Assessment of fine particulate matters in the subway system of Istanbul

Abstract

The purpose of this study was to determine the concentrations of fine particulate matter (PM_2.5) at six subway stations. The PM_2.5 levels were compared at the platforms, inside the trains’ cabin and at the train driver’s cabin in the first section of the train. The measurements were grouped into three sections: (1) six underground platforms, (2) trains’ cabin and (3) train driver’s cabin in the Aksaray-Airport light metro line (M1) and the Taksim-4.Levent metro line (M2). On average, the highest PM_2.5 concentrations were recorded in the underground platforms. The average daytime PM_2.5 concentrations varied from 49.3 to 181.7 µg·m⁻³ at the two subway line platforms. The PM_2.5 levels measured at five stations were higher than the ambient air PM_2.5 standard declared by US-EPA, given as 35 µg·m⁻³ for a 24-h arithmetic mean. The second highest PM_2.5 concentration was recorded inside the trains (61 to 73 µg·m⁻³), followed by train driver’s cabin (50 to 74 µg·m⁻³) of the M1 and M2 lines. When the train was in the subway tunnels, the PM_2.5 concentrations increased. The correlation (R²) between the PM_2.5 level on the platform and the depth of the underground stations was 0.88 (p<0.01). The highest PM_2.5 levels were recorded in the morning and evenings.

Keywords

Istanbul Subway Train cabin Platform Train driver’s cabin

Introduction

Several studies have revealed that daily exposure to elevated levels of respirable particles is closely linked to an increase in mortality, hospital admissions and respiratory problems.1^–4 The daily human exposure to atmospheric pollutants is increased by the effect of traffic emissions.5^–7 The efficiency of inhalation and the respiratory deposition of particulate matters (PM) are dependent on the size of the particles.8 Some studies especially after 2005, focused on measuring the PM₁₀ and PM_2.5 concentrations at urban sites.9^–11 However, people spend most of their time at homes, at their workplaces or in transit, and concern over the air quality of indoor microenvironments and its influence on public health is increasing. Therefore, many studies are devoted to assessing the particle pollution in transportation, and some of these studies focused on subway systems. The total suspended particulate matters are slightly higher in underground systems compared with other transportation types.5,12,13

In metropolitan cities, many people prefer subways to other forms of transportation and are thus exposed to pollutants. The main sources of PM in subway system are (1) particles from abrasive forces acting on rails, (2) wheels from traction and braking (likely to contain an abundance of iron), and (3) particles shed from people and their clothing.14,15 Many researchers have emphasised the relationship between fine particles and adverse health effects.16^–19 The particles in subway systems are nearly four times more likely to cause oxidative stress and are eight times more genotoxic in lung tissues.20 Few studies have been performed in subway systems around the world though PM concentrations have been determined to be substantially high in London, Stockholm, Berlin, New York, Prague, Mexico City, Helsinki, Hong Kong and Seoul.6,13,16,21^–28 In the London subway system, PM₁₀ and PM_2.5 levels were observed to be 3 and 8 times higher, respectively, than surface transport concentrations.5 Similarly, the PM₁₀ and PM_2.5 concentrations in the Stockholm subway were observed to be 5 and 10 times higher respectively, than the measurements at the busiest street in the city centre with PM_2.5 levels of 199 (±104) µg·m⁻³ on weekdays and 148 (±82) µg·m⁻³ on weekends on the platforms.13 The average PM_2.5 concentrations at the underground and ground stations and in the subway cars were 47–60, 19 and 21 µg·m⁻³.26 Park and Ha28 reported that the PM_2.5 levels inside trains and on platforms were 121.7 (±16.1) µg·m⁻³ and 115.6 (±8.6) µg·m⁻³, respectively, in Seoul, Korea.

The purpose of this study was to determine the concentrations of PM_2.5 at six subway stations, comparing the PM_2.5 levels at the platforms, inside the trains and at the train driver’s cabin in the first section of the train. No study of these measurements has previously been performed in the subway system of Istanbul. Fine particulate matter (PM_2.5) was monitored between September 2007 and January 2008, and the samples were taken 66 times during the study period in the Istanbul subway system.

Materials and methods

Site description

PM_2.5 measurements were performed on two lines of the Istanbul subway: (1) the Aksaray-Airport light metro line (M1), which opened in 1989, and (2) the Levent-Taksim metro line (M2), which opened in 2000.

The samples were taken 66 times during the study period. The length of the M1 line is 19.6 km (8 km underground, 11.5 km on ground). The total length of the M2 tunnel is 14.5 km. The number of stations on the M1 and M2 lines is 18 and 10, respectively. PM measurement on the platforms was performed at six stations: the Aksaray, Otogar and Airport stations on the M1 line and the 4.Levent, Şişli and Taksim stations on the M2 line (Figure 1). The frequency of trains is 5 min for M1 and 6.5 min for M2. A one-way trip is approximately 32 min for M1 and 12 min for M2. The average capacities of the M1 and M2 lines are 245,000 and 195,000 commuters per day, respectively. The ventilation systems are different in two subway lines on the platforms: the natural ventilation on M1 line and the air-conditioning on M2 line. The trains of M1 and M2 lines have the same air-conditioning system.

Measurements on the station platforms

A portable real-time aerosol monitor (MIE DataRAM, Model 2000, Thermo, USA) was used to measure PM_2.5. The flow rate of the monitor was 0.102–0.138 m³·h⁻¹ and the data logging interval was set at 15-min intervals. The monitoring was conducted at a platform close to the centre, and the sampling head was placed approximately 1.5 m above the platforms. The measurements were performed between 28 September 2007 and 18 January 2008. The measurements were performed between 6 am and 12 pm, on seven sequential days.

Measurements inside the train driver’s cabin and the trains

Personal exposure inside the train driver's cabin and in the passenger’s cabins of the trains was measured with a portable real-time aerosol monitor (pDR 1200, Thermo, USA). The flow rate of the monitor was 4.0 L·min⁻¹, and the data were recorded every 30 s. The PM_2.5 measurement in the train driver’s cabin was performed for three days on the M1 and M2 lines. The monitor was placed in the interior of the cabin at the level of the driver’s respiration zone.

Monitoring took place between 7 am and 12 pm in the working period of the train. The measurements of the PM_2.5 concentrations were conducted in the trains for three working days on the M1 and M2 lines. The inlet of the portable monitor was positioned on the collar of the researcher’s clothes. The researcher stood in the middle of the cabin. The second cabin of the train was preferred for all measurements in the trains. The measurements were conducted 3 times a day during a one way trip: in the morning (7.30–9:30 am), at noon (12 am–2 pm) and in the evening (6–8 pm). The times were recorded when the train was underground and above ground. The portable real-time aerosol monitors (MIE Dataram and pDR 1200) were calibrated against a Partisol FRM Air Sampler (Model 2000, Thermo, USA) in the laboratory. The correlation (R) between the two methods was 0.98 for the MIE and 0.99 for the pDR 1200, and the slope of the regression is given in Figure 2. Real-time monitoring was performed to observe hourly fluctuations of PM_2.5 concentration.

Results and discussion

Station platform

No study on particulate matters in the Istanbul subway system has been previously reported. This study was undertaken to examine the concentration of fine particulate matters inside the trains’ passenger cabins, the train driver’s cabin and on platforms at the subway stations. Figures 3 and 4 show the PM_2.5 concentrations at the station platforms of M1 and M2 in Istanbul. The values were measured for workdays and weekend days between 6 am and 12 pm. The subway trains travel at 5-min intervals during rush hour, 6-min intervals during non-rush hour on weekdays and on Saturdays, and 7-min intervals on Sundays. As shown in Figures 3 and 4, the PM_2.5 fluctuations at all stations were slightly higher between 7 am and 10 am in the morning and 6 pm and 10 pm in the evening. The differences of the concentrations were analysed with a t-test. The results showed that there was no statistically significant difference between PM_2.5 concentrations on weekdays and weekends (p > 0.05). However, there was a statistically significant difference in PM_2.5 concentrations between peak hours and midday hours (p<0.01). The PM_2.5 concentration increased during rush hours due to the increased number of passengers in the subway and the increased traffic load on the ground.

Table 1 summarises the results of the continuous measurements performed at the platforms of the stations on the M1 and M2 lines, inside the passengers’ cabins of the trains and in the train driver’s cabin. The average daytime PM_2.5 concentrations varied from 49.3 to 181.7 µg·m⁻³ at the two subway line platforms. In Turkey, the target limit value for daily average of PM₁₀ is given as 50 µg·m⁻³ (which should be achieved in 2019), and a limit value for PM_2.5 has not specified yet. Therefore, the results of this study were compared to the US EPA limit values. The PM_2.5 levels measured at 5 stations were higher than the ambient air PM_2.5 standard declared by United States Environmental Protection Agency (US-EPA), given as 35 µg·m⁻³ for a 24-h arithmetic mean. The PM_2.5 concentrations measured in the underground stations on the M2 line were notably higher than those on the ground and the underground stations on the M1 line. The highest PM_2.5 concentration levels were observed at the Taksim subway station (181.7 µg·m⁻³) and were significantly different from the other stations (p<0.01). The Taksim station is near Taksim square, which is the cultural and entertainment centre of Istanbul and is a major commercial area. This site is also surrounded by bus stops and roads. It is estimated that the bus frequency is as high as 200 per hour; therefore, traffic sources are a major contributor to air pollution at this location.

Table 1.

Summary of PM_2.5 measurements in the platform of subways, inside the passengers’ cabins of the trains and train driver’s cabin and comparison of PM_2.5 concentrations (means and ranges) in the different subways.

City	Station	Micro environment	Time period	PM_2.5 concentration Means and ranges (µg m⁻³)	Reference
Istanbul	M1 line	Inside the passengers’ cabin of the train	1 trip (35–45 min)	72.9 (daily average)	This study
			Morning	60.1 (32.0–105.0)
			Midday	50.8 (22.0–138.0)
			Evening	107.9 (85.2–240.5)
	M2 line	Inside the passengers’ cabin of the train	1 trip (20–30 min)	61.2 (daily average)
			Morning	45.3 (19.1–140.2)
			Midday	33.2 (1.0–96.0)
			Evening	105.1 (27.2–118.3)
	M1line	Platform	18 h
	Airport			49.3 (20.5–136.7)
	Otogar			98.3 (14.5–291.4)
	Aksaray			88.8 (10–372.8)
	M2 line	Platform	18 h
	Taksim			181.7 (26.7–1014.7)
	Şişli			106.8 (30.9–391.6)
	4.Levent			105.3 (14.5–318.9)
	M1 line	Train driver’s cabin	12 h	73.5 (20.7–138)
	M2 line	Train driver’s cabin	16 h	49.5 (10.1–85.4)
Stockholm	Mariatorget	3 m above platform	Weekdays	199 ± 104	Johansson and Johansson13
			Weekends	148 ± 82
London		Platform	8-h shift	270–480	Seaton et al.31
		Inside the train	8-h shift	130–200
Mexico city		Personal exposure	Approx. 1 h	61 (31–96)	Gomez-perales et al.25
Helsinki		Stations	Working day	47 ± 4 to 60 ± 18	Aarnio et al.26
Hong Kong		Inside the train	25–50 min	33 (21–48)	Chan et al.6
Guangzhou		Inside the train	2.5 h	44 (NA)	Chan et al.27
Seoul		Platform	Working day	105.4 ± 14.4	Park and Ha28
		Inside the train		116.6 ± 14.2

The highest average PM_2.5 concentrations on the M1 were recorded at the Otogar ground station (98.3 µg·m⁻³) and were significantly different from the Airport station (p<0.01) but not from the Aksaray station. The Otogar ground station is adjacent to the interurban bus terminal. The average road traffic flow was approximately 30,000 vehicles per day. The lowest average PM_2.5 level recorded at the Airport underground station was 49 µg·m⁻³. There is heavy traffic at the street level of the stations except at the Airport station. The PM_2.5 levels at the Otogar ground station were two times higher than the PM_2.5 levels at Airport station, as these levels were influenced by traffic-related particulate pollution. The Aksaray, 4.Levent and Şişli underground stations are in commercial areas in Istanbul. The commuter and traffic density is very high on weekdays and on weekends. Local bus stops are situated near the subway station allowing the passengers to connect without difficulty. The local characteristics of the stations affected the PM_2.5 levels, but there was an irregular pattern on the stations.

The PM_2.5 concentrations measured between September 2007 and January 2008 at the subway stations of Istanbul are shown in Figure 5.

We calculated the daily average PM_2.5 concentration for each station. There was no continuous PM_2.5 measurement in the urban area of Istanbul. The Istanbul Metropolitan Municipality's Directorate of Environmental Protection (IMM-DEP) has conducted PM₁₀ measurements at 10 urban air quality monitoring (UAQM) stations located at various key topographic points around the city since 1992. In this study, the PM₁₀ concentrations measured at the UAQM stations that are closest to the subway stations were considered. The main results show that there is a strong correlation between the PM_2.5 and PM₁₀ concentrations. The highest correlations (0.97) were observed for Otogar and the lowest correlations (0.70) were observed for the 4.Levent and Taksim stations. The Otogar subway station is at the ground level. The UAQM station is very close to the Otogar ground station but the other UAQM stations are far from the 4.Levent and Taksim metro stations. In our previous study,29 we observed that PM₁₀ concentration at the subway stations correlated well with those measured at the UAQM stations. The highest correlation observed was 0.96 for Otogar and the lowest correlations were 0.55 and 0.62 for the Şişli for 4.Levent stations, respectively. The PM_2.5 concentrations monitored at the Taksim, Şişli, 4.Levent and Aksaray stations were higher than the PM₁₀ concentrations monitored on the ground stations with no regard to location (Figures 1 and 5). This result was similar to the results obtained by several previous studies.6,26,28,30 The possible reasons for having high PM_2.5 may have been the fine particles ejected from the brake system into the air and the increasing number of passengers. In our previous study, we observed that PM₁₀ levels were also high at the platforms (the average concentrations were between 58 ± 28 µg·m⁻³ and 213 ± 125 µg·m⁻³), and the subway to outdoor ratio of the PM₁₀ measurements varied between 1 and 4.2.29

The studies have shown that PM_2.5 concentrations are highly variable. Table 1 shows a comparison of the PM_2.5 concentrations at the different microenvironments (means and ranges) in the metro systems.

Park and Ha28 and Nieuwenhuijsen et al.32 reported that the difference in the results for PM levels in subway systems between cities may be due to seasonal features, time of day, type of brake system, and depth and ventilation type of the tunnel. In this study, the correlation between the average PM_2.5 concentration and the depth of the underground stations was found to be high (R²= 0.88, p<0.01) (Figure 6).

However, further investigations should be performed to prove the significant relation between depth and PM concentration. In Helsinki, the average PM_2.5 concentration was 60 µg·m⁻³ at an underground station.26 In Seoul, Park and Ha28 measured PM_2.5 concentrations of 105.4 ± 14.4 µg·m⁻³ at an underground station. The impact of location was examined using general linear model statistical analysis, and it was found that the monitoring locations could affect PM₁₀ and PM_2.5 concentration measurements. The exposure level obtained in this study is substantially lower than those measured on the platforms in the London subway (270–480 µg·m⁻³).31 One possible reason for this decrease could be the differences in the general characteristics of the subways. Particulate matters may have originated from the wear of steel during friction periods between wheels and rail, wear of brakes and vapourisation of metals due to sparkling. The London subway has a blocks brake system and natural ventilation, while as the Istanbul subway has a pneumatic brake system and two different ventilation system: air conditioning and natural ventilation.32

The PM concentrations rose when the train was in the subway tunnels because the underground parts of the trip were more polluted. The subway tunnels in Istanbul are not washed. The effect of washing was investigated by Johansson and Johansson in the Stockholm subway system.13 They reported that tunnel washing reduced the concentrations by approximately 10%. The effects of tunnel washing should also be investigated in the Istanbul subway. Further factors such as the number of passengers, the type of brake system, the season, and the efficiency of ventilation should also be investigated comprehensively.

Inside the train and the train driver’s cabin

The statistical values of PM_2.5 inside the train and the train driver’s cabin are given in Table 1. The PM_2.5 concentrations inside the trains are shown in Figure 7. The journey time on the M1 line at the ground and underground levels was approximately the same. The PM_2.5 levels tend to decrease at the ground level (Figure 7(a)).

The PM_2.5 concentrations inside the train ranged between 22 and 240 µg·m⁻³ on the M1 line and between 1 and 140 µg·m⁻³ on the M2 line, and the daily average of PM_2.5 was recorded as 72.9 µg·m⁻³ and 61.2 µg·m⁻³, respectively (Table 1). The concentrations were higher in the rush hours in the mornings and evenings than during midday. As shown in Table 1, the PM_2.5 levels inside trains in Seoul were 115.6 ± 8.6 µg·m⁻³ in the above ground stations and 105.4 ± 14.4 µg/m³ in the underground stations.28 In a study conducted at the subways in Hong Kong and Guangzhou, the PM_2.5 concentration averages were 33 ± 10 µg·m⁻³ and 44 ± 11 µg·m⁻³, respectively, inside the trains.6,27 In London, the PM_2.5 level inside the trains ranged from 130 to 200 µg·m⁻³.31 Our results are higher than those measured in Hong Kong and Guangzhou but lower than the results that were obtained in London and Seoul (Table 1). In the evenings, more passengers would prefer to travel by subway due to the heavy traffic on the road in Istanbul. At the platforms and inside the trains, the PM_2.5 concentrations measured in the evenings were higher than those observed in the mornings and midday. The increased number of passengers in the subway could have caused this rise of PM_2.5 concentrations.

On the M2 line, the average PM_2.5 levels in the train and train driver’s cabin were approximately half of the total measured on the platforms. The use of ventilation systems inside the trains may be effective in this reduction. The PM_2.5 concentration decreased continuously during the trip from the Taksim underground station to the 4.Levent underground station (Figure 7(b)).

Figure 8 demonstrates continuous data for the PM_2.5 concentrations for the period of work time in a day in the train driver's cabin. The highest PM_2.5 concentrations were recorded in the mornings and evenings, similar to the train and the platforms. The PM_2.5 level in the train driver’s cabin on the M1 line was almost the same as the measurements inside the train (73.5 µg·m⁻³) (Table 1). In both the train and the train driver’s cabin on the M2 line, the concentrations were lower than those measured on the M1 line.

Conclusion

This study examined the PM_2.5 concentrations in the Istanbul subway. The real-time particulate measurements were performed inside the passengers’ cabins of the trains, in the train driver’s cabin and on the platforms between September 2007 and January 2008. This is the first study to investigate PM levels on the Istanbul subway system. The PM_2.5 concentrations inside the passengers’ cabins of the trains and inside the train driver’s cabin were found to be lower than those observed at the platforms. The in-train particulate level could be affected by the type of ventilation. The depth of the underground station could affect the PM_2.5 levels at the platform. The results of this study indicate that commuters may be exposed to high levels of PM_2.5 in the subways of Istanbul. Measurements should also be performed after tunnel washing to determine the subsequent decline of particle levels.

Footnotes

Acknowledgements

This study was supported by the Istanbul Transportation Co., which is the in partnership with the Istanbul Municipality, and by the Research Fund of the University of Istanbul: Project Number: 547.

References

Dockery

Pope

. Acute respiratory effects of particulate air pollution. Ann Rev Public Health 1994; 15: 107–132.

HKEPD. Guidance Notes for the Management of Indoor Air Quality in Offices and Public Places. Hong Kong: Hong Kong Environmental Protection Department Environment, Hong Kong Printing Department, 1999.

Ostro

. The association of air pollution and mortality: examining the case for inference. Arch Environ Health 1993; 48: 336–342.

Tony

. Weighing the health risks of airborne particulates. Environ Sci Technol 1995; 29: 360–364.

Adams

Nieuwenhuijsen

Colvile

. Determinants of fine particle (PM_2.5) personal exposure levels in transport microenvironments, London, UK. Atmos Environ 2001; 35(27): 4557–4566.

Chan

Lau

Lee

Chan

. Commuter exposure to particulate matter in public transportation modes in Hong Kong. Atmos Environ 2002; 36: 3363–3373.

Gee

Raper

. Commuter exposure to respirable particles inside buses and by bicycle. Sci Total Environ 1999; 235: 403–405.

Hinds

. Aerosol Technology: Properties, behavior and measurement of Airborne Particles, 2nd edn. Canada: Wiley-Interscience Publication, 1999, pp. 75–100.

Giugliano

Lonati

Butelli

Romele

Tardivo

Grosso

. Fine particulate (PM_2.5-PM₁) at urban sites with different traffic exposure. Atmos Environ 2005; 39: 2421–2431.

10.

Birmili

Allen

Bary

Harrison

. Trace metal concentrations and water solubility in size-fractioned atmospheric particles and influence of road traffic. Environ Sci Technol 2006; 40: 1144–1153.

11.

Khwaja

Parekh

Khan

Hershey

Naqvi

Malik

Khan

. An in-depth characterization of urban aerosols using electron microscopy and energy dispersive X-ray analysis. CLEAN 2009; 37(7): 544–554.

12.

Sitzmann

Kendal

Williams

. Characterization of airborne particles in London by computer-controlled scanning electron microscopy. Sci Total Environ 1999; 241: 63–73.

13.

Johansson

. Particulate matter in the underground of Stockholm. Atmos Environ 2003; 37: 3–9.

14.

Hurley JF, Cherrie JW, Donaldson K, Seaton A, Tran CL. Assessment of health effects of long-term occupational exposure to tunnel dust in the London Underground. IOM (Research Report: TM/03/02), Edinburg, UK, December 2003.

15.

Birenzvige

Eversole

Seaver

Francesconi

Valdes

Kulaga

. Aerosol characteristics in a subway environment. Aerosol Sci Technol 2003; 37: 210–220.

16.

Chen

Kan

Huang

Zhang

. Impact of ambient air pollution on public health under various traffic policies in Shanghai. China Biomed Environ Sci 2009; 22: 210–215.

17.

Costa

Dreher

. Bioavaliable transition metals in particulate matter mediate cardiopulmonary injury in healthy and compromised Animal Models. Environ Health Perspect 1997; 105(5): 1053–1060.

18.

Karthikeyan

Joshi

Balasubramanian

. Microwave assisted sample preparation for determining water-soluble fraction of trace elements in urban airborne particulate matter: evaluation of bioavailability. Anal Chim Acta 2006; 576: 23–30.

19.

Laden

Neas

Dockery

Schwartz

. Association of fine particulate matter from different sources with daily mortality in six US cities. Environ Health Perspect 2000; 108: 941–947.

20.

Karlsson

Nisson

Möller

. Subway particles are more genotoxic than street particles and induce oxidative stress in cultered human lung cells. Chem Res Toxicol 2005; 18: 19–23.

21.

Fromme

Oddoy

Piloty

Krause

Lahrz

. Polycyclic aromatic hydrocarbons (PAH) and diesel engine emission (elemental carbon) inside a car and a subway train. Sci Total Environ 1998; 217: 165–173.

22.

Chillrud

Epstein

Ross

Sax

Pederson

Spengler

Kinney

. Elevated airborne exposures of teenagers to manganese, chromium, and iron from steel dust and New York City’s subway system. Environ Sci Technol 2004; 38: 732–737.

23.

Chillrud

Grass

Ross

Coulibaly

Slavkovich

Epstein

Sax

Pederson

Johnson

Spengler

Kinney

Simpson

Brandt-Rauf

. Steel dust in the New York City subway system as a source of manganese, chromium, and iron exposures for transit workers. J Urban Health: Bull NY Acad Med 2005; 82(1): 33–42.

24.

Branis

. The contribution of ambient sources to particulate pollution in spaces and trains of the Prague underground transport system. Atmos Environ 2006; 40: 348–356.

25.

Gomez-Perales

Colvile

Neuwenhuijsen

Bremautnz

Avedoy

GVJ

Paramo-Figueroa

Blanco-Jimenez

Bueno-Lopez

Mandujano

Bernabe-Cabanillas . Commuters’ exposure to PM_2.5, CO and Benzene in public transport in the metropolitan area of Mexico City. Atmos Environ 2004; 38: 1219–1229.

26.

Aarnio

Yli-Tuomi

Kousa

Makela

Hirsikko

Hameri

Raisanen

Hillamo

Koskentalo

Jantunen

. The concentrations and composition of and exposure to fine particles (PM_2.5) in the Helsinki subway system. Atmos Environ 2005; 39: 5059–5066.

27.

Chan

Lau

Lee

Chan

. Exposure level of carbon monoxide and respirable suspended particulate in public transportation modes while commuting in urban area of Guangzhoui China. Atmos Environ 2002; 36: 5831–5840.

28.

Park

. Characteristics of PM₁₀, PM_2.5, CO₂ and CO monitored in interiors and platforms of subway train in Seoul, Korea. Environ Int 2008; 34: 629–634.

29.

Alver Şahin

Onat

Stakeeva

Ceran

Karim

. PM₁₀ concentrations and the size distribution of Cu and Fe-containing particles in Istanbul's subway system. Transport Res Part D. Transport Environ 2011; 17: 48–53.

30.

Kim

Roh

Lee

Kim

. Spatial distribution of particulate matter (PM₁₀ and PM_2.5) in Seoul Metropolitan Subway stations. J Hazard Mater 2008; 154: 440–443.

31.

Seaton

Cherrie

Dennekamp

Donaldson

Hurley

Tran

. The London underground: dust and hazards to health. Occup Environ Med 2005; 34: 332–346.

32.

Nieuwenhuijsen

Gomez-Perales

Colvile

. Levels of particulate air pollution, its elemental composition, determinants and health effects in metro systems. Atmos Environ 2007; 41: 7995–8006.