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Tiếp xúc cá nhân với Benzene của các nhóm dân lựa chọn, và tác động của giao thông trên hành khách tại thành phố Hồ Chí Minh đã được nghiên cứu. Nghiên cứu được thực hiện trong tháng Sáu, tháng Bảy và tháng 11 năm 2010. Số liệu sơ bộ cho thấy rằng trung bình, tiếp xúc cá nhân với Benzene cho những người không nghề nghiệp tại TP Hồ Chí Minh là ~ 18 mg m 3 và hầu hết các tiếp xúc là do đi lại . Tiếp xúc với benzene trong đi du lịch bằng xe buýt, taxi và xe gắn máy là, tương ứng, 2230, 2239 và 185240 mg m 3 . Điều khiển xe máyxe taxi, nhân viên làm đầy xăng và bán hàng rong bị phơi nhiễm cao trong ngày tại 116, 52, 32 mg m 3 , tương ứng. Đo thêm là cần thiết để đánh giá rủi ro tốt hơn và việc tìm kiếm các biện pháp hiệu quả để giảm tiếp xúc. Điểm nổi bật ► tiếp xúc cá nhân với Benzene tại TP Hồ Chí Minh là ~ 18 mg m 3 . ► phơi sáng trong thời gian đi lại là 2239 mg m 3 cho xe buýt và taxi chế độ. ► phơi sáng trong thời gian đi lại là 185240 mg m 3 cho chế độ xe gắn máy. Benzen tiếp xúc ► trong thời gian đi lại góp phần chính của tiếp xúc hàng ngày. Trình điều khiển ► xe máyxe taxi, nhân viên xăng đầy và người bán hàng rong là những người tiếp xúc cao.
Personal exposure to benzene of selected population groups and impact of commuting modes in Ho Chi Minh, Vietnam Tran Thi Ngoc Lan a , * , Ngo Quang Liem a , Nguyen Thi Thanh Binh b a University of Science, Vietnam National University, 227 Nguyen Van Cu, HCMC, Viet Nam b Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore article info Article history: Received 27 March 2012 Received in revised form 13 December 2012 Accepted 16 December 2012 Keywords: Benzene Personal exposure Ho Chi Minh Motorcycle exhaust Commuter abstract Personal exposure to benzene of selected populatio n groups, and impacts of traffic on commuters in Ho Chi Minh City were investigated. The study was carried out in June, July and November 2010. The preliminary data showed that on average, personal exposure to benzene for non-occupational people in Ho Chi Minh is w18 m g/m 3 and most of the exposure is due to commuting. Benzene expo- sure during travelling by bus, taxi and motorcycle is, respectively, 22e30, 22e39 and 185e 240 m g/m 3 . Motorcycleetax i drivers, petrol filling employees and street vendo rs suffer high daily exposures at 116, 52, 32 m g/m 3 , respectively. Further measurements are needed for a better risk assessment and fi nding effective measures to reduce exposure. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Air pollution in urban areas is a serious problem of many developing countries. The rapid increase of the urban population and low transport infrastructure in East Asia has resulted in formation of megacities with dense motorcycle fleets like Kuala Lumpur, Bangkok, Delhi, Hanoi and Ho Chi Minh City. A conse- quence of this is decline of air quality due to volatile organic compounds (VOCs) and particulate matters discharged from vehi- cles. Many VOCs are considered toxic to humans, especially benzene since it is known as a carcinogenic substance. Benzene is highly volatile, so most exposure is via inhalation. The toxicity of benzene is stated on the US Environmental Protection Agency (EPA) website (2012b). Exposure to benzene increases the risk of leukaemia in humans (Bois et al., 1996; Crump, 1994; Rinsky et al., 1987). Experimental animal studies, both in inhalation and oral ingestion, showed evidence of increased risk of cancer in multiple organ systems including the haematopoietic system, oral and nasal cavities, liver, forestomach, lung, ovary, and mammary gland (Cronkite et al., 1985; Snyder et al., 1980, 1993). The risk of leukaemia associated with lifetime exposure to benzene at 17, 1.7 and 0.17 m g/m 3 is 10 À4 ,10 À5 and 10 À6 , respectively (World Health Organization [WHO], 2000). Benzene is carcinogenic; therefore, WHO and the US EPA do not recommend any safe level of exposure. Benzene in urban areas mainly originates from vehicle exhaust and evaporation from fuel tanks. Petrol-filling stations and garages contribute a significant amount of atmospheric benzene. Other sources that significantly increase indoor benzene levels are coal burning, tobacco smoking, off-gassing from building materials (paints, adhesives, etc.), use of benzene-containing consumer products and unflued oil and petrol heating. Evaporation from fuel tanks of motorcycle might be important source of benzene in Vietnam since each family owns several motorcycles and a guest room is common parking place for motorcycles. Human exposure to a pollutant is considered as the concentration of the pollutant in the air that one individual inhales, and differs from outdoor and indoor pollutant concentration. Exposure depends on pollution level in the urban air, as well as in microenvironments that an individual is exposed to, and the duration of exposure. Personal exposure to benzene of non- occupational non-smoking population was found to be higher than the outdoor ambient benzene level in Barcelona City metro- politan area and Catalan rural areas, Spain (Gallego et al., 2008); in Rouen, Île de France (Paris area), Grenobleand and Strasbourg, France (Gonzalez-Flesca et al., 2007); and in Copenhagen, Denmark (Skov et al., 2001). The right method forthe assessment of toxicity of benzene to humans is to relate benzene exposure to health effects. Ho Chi Minh City, with a population of around 8.5 million, is one of the most crowded cities in East Asia. Public transportation consists of buses and taxis; however, the usage rate is low. * Corresponding author. E-mail address: ttnlan@hcmus.edu.vn (T.T.N. Lan). Contents lists available at SciVerse ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2012.12.017 Environmental Pollution 175 (2013) 56e63 Motorcycles are preferred for their flexibility. The number of motorcycles and automobiles in March 2008 were 3,4 44,86 8 and 346,355, respectively, increasing to 3.9 million and 386,000 by June 20 09. On average, vehicle population increases about 10% per year. Following this trend, the number of motorcycles at the end of 2011 was probably around 5 million. Traffic volume in the city is extremely high. Our traffic survey at 29 major and 22 minor roads on 24 November 2010 showed daily traffic volume in a range of 1.7  10 4 e4.7  10 5 vehicles/day with an average at 1.4  10 5 vehicles/day. Traffic volume in rush hours (7 AMe8AM and 5 PMe6 PM) was 1.4  10 3 e4.8  10 4 , with average of 1.2  10 4 . Motorcycles contributed 90%e91% of a traffic fleet. On average, the moving speed of a motorcycle is 20 km/h in non-rush hours and 17 km/h in rush hours, road width is 18 m and buildings are mainly three-storey. Road area in rush hours was 10e200 m 2 / vehicle with a same median and geometric value of 29 m 2 / vehicle (unpublished data). Traffic jams are frequent. A large percentage of transportation means is made up of old technology. EURO II standard takes power in Vietnam since July 2008, but it is only applied for new imported vehicles. According to Lan et al. (2011), a daily benzene concentration in street urban air in Ho Chi Minh was 7e117 m g/m 3 with a geometric mean value of w45 m g/m 3 . A high concentration of benzene may badly affect human health. Up to date, there was not any published data on exposure to pollutants in Vietnam. This study aims to investigate personal exposure to benzene of selected population groups in Ho Chi Minh. The study was conducted from the end of 20 09 to the end of 2010. 2. Material and methods 2.1. Study population Ten people in three groups were recruited. The first group consisted of two housewives. Exposure in this group is equivalent to indoor benzene levels. Two houses were chosen. The first house was located in a narrow residential lane located w200 m away from a main street. The other house is a small shop located on and facing a main street. An entrance to the first house was just opened and closed few times in a day, while an entrance to the second house was opened from 8 AM to 9 PM. Windows on the second and third floors of the two houses were opened day around. None of houses had an air conditioner and/or ventilator. In Vietnam, most houses and building facing streets are shops. Commonly, shops are opened 12e 14 h per day. The two houses are designated as off-road and roadside indoors, respectively. Two students and a lecturer were in the second group for the study on exposure during commuting and working in university. Motorcycles are the most common means of urban transportation in Vietnam. Buses are mainly for university students and temporary visitors. Taxis and private cars are for high-income people. One student commuted by a city bus, another drove a motorcycle and the lecturer commuted by a 4-seat taxi. Departure and destination were two university Campuses, but routes were different. One-way mileage was about 25 km for the motorcycle and taxi, 32 km for the bus following definite route. One-way commuting time was 1.5 h by bus, 1 h by taxi and 1 h 10 min by motorcycle in the morning (6:30 AMe8 AM). The commuting time in the evening (after 6 PM) was a10e15 min shorter than in the morning. About one third of the route was in crowded narrow streets, and the other two thirds were in open-space roads. From w8AMtow7:30 PM, the students and lecturer stayed in the university campus, which is about 200 m away from a main road. Benzene exposure in the second group was designated as commuter/bus, commuter/taxi and commuter/motorcycle. Individuals in the third group were persons at high risk of exposure: a street vendor, a motorcycleetaxi driver, a bus driver, a taxi driver and a petrol-filling employee. Working times of street vendors varied depending on cases. The vendor was working from 6 AM to 10 PM in the front of the roadside house. This selection enabled evaluation of influence of proximity to road to indoor benzene levels. Motorcycleetaxis are common in Vietnam. Motorcycleetaxi drivers have no definite working time, from a few to 14 h per day. The motorcycleetaxi driver in the study was on the streets for about 10 h/day. In this time, he was driving for 7 h, waiting in front of university about 2 h, and having lunch and dinner for more than 1 h. The remaining time in the day, he was at home. Commonly, street vendors and motorcycleetaxi drivers are low-income people and most of them are temporary residents of the city. They usually live in houses located in narrow lanes in dense residential areas. In Ho Chi Minh City, departure of the first and last bus from a terminal is 5 AM and 8 PM. The bus in the study was a diesel 47-seater, and was air-conditioned like most of buses in Ho Chi Minh. The bus route was about 30 km. Working time of the bus driver is from 5 AM to 10 PM with a break after each round. Taxis in Ho Chi Minh run on petrol and operate throughout the day. A working shift of a taxi driver is normally 12 h. A 7-seat air-conditioned taxi was employed for a whole day for the study. All taxis and <9-seat cars in Ho Chi Minh are air-conditioned. Working time of the petrol-filling employee was 6 AM to 10 PM. The petrol-filling employee wore a facemask made of cotton-cloth layers. This type of mask is very common in Vietnam for protection from sunlight, particles and pollutants, and Vietnamese citizens wear them when commuting by motorcycles. The protection ability of facemasks from gas pollutants is not clear. Normally, petrol-filling employees, taxi drivers and bus drivers work one week on and one week off. 2.2. Sampling Passive sampling was applied for measurement of daily exposure of taxi and motorcycleetaxi drivers, while active sampling was used for investigation of hourly exposure in other cases. Active sampling was performed according to the NIOSH 1501 method (NIOSH, 2003). Air was drawn into sample tubes (Sibata 80150-0541, 70 mm  f 6 mm/4 mm, 200 mg of activated carbon) at a flow of 100 ml/min in 55 min or 110 min using a programmable minipump (MP P 30, Sibata, Japan). The pump was calibrated using a bubble flow meter. A sample tube holder was attached on a breast pocket. Sampling was conducted throughout the day for the off-road house, 6 AMe10 PM for the roadside house, street vendor, commuters, bus driver and petrol-filling employee. To simulate the air that petrol-filling employee inhaled, sampling tube was attached through a small hole into a PVC bottle (6 cm diameter and 10 cm height), the mouth of which was covered by the same facemask that the petrol-filling employee had. Sample tubes after sampling were sealed with plastic caps. Passive sampling was performed using Lanwatsu passive samplers (Lan and Binh, 2012). Passive samplers were attached on breast pockets. Sampling duration was 24 h. Sampling rates of the Lanwatsu passive sampler at 30 C were 17.7, 16.2, 15.3, 15.1 and 14.4 ml/min, respectively, for benzene, toluene, ethylbenzene, p,m- xylenes and o-xylene. Passive sampling was used in the second sampling campaign (November 2010) for taxi and motorcycleetaxi drivers, while active sampling was applied in the first sampling campaign (from June to July 2010) in other cases, Sampling was done in working days. Totally, 65 samples were taken. All samples were stored in a plastic bag sealed with a zippered laminar aluminium envelope and kept in an airtight box, cold-stored and brought to the laboratory. Sampling campaigns, climatic conditions obtained from Ho Chi Minh City meteorological station (VVTS) and mixing layer depth obtained from HYSPLIT Trajectory Model (NOAA) are given in Table 1 and Fig. 1. Ho Chi Minh has tropical monsoon climate. A year has distinct dry season (DecembereApril) and rainy season (MayeNovember). Temperature is stable all year round with a monthly average of 26 Ce28 C. The difference between daytime and nighttime temperature is 8 Ce 10 C. This difference is greater during the dry season compared with the rainy season. Commonly, wind velocity in the evening is stronger than in the morning. Surface thermal inversion is rather frequent in early morning in Ho Chi Minh. The frequency of surface stable layers of several hundred metres at 7 AM is up to 30% for any month in a year. Moreover, the thickness of surface stable layers during boreal winter time is double or triple that of the rainy season. From March to May, the frequency of surface stable layers decreases while their elevation above the surface increases and reaches 1.2 km. In addition, additional stable layers develop at 1.5e 3.5 km height above unstable layers during the dry season (Nodzu et al., 2006). The above conditions enable accumulation of pollutants on the surface in the early morning. 2.3. Instrumental methods and materials 2.3.1. Chemicals and standards All chemicals (puriss, anhydrous, analytical standard grade; with a purity >99.5%) were purchased from SigmaeAldrich. Carbon disulfide was treated by adding 20 mL of concentrated sulphuric acid and 10 drops of concentrated nitric acid to 1 L of the solvent and shaking for two days. A CS 2 layer is then decanted off, dried with anhydrous sodium sulphate and distilled. The treated CS 2 was checked for benzene by gas chromatography (GC) analyses. Usually, one treatment is enough for complete benzene removal. The bottle with benzene-free CS 2 was stored in a steel box containing activated charcoal at 5 C to avoid recontamination. A calibration curved was constructed using six working standards containing benzene (0.110e3.52 m g/ml), toluene (0.215e6.88 m g/ml), ethylbenzene (0.0542e 1.72 m g/ml), p,m-xylenes (0.108 e3.46 m g/ml) and o-xylene (0.0550e1.76 m g/ml) and two internal standards (IS), fluorobenzene (3.20 m g/ml) and chlorobenzene (3.46 m g/ml). The working standard solutions were stored in darkness at À5 C. 2.3.2. Analyses Analysis was carried out within a week after sampling as it was described in our previous report (Lan and Binh, 2012). A six-point linear calibration curve showed correlation coefficients above 0.999 for all analytes. The limit of detection was 1.42, 1.35, 1.40, 1.41, 1.42 ng/ml, respectively, for benzene, toluene, ethylbenzene, T.T.N. Lan et al. / Environmental Pollution 175 (2013) 56e63 57 p,m-xylenes and o-xylene. One working standard was routinely injected after each ten injections to correct a change in calibration curves. The change was not more than 2.6% for all analytes. Pollutant concentrations in the air were evaluated using Eq. (1) for both active and passive sampling C ¼ ðW À W blank ÞÂ10 6 DE  A  t (1) Where W and W blank ( m g) are the amounts of pollutant in sample and blank sample by analyses, DE is desorption efficiency given by the producers for each lot of charcoal and was 98% in this study, A is the sampling rate (ml/min), t is sampling duration (hour). Sampling rate of passive samplers was adjusted by using Eq. (2). A 1 ¼ A 2  T 1 303 1:5 (2) Where A 1 is a sampling rate at site temperature T 1 (K), A 2 is sampling rate at 30 C. Two petrol types marketed in Vietnam, RON92 and RON95, were analysed. Petrol was diluted in benzene-free carbon disulphide containing IS and injected into the gas chromatograph for quantification. 2.4. Evaluation of daily benzene exposure Daily exposure to benzene is evaluated using Eq. (3). Daily exposure ¼ P 24 1 Hour Concentration  Duration of exposure 24 (3) Sampling in the case of the taxi driver was conducted for two 12-h working shifts. Actually, the taxi driver was exposed to the measured daily concentration for only half a day, while the other half day he was exposed to indoor benzene levels in his house. Sampling was not conducted in the evening and at night for the commuters, bus driver, taxi driver, street vendor and petrol-filling employee. Approximately, the indoor benzene concentration in the evening and at night in the off-road house was assigned to the in-house exposure of the above individuals. This way of evaluation was reasonable since most of the houses in Ho Chi Minh are located on narrow residential lanes away from main roads, and the percentage of roadside houses and buildings is very small. The risk of leukaemia was roughly estimated using Eq. (4): here, the entire population groups were considered to be exposed to benzene levels found in this study for all their life. Risk ¼ 10 À4  Daily exposure 17 (4) An average daily exposure to benzene for the non-occupational population in the city was approximately using Eq. (5), where a transportation usage rate was evaluated using Eq. (6). Average exposure ¼ Daily exposure commuting mode i  usage rate commuting mode i (5) Usage rate commuting mode i ¼ Number of persons in vehicle i  traffic volume of vehicle i P 3 1 Number of persons in vehicle i  traffic volume of vehicle i  100 (6) 3. Results and discussion 3.1. Daily exposure to benzene in Ho Chi Minh City Daily exposure to benzene was given in Fig. 2. The housewife in the off-road house was exposed to benzene levels of 2.7 m g/m 3 ; while the one in the roadside house suffered 8.5-folds higher benzene exposure at 23 m g/m 3 . Exposure of the motorcycleetaxi driver and the petrol-filling employee was 116 and 52 m g/m 3 , and is higher than roadside benzene levels of 45 m g/m 3 . The daily exposure for taxi and bus drivers was 21 and 15 m g/m 3 , lower than the exposure of the street vendor at 32 m g/m 3 . Exposure of the student commuting by a motorcycle was 19.6 m g/m 3 , about four folds of the exposure of the student commuting by bus (4.1 m g/m 3 ) and of the lecturer commuting by a taxi (4.3 m g/m 3 ). Should the obtained values hold for the entire group population, the risk of leukaemia would be 1.2  10 À5 , 2.4  10 À6 , 2.5  10 À6 , 1.6  10 À6 ,1.4 10 À5 , 9.0  10 À6 ,1.2 10 À5 ,3.1 10 À5 ,1.9 10 À5 , respectively for motorcycle commuter, bus commuter, taxi commuter, indoor/off-road housewife, indoor/roadside housewife, bus driver, taxi driver, petrol-filling employee, and street vendor. Up to now, there is not any published data on personal exposure to pollutants and commuting behaviour in Ho Chi Minh City. Ho Chi Minh is a big city in Vietnam with an area of 2095 km 2 . The distance from the northernmost point to the southernmost point is 102 km, and from the easternmost point to the westernmost point is 47 km. Approximately, the average commuting time is considered to be 2 h/day/person. In this situation, the students and the lecturer can Fig. 1. Daily variation in mixing layer depth during sampling campaigns. Table 1 Climatic conditions of the exposure campaigns. Rain Temperature ( C) Relative humidity (%) Pressure (atm) Wind speed (km/h) Wind direction Mixed layer depth (m) Range Average Range Average Off-road June 14the15th 2010 No 23e34 28 47e100 80 0.9960 7 S, SW (daytime) W, NW (night) 86e985 Roadside a June 21st 2010 No 25e31 28 62e100 85 0.9943 9 E, SE, S 172e1296 Vendor a Bus driver June 25th 2010 No 26e30 30 46e94 75 0.9927 13 S, SW 139e1157 Petrol-filling employee June 30th 2010 No 24e34 29 41e100 80 0.9939 9 S, SW, W 138e1371 Commuter/bus, commuter/ motorcycle July 6th 2010 No 24e33 28 51e100 79 0.9960 8 S, SE (daytime) NE (night) 84e1115 Commuter/taxi July 8th 2010 No 23e34 28 49e100 78 0.9954 6 NE, NW 128e1750 Taxi and motorcycleetaxi driver Nov 24the25th 2010 No 23e31 27 51e100 80 0.9945 5 S (daytime) NW (night) 76e900 a Short-time traffic jam was from 7 AM to 8 AM. T.T.N. Lan et al. / Environmental Pollution 175 (2013) 56e6358 be considered as representative of the three non-occupational population groups working indoors and commuting by motor- cycle, bus and car. A percentage of the above population groups can be roughly estimated from the traffic volume of the above means of transport. The survey on the 24th November 2010 showed daily traffic volumes of motorcycles, 2e5-seat cars, 7 e12-seat cars, 13e 29-seat passenger cars and >29-seat buses on 49 major and minor roads in Ho Chi Minh were, respectively, 6,092,086, 122,292, 224,187, 33,516 and 30,335 (unpublished data). Approximately, the number of 7-seat cars was about half of 7e12-seat cars. Normally, 12e29-seat passenger cars are long-distance intercity buses; therefore, only numbers of motorcycles, 2e7-seat cars and >29- seat buses are used for evaluation of the usage rate of commuting modes. On average, the in-vehicle number of people in one vehicle is 1.3, 2 and 35 for a motorcycle, 2e7-seat cars, and >29-seat buses. Thus, the usage rate of motorcycle mode, car mode and bus mode is, respectively, w87%, w1.3% and 11.7%. Thus, an average exposure to benzene for the non-occupational population that has to commute would be 19.6  87% þ 4.3  1.3% þ 4.1  11.7% m g/m 3 ¼ 18 m g/m 3 ; here the non-commuting population group was neglected since this group is composed of elderly and under-1-year-old children. 3.2. Daily profile of benzene exposure It is important to know diurnal variation in benzene exposure and sources of benzene. This information is essential for effective solutions for reducing exposure. Measurement of diurnal variation taken on a typical weekday showed that indoor exposure in the off- road house was w4 m g/m 3 from 6 AM to 20 PM, four times higher than 0.96 m g/m 3 at night. Indoor exposure in the roadside house was similar to outdoor exposure, although commonly the indoor exposure was lower than the outdoor exposure, as the house had its doors open. Exposure was high in the morning and evening peak- hours (7 AMe9 AM, 5 PMe7 PM) and low at midday (Fig. 3(a)). Should this hold for other days, we could assume that diurnal variation of indoor exposure in the roadside house is the same as diurnal variation of roadside benzene concentration found in our previous study (Lan et al., 2009, 2010); accordingly, roadside benzene concentration was well correlated with the number of on- road motorcycles. Indoor benzene levels at midday were higher than outdoor benzene levels due to indoor stagnant conditions depressing dispersion of pollutants. A very high benzene concen- tration in the early morning was due to the short-time traffic jam in the sampling day. Fig. 3b gives the variation of exposure of the bus drive and petrol-employee during the working day. Exposure of the petrol- employee varied a lot (30ew130 m g/m 3 ), and was high in the Fig. 2. Daily benzene exposure of different population groups in Ho Chi Minh. a b c Fig. 3. Hour variation in benzene exposure. T.T.N. Lan et al. / Environmental Pollution 175 (2013) 56e63 59 morning and at night, and low at midday. This phenomenon is difficult to explain since exposure of the petrol-employee depends on many factors like traded amount of petrol, climatic conditions, and working behaviour. Benzene exposure of the bus driver was 15 m g/m 3 e35 m g/m 3 with an average of 22 m g/m 3 (Fig. 3b), and was high in the morning and evening, and low at midday. It is difficult to explain variation of the exposure of the bus driver, since a bus is a moving object. Benzene levels inside a bus depends on many factors like moving speed, number of stops, ventilation conditions, outside benzene concentration, etc. Low mixing layer depth or the possible formation of a stable surface layer in the early morning and at nighttime could be one of the many reasons for the observed high exposure in morning and nighttime for the petrol-filling employee and bus driver. Fig. 3c reports the daily profile of exposure to benzene of the students and the lecturer. Exposure during commuting by bus, taxi and motorcycle was 22e30 m g/m 3 ,22e39 m g/m 3 and 185e240 m g/ m 3 , respectively. The high exposure of the motorcyclist is due to direct exposure to highly polluted streams of exhaust gas from his own motorcycle and other on-road vehicles in a dense traffic fleet. According to Lan et al. (2010), benzene concentration in exhaust gas from 23 in-use motorcycles in Ho Chi Minh is in a range of 11e 1078 mg/m 3 and largely depends on vehicle technology, moving speed and accumulated mileage. Benzene exposure should be higher for lower technology and higher fleet density, depend largely on climatic conditions. Eye irritation and headaches are symptoms that motorcyclist in big cities in Vietnam often suffer; therefore, they normally wear protective facemasks. On average, exposure to benzene on the motorcycle was 7e8 times of those in the bus and taxi. Benzene concentration was w3 m g/m 3 in the university campus, and w1 m g/m 3 at home during nighttime. The contribution of commuting in personal daily benzene exposure of university students and staffs was almost 99% for motorcycles, and 60%e65% in the bus and taxis. In other words, commuting contributes a main part of personal exposure to benzene. This was reported by Kuo et al. (2000) and Horton et al. (200 6) for Taichung and Perth. 3.3. Comparison of benzene exposure in Ho Chi Minh and in the world Studies on benzene exposure were conducted over the world. Examples are the EXPOLIS (Hanninen et al., 2004) and PEOPLE (Ballesta et al., 2006, 2007) projects. EXPOLIS is a population-based study of urban adult personal exposures to multiple pollutants conducted between 1996 and 1998 in Athens, Greece; Basel, Switzerland; Grenoble, France; Helsinki, Finland; Milan, Italy; and Prague, Czech Republic. The PEOPLE (Population Exposure to Air Pollutants in Europe) project is a study of outdoor, indoor and human exposure to benzene. The PEOPLE project involved sixcities, namely Brussels, Lisbon, Bucharest, Ljubljana, Madrid and Dublin. A similar project named UATMP (US EPA website, 2012a) was carried out in the US. A study on benzene exposure was also conducted in Asia (Liu et al., 2009; Navasumrit et al., 2005; Tunsaringkarn et al., 2012). Table 2 gives an overview of personal exposure to benzene reported in the literature. Ho Chi Minh City is in a group of hi gh- level exposure cities together with Athens and Bucharest. In Ho Chi Minh, only a small percentage of the population, living far from roads and commuti ng by taxi or by bus, has almost the same exposure as in Europe, th e US and Asian developed countries, while most of population is exposed to much higher benzene levels. Proximity to main roads and motorcycle commuting significan tly increase benzene exposure. Benzene exposure of the petrol-filling employee during working time in this study was 30e120 m g/m 3 , in the same order with the reported values of 17.5 m g/m 3 e51 m g/m 3 for urban areas in Ioannina, Greece (Karakitsios et al., 2007), 92.75e121.67 pp b (290e390 m g/m 3 )in Bangkok, Thailand (Navasumrit et al., 2005; Tunsaringkarn et al., 2012) and 530 m g/m 3 in Rome (Carere et al., 1995). Daily benzene exposure of the bus driver was 15.7 m g/m 3 , almost half of 24.7 m g/m 3 found in Athens (Chatzis et al., 2005). Table 3 summarises mean in-vehicle benzene concentrations reported. Motorcycle commuting is popular in Thailand, Malaysia, Vietnam, Pakistan and India. On-motorcycle benzene levels found in this study were in the same range of that reported for Taiwan. In general, on-motorcycle benzene levels are higher than in-bus and in-car benzene levels, and in-car benzene levels were often found to be higher than that of in-bus levels. Except values obtained for Taipei, Taichung (Taiwan) and Sydney (Australia), benzene levels in buses and cars were from 5 m g/m 3 to 50 m g/m 3 . 3.4. Relationship between benzene and toluene In urban areas, benzene is mainly airborne and generated by vehicular traffic; therefore, toluene/benzene (T/B) ratio is often used to evaluate the contribution of sources to atmospheric benzene and toluene. A good relationship between species indi- cates a single source. A clearly distinguishable ratio indicates significant contributions of different sources. T/B values below 3 have been found to be characteristic of traffic emissions worldwide (Perry and Gee, 1995; Brocco et al., 1997; Heeb et al., 2000; Monod et al., 2001; Chan et al., 2002; Hiesh et al., 2006; Kumar and Tyagi, 2006; Khoder, 2007; Truc and Oanh, 2007; Hoque et al., 2008; Hoshi et al., 2008; Liu et al., 2009; Matysik et al., 2010). T/B values of 1.5e4.3 are considered an indicator of traffic emissions, as reported by Hoque et al. (2008) and Liu et al. (2009). For T/B values >4.3, solvent source impacts are probable. A specific B/T ratio below 0.20 Table 2 Personal exposure to benzene ( m g/m 3 ) reported in literature. Average for population Population commuting by bus Population commuting by bus car Athens, Greece 13.1e24.6 Chatzis et al., 2005 Helsinki, Finland 2.6 n e4.7 TSE Edwards et al., 2001 Rouen, France 4.7 Gonzalez-Flesca et al., 2007Grenoble, France 6.4 Ile de France, France 5.3 Strasbourg, France 5.9 Florence, Italy 2.4 s ,7 w 2.3 s , 6.6 w Fondelli et al., 2008 Perth (Australia) 1.76 s , 1.98 w Horton et al., 2006 Copenhagen, Denmark 5.22 Skov et al., 2001 Windsor, Ontario (Canada) 1.69 s , 1.96 w Stocco et al., 2008 Oxford (England) 4.6 Lai et al., 2004 Brussels (Belgium) 5.1 4.5 5 Ballesta et al., 2006Lisbon (Portugal) 6.1 5 5.3 Bucharest (Hungary) 12.9 13.2 18.8 Ljubljana 5.5 4 5 Madrid 5.1 8.8 10.9 Dublin, Ireland 2.9 2.4 2.4 Daegu, Korea 2.6e8.1 Park and Jo, 2004 Madrid, Spain 6.5 10 Ballesta et al., 2008 California, USA 5.1 Marshall et al., 2006 s : spring, w : winter, n : non-smoking environment, TSE : tobacco smoking environment. T.T.N. Lan et al. / Environmental Pollution 175 (2013) 56e6360 has been proposed as an indicator of samples strongly affected by industrial emissions in Dongguan, China (Barletta et al., 2008), while a ratio of 0.4e1.0 has been used as an indicator of air propelled by vehicular exhaust in Beijing (Wang et al., 2012). T/ B > 4.3 was used to identify sources influenced by solvent use in Windsor, Ontario, Canada (Xu et al., 2010). A high T/B ratio (8.6) in Taiwan suggested large additional sources of toluene from industry (Hiesh et al., 2006). Monod et al. (2001) reported an overview of inter-species ratios between BTEX species in different environ- ments in Asia, Europe and South America, accordingly T/B value was 2.3 (R 2 ¼ 0.91) for a traffic microenvironment. The contents of benzene and toluene in RON92 and RON95 petrols were 1.81, 4.55 and 1.85 and 4.20 wt%, respectively. Thus, the T/B ratio in A92 and A95 gasoline were 2.5 and 2.3. Fig. 4 shows daily profile of T/B ratio. T/B was 1.9e2.3 for the motorcycle mode and 3.1e3.7 for the taxi mode, but was not put on Fig. 4 due to limited data. T/B ratios for motorcycle, daytime outdoor air (vendor) and indoor air, and bus microenvironment were similar to that in petrol and in motorcycle exhaust (Lan et al., 2009), implying the same origin of benzene and toluene from gasoline vehicles. The higher T/B for indoor air and the street vendor in late evening and nighttime can be explained by a change in fleet composition and increase of the contribution of off-gassing from construction materials due to decreases in traffic volume. Heavy trucks in Ho Chi Minh are allowed from 8 PM to 6 AM. T/B was 3.1e3.7 for taxi, and 5e6 for the off-road house at nighttime, implying an additional source of toluene, possibly from fragrances. T/B ratio for the petrol- filling employee was the low at 1.28e1.57. The low T/B ratio at the filling station is due to direct evaporation of petrol. Light benzene evaporates more than toluene owing to higher vapour pressure and results in a lower T/B ratio in comparison with the ratio in petrol. This phenomenonwas observed by Correa et al. (2012), accordingly, the presence of lighter BTEX was more pronounced in the atmo- sphere of gas stations than in the gasoline vapour in equilibrium with the liquid petrol. 4. Conclusions Personal exposure to benzene was measured in Ho Chi Minh City. The preliminary data showed that (1) Benzene exposure was 22e30 m g/m 3 for bus commuter, 22e 39 m g/m 3 for taxi commuter, and 185e240 m g/m 3 for motor- cycle commuter. (2) Average personal exposure to benzene in Ho Chi Minh is around 18 m g/m 3 , about 40% of the benzene level in outdoor roadside air. (3) Benzene exposure during commuting time contributes as the main part of daily exposure. (4) Motorcycleetaxi drivers, petrol-filling employees and street vendors are people of high exposure to benzene. 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