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Benzene (C₆H₆) with molecular weight 78.1 g/mol is an organic hydrocarbon (consists of carbon (C) and hydrogen (H) atoms) and an aromatic compound with a single six-member unsaturated carbon ring. It is a clear, colorless, volatile, highly flammable liquid with a characteristic odor and density of 874 kg/m³ at 25°C. Benzene has a melting point of 5.5°C which is relatively low boiling point of 80.1°C at 1 atmosphere of pressure. It has a high vapor pressure which is 12.7 kPa at 25°C, causing it to evaporate rapidly at room temperature. It is slightly soluble in water (1.78 g/l at 25°C) and is miscible with most organic solvents (WHO, 2000). Benzene in air exists in the vapor phase, with residence times varying between 1-14 days, depends on the environment, climate and other pollutants concentration (WHO, 2010).

Standards and guidelines on safe exposure level of indoor air pollutants have been developed by government, professional health and safety organizations based on research and epidemiologic studies for many years. These are to ensure the best air quality inside a building for the occupants in order to control and minimize the potential health effect to occur in humans.

Regulatory guidelines for acceptable VOC concentrations within indoor environments do not currently exist in Malaysia. There is no safe level of benzene that can be recommended as benzene is a genotoxic carcinogen in humans (WHO, 2010). There is no reason that the guidelines for indoor air should differ from ambient air guidelines since the risk of toxicity from benzene inhalation is the same whether the exposure was indoors or outdoors. The geometric mean of the range estimates of excess lifetime cancer risk at concentration of 1 μg/m³ is 6×10^-6. The concentrations of airborne benzene associated with lifetime cancer risk of 1/10 000, 1/100 000 and 1/1000 000 are 17, 1.7 and 0.17 μg/m³, respectively (USEPA, 2011; WHO, 2010). OSHA set an exposure limit of 1 ppm in the workplace during an 8 hours working day and 40 hours working week. While, short-term exposure levels for 15 min are set at 5 ppm (OSHA, 1998).

Based on the papers selected as shown in Table 1, there were 15 countries that involved in determination of benzene in indoor environments. Generally, the highest number of studies that were conducted in these countries were included Portugal, United States of America, Turkey and Korea. The variation in the climate and location of each country worldwide, varies in temperature and humidity can significantly influence the indoor benzene levels.

The selection of the year was based on the date of publication since year 2000 until 2018. As shown in Table 1, earlier study was conducted in the year 2000 in two urban primary schools (Adgate et al., 2004) and the recent study was conducted in the year 2014 (Mainka et al., 2015) in two urban nursery school. However, the most recent study was published in the year 2018 by Villanueva et al. (2018) that covered 18 primary schools in urban, rural and industrial areas. Based on the list, study on exposure to benzene in children has become as one of the interesting topics to be investigated from across the countries from the last two decades.

As referred to Table 1, the most frequently studied indoor environment was primary schools (54%), followed by daycare centres (31%) and preschools (15%). Most of the studies conducted in schools were focusing on the differences between the type of sampling areas, for example the differences between schools located in urban and suburban areas (Villanueva et al., 2018; Demirel et al., 2014; Pegas et al., 2012; Sofuoglu et al., 2011; Bartzis et al., 2008; Godwin and Batterman, 2007; Adgate et al., 2004). Meanwhile, as for the preschool and daycare centre, Yoon et al. (2011) and Mainka et al. (2015) also focused on the differences between sampling areas. However, the other studies did not mention clearly the exact location of the sampling sites (i.e. city, urban, suburban, rural, industrial areas).

Based on Table 1, the sampling methods of the studies were varying by approach, time, flow rate, number of sampling sites and sampling techniques. Active sampling requires a pump whereas passive sampling is through diffusion controlled (Goodman et al., 2017). For the exposure assessment of benzene, active and passive samplings were used at almost equal percentages; 58% for passive sampling and 42% for active sampling of all studies. Passive sampling generally used single sorbent due to the lower diffusion-controlled sorbent adsorption rates. Radiello passive sampler (RAD 130, activated charcoal), Summa^TM canister passive samplers and 3M OVM 3500 organic vapor monitors were used in passive sampling. Meanwhile, active sampling techniques were included the used of charcoal and thermal desorption tubes; i.e. Anasorb Coconut Shell Charcoal tubes and Tenax TA thermal desorption tubes. Active sampling technique was used in studies that collect sufficient volumes of air in shorter time periods, as low as 30 minutes (Norbäck et al., 2017; Sofuoglu et al., 2011; Jang et al., 2007).

There were variations of sampling duration in the studies of exposure to benzene based on Table 1. Most of the papers had a sampling time more than 4 hours per day (Axelrad et al., 2013; Bureau, 2011). Few studies had sampling duration of 24 hours for 5 days per week (Kalimeri et al., 2016; Madureira et al., 2015; Demirel et al., 2014; Zhang et al., 2013; Pekey and Arslanbaş, 2008), 7 days per week (Villanueva et al., 2018; Geiss et al., 2011; Bartzis et al., 2008) and less than 6 hours per day (Norbäck et al., 2017; Noguchi et al., 2016; St-Jean et al., 2012; Sofuoglu et al., 2011; Yoon et al., 2011; Godwin and Batterman, 2007; Jang et al., 2007). Due to their differ in sampling durations comparison between these studies were limited.

However, some studies had reported the same sampling durations of 24-hours period. For example, five studies were conducted in indoors with the presence of children and the range mean values were reported as followed: 1.5-2.7 μg/m³ (Madureira et al., 2015); 0.39-13.2 μg/m³ (Demirel et al., 2014); 2.5-148.0 μg/m³ (Zhang et al., 2013); <1.0-1.63 μg/m³ (Madureira et al., 2012) and 7.5-19.77 μg/m³ (Pekey and Arslanbaş, 2008). Meanwhile, the other studies were reported in varieties of sampling durations from 30 minutes (Jang et al., 2007) to 7 days per week.

Analytical method protocol used by most of the authors in their studies had cited US EPA Compendium Method TO-17 for the analysis of benzene (Norbäck et al., 2017; Quirós-Alcalá et al., 2016). All studies reported to use gas chromatography/mass spectrometry (GC/MS) as the principal method of analysis. Meanwhile, automated thermal desorption GC/MS and flame ionization detector were used as the principal mode of detection, however they were not uniformly specified in every study. In general, the methods used for analyzing benzene in indoor environments are consistent with international protocol.

For the details on the concentrations of benzene, the indoor arithmetic mean (AM), median, minimum (min) and maximum (max) concentrations are reported in Table 2. The min, max and median values were reported only in some studies and the AM values were reported in all the selected studies. In the comparison on benzene levels among the studies, the differences in sampling approach and duration were not considered. Thus, the evaluation in this study is provided with that following criteria (Goodman et al., 2017).

Based on Table 2, the maximum concentration of benzene in school was recorded at 19.77 μg/m³ which was found during winter season (Pekey and Arslanbaş, 2008). The study concluded that the indoor activity, ventilation and the duration of human occupancy during the winter season had influenced the indoor air quality in the building. High level of air pollutant during winter season was due to the decrease in ventilation since windows were opened less frequently and air conditioners were seldom used, resulted in persistent benzene sources from indoors. Meanwhile, the highest concentration of benzene reported in preschool in Nanjing, China at 148.0 μg/m³ (Zhang et al., 2013). However, the explanation on the level of benzene recorded in this study was not reported. As for the benzene concentration in daycare centre, the maximum value was found at 32.7 μg/m³ (Zuraimi and Tham, 2008). This study was conducted to identify the effects of ventilation strategies on VOCs in 104 daycare centres in Singapore. Based on the source factor analysis, benzene was loaded together with compounds dominantly associated with traffic emissions from outdoor and can be emitted from indoor sources and human related activities (Sax et al., 2004; Zuraimi et al., 2003).

Based on Table 2, 56% of studies reported I/O ratios and 44% did not reported on the I/O ratios. This is because there were no available data on the outdoor air measurements. I/O ratios that were ≤1 indicate the absence of indoor sources or dilution effects of indoor sources. While ratios that were >1 indicate strong indoor sources or poor ventilation (Zuraimi et al., 2008, 2003). Few of the studies evaluated the I/O ratio based on each study site (Villanueva et al., 2018; Kalimeri et al., 2016; Demirel et al., 2014; Yoon et al., 2011; Pekey and Arslanbaş, 2008) and some studies reported I/O ratios generally.

Generally, I/O ratios in the range of 1.1-3.3 revealed that much of benzene sampled were several times higher in indoors compared to the outdoors. Meanwhile, the I/O ratios ranged from 0.61 to 1.0 indicated strong outdoor sources of benzene. The lowest I/O ratio at 0.61 was reported by Guo et al. (2003) due to the motor vehicle emission as the significant outdoor benzene source. The highest I/O ratio at 3.3 was found during summer season in preschool by Kalimeri et al. (2016). The study found that the source of benzene was detected from the wall paint in the building.

Many studies had reported on the effects of the seasonal variations during the sampling were conducted. These variety of the seasons had influenced the concentrations of benzene in indoor environments (Kalimeri et al., 2016; Madureira et al., 2015). 44% of the studies did not reported on the type of season, 28% were reported on the benzene levels from a single season, 24% were recorded in two seasons (winter/summer; spring/summer; winter/spring) and only one study that reported in three seasons (winter/spring/autumn) (Sofuoglu et al., 2011).

Based on the studies that were conducted within two seasons, concentrations of benzene were found to be higher in indoors during winter season as compared to the warmth season (Kalimeri et al., 2016; Geiss et al., 2011; Roda et al., 2011; Pekey and Arslanbaş, 2008; Godwin and Batterman, 2007). This might be due to the low ventilation rate as air conditioners are seldom used and short window opening periods with low opening frequency had increased the benzene levels (Kalimeri et al., 2016). Besides that, indoor activity, sources of benzene indoors and duration of human occupancy also influenced the indoor air quality during winter (Pekey and Arslanbaş, 2008).