust samples of this study to be higher. This is hardly surprising since the sampling locations for this study are nearer to urban and industrial areas than is the case for the small town of Dungun. It is this proximity to urban and industrial areas that can be expected to exacerbate heavy metal contamination in our samples, especially from street and blown-soil dust. Nevertheless, comparison with a study by Tong and Lam (1998) and a more recent study by Wang et al. (2011) in nursery schools (childcare centres) and kindergartens (preschools) in Hong Kong and China, respectively, showed that the concentration of heavy metals in this study are still low, apart from Pb. Further comparison with the composition of heavy metals in households, e.g. by Al-Rajhi et al. (1996) in a wide range of rural, suburban, urban, industrial and motorway environments in Riyadh, Saudi Arabia; Rasmussen et al. (2001) in Ottawa, Canada; Chattopadhyay et al. (2003) in Sydney, Australia; Turner and Simmonds (2006) in four regions (Birmingham, the Midlands, Plymouth and Devon) in the UK; Jaradat et al. (2004) in industrial areas of Jordan; and Habil et al. (2013) near roadside and residential areas in Agra, India, indicated that the concentrations of heavy metals in indoor dust from this study are still within the range of those recorded in many other areas around the world.

Table 5 Comparison of the heavy metals (in μgg−1) from this study to other studies
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Heavy metals in dust on interior walls

The concentrations of the five heavy metals studied in the dust collected from the interior walls are summarised in Table 6 and it can be noted that Fe remained the dominant heavy metal in these samples. The average concentration of Fe in the dust collected from interior walls was 1,865 ± 756 μgm−2. The concentrations of the other heavy metals determined were lower, with Zn recording the second highest concentration (236.1 ± 335.4 μgm−2 ) followed by Cr (120.8 ± 26.0 μgm−2), Pb (5.85 ± 2.75 μgm−2) and Cd (0.32 ± 0.14 μgm−2).

Table 6 Heavy metals concentration in dust on interior walls (n = 9) and its range (in parenthesis)
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The dust on the interior walls consisted of very small particles which adhered to the surface. It is possible that heavy metals accumulated on the indoor walls as a result of the re-suspension of the indoor and outdoor environment of the classroom. The amount of Fe recorded in the indoor wall dust samples suggests that wind-blown dust, particularly from surface soil and road dust are the main contributors to the amount of heavy metals found. While heavy metals may originate from paint along with gaps and cracks in walls (Harney et al. 2000; Tong and Lam 2000), this study has no way of confirming this. The study by Tong and Lam (2000) also found that the colour of the paint determined the type of heavy metal contaminant on the indoor wall surface. A comparison of Pb concentrations (from the surface of interior walls) between this study and one by Decker et al. (1999) indicates that the average concentration of Pb on interior walls (5.85 ± 2.75 μgm−2) is far lower in this study than is the case for surface areas in the inner-city old high school in the USA, which recorded a lead concentration of between 102 to 342 μgft−2 (1,098–3,261 μgm−2).

The EF value for the composition of heavy metals on interior walls showed that Zn, Cd and Cr are moderately enriched compared to Pb, which has a very low EF value (Table 3, Fig. 2c). The results also demonstrated that the level of Pb on the indoor walls correlates to the amount of Pb in both soil and indoor dust. Moreover, the quantity of Zn, Cd and Cr are contributed to by anthropogenic factors, particularly from building materials and outdoor dust intrusion. According to a study by Davis et al. (2001), heavy metals such as Zn, can be attributed to various building siding materials, such as: brick, both painted and unpainted wood and concrete.

Heavy metals on children’s palms

The concentration of heavy metals found on children’s palms is shown in Table 7. Of the five heavy metals determined, Fe showed the highest average concentration (3,882 ± 3,401 μgm−2), followed by Zn (919.9 ± 393.8 μgm−2), Cr (383.5 ± 393.2 μgm−2), Pb (72.97 ± 112.07 μgm−2) and Cd (2.36 ± 3.58 μgm−2), respectively. The high concentration of Fe indicates that surface soil had an influence on the concentration of heavy metals found on the children’s palms. These heavy metal concentrations were also discovered to have a similar pattern to those recorded in the interior wall dust. Except for Cd, the concentration of heavy metals on the children’s palms is higher compared to the concentrations determined on the indoor walls. The size of the children hands and the variety of sources of dust on their palms (fine and coarse mode dust) may have contributed to the higher concentration of heavy metals on their palms. The children seem to be affected by all forms of indoor dust and their palm concentrations are indicative of both interior wall dust and indoor floor dust.

Table 7 Heavy metals concentration on children’s palms (n = 9) and its range (in parenthesis)
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From the ANOVA analysis, it can be noted that there is a significant difference (p < 0.05) in the concentration of heavy metals on the children hands from the samples collected at different stations. For example, the concentrations of Pb and Fe as recorded on the children’s palms at S6 were higher compared to the concentrations recorded at other stations. The activities within the schools, especially relating to outdoor physical activities, contribute to the amount of heavy metals on the children’s palms. The touching of indoor walls may also augment the level of heavy metals which originate from the wall paint and are transferred onto their hands. In the case of S6, the concentration of Pb on the children’s palms may be due to the corresponding high concentration of it on the indoor walls.

Further investigation using the EF (Table 3, Fig. 2d) indicated that the Zn and Cd found on the children’s palms was significantly enriched by anthropogenic sources. In this case, the influence of anthropogenic sources, such as: indoor dust from building materials and outdoor fine dust sticking on indoor walls and materials can be considered to be the main contributors to the amount of heavy metals on children’s palms. The moderately enriched Zn and Cd, which were also found on the indoor walls, can explain the contribution of fine particles from suspended dust on the indoor walls (Fig. 2c) to the children’s palm.

Correlation between heavy metals determined at different substrates

The correlation between heavy metals in different substrates is presented in Table 8. A strong significant correlation between the metals on children’s palms was noted, for example, Cr and Fe (r = 0.893, p < 0.01) and Cd and Pb (r = 0.773, p < 0.01). Most of the concentrations of heavy metals on children’s palms are well correlated with those found in surface soil dust, for example, Fe (r = 0.782, p < 0.01) and Cr (r = 0.679, p < 0.01) in children’s palms strongly correlate with Cr in surface soil dust. Likewise, the majority of the heavy metal concentrations in indoor dust are significantly correlated with each other, e.g. a strong significant relationship was found between Pb and Zn (r = 0.761, p < 0.01) in indoor dust. Besides, Cr and Zn have a positive significance with other heavy metals (p < 0.01). However, correlations between concentrations of heavy metals in indoor dust and those in dust on interior walls were weaker, except for the correlation of Zn in indoor dust with Fe in dust on interior walls (r = 0.272, p < 0.01), followed by Cd in indoor dust and Fe in dust on interior walls (r = 0.416, p < 0.01). Concentrations of Fe in indoor dust were found to have a strong correlation with those of Fe (r = 0.709, p < 0.01) and Cr (r = 0.521, p < 0.01) in surface soil dust, suggesting that the amount of Fe in the indoor environment may be related to the composition of Fe in the surface soil dust and also to other Fe materials in the indoor environment.

Table 8 Correlation matrix between heavy metals in indoor dust, in dust on interior walls, in surface soil dust and on children’s palms
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The outdoor-to-indoor movement of children during class activities, such as: playing in the playground, can also contribute to the amount of heavy metals in the indoor environment. It is highly likely that mineral dust caused by the suspension or re-suspension of settled dust is brought in by the children’s footwear (Tong and Lam 2000; Almeida et al. 2010). Bero et al. (1993) also showed that floor surfaces are good reservoirs for dust and soil. Hence better housekeeping with more frequent sweeping, dusting and vacuuming may alleviate the level of heavy metals found in indoor dust. Likewise, good ventilation systems should distribute sufficient fresh air to occupants, particularly children, as well as provide thermal comfort (Clements-Croome et al. 2008).

Conclusions

This study presents an analysis of selected heavy metals (Zn, Fe, Pb, Cd and Cr) which are found in indoor dust and which are associated with surface soil and interior walls. The average concentrations of the selected heavy metals in the indoor dust were dominated by Fe with a concentration of 4,801 ± 1,873 μgg−1 followed by Pb > Zn > Cr and Cd with an average concentration of 253.5 ± 83.2, 144.9 ± 73.4, 11.9 ± 6.8 and 0.23 ± 0.10 μgg−1, respectively. Fe also showed the highest concentration of the heavy metals investigated in samples of soil dust (8,225 ± 6,800 μgg−1), interior walls (1,865 ± 756 μgm−2) and children’s palms (3,882 ± 3,401 μgm−2).

The enrichment factor results showed that most of the heavy metals determined in indoor dust are influenced by anthropogenic sources. The re-suspension of accumulated heavy metals in road dust is expected to contribute to the amount of heavy metals in the indoor environment. The high EF value for heavy metals, such as: Zn, Cd and Cr on the inside walls and children’s palms, shows that particulate matter from outdoor dust, building materials, wall surfaces and other indoor materials touched by the children can influence their exposure to heavy metals. The high levels of heavy metals on their palms could be an important indicator, signalling the exposure of heavy metals to the children. These toxic heavy metals are a source of health concern as they can be transferred to other organs in the body through ingestion via children’s hand to mouth activities. According to Järup (2003) and Llop et al. (2013), the effects of heavy metals to children include neurotoxicity.

The study showed that preschool children can be exposed to heavy metals from a variety of sources from the outdoor and indoor environment. Comprehensive investigation into the existence of heavy metals in and around the school environment is crucial if children are to reduce being exposed to such metals. At the same time, various mitigation procedures, such as: the daily cleaning of indoor dust and a reduction in the use of materials containing heavy metals should be implemented so as to reduce the exposure of heavy metals to those children in a preschool environment.