BIBLIOGRAFÍA

Air quality standards for pollutants are based on outdoor averaged levels of pollution (Dor, 1995). Yet there is evidence that pollution exposure varies both spatially and temporally (Kingham et al., 1999). A “hot spot” of air pollution can be defined as an ‘area where the average concentrations of air pollutants are higher than those in

surrounding areas.’ (Zhu et al., 2008 p. 7329). Past research has shown that in such “hot spots”, localised concentrations of air toxins can occur due to large or small emission sources (Sweet and Vermette, 1992; Hung et al., 2005, Smith et al., 2007). It is important to gain an understanding of the spatial and temporal distribution of air toxins in a “hot spot” for conducting accurate assessment of personal pollution exposure (Burnett et al., 2001; Leikauf, 2002; Jerrett et al., 2005; Weis et al., 2005).

2.5.2 Spatial and Temporal Variation in Commuting Journeys

Several scientific research projects have suggested that personal exposure to pollution levels depend on temporal and spatial changes on journeys (Kingham et al., 1999; Zhang et al., 2006; Cheng et al., 2008). A report published by the World Health Organisation (2005) similarly contends that pollution levels are affected by the volume and spatial distribution of emissions, as well as its dispersion conditions. The publication further reports that pollution intake is also determined by how long people stay in polluted areas and what they do there. A vast amount of research has been conducted to measure commuters’ personal pollution exposures on different transport modes (Alm et al., 1999; Praml and Schierl, 2000; Chan et al., 2001). However; there has been little scientific work done to investigate pollution level variations within a journey that might lead to commuters being exposed to short-term peak levels of pollutants while commuting. For instance, commuters spend a part of their commuting journeys in car parks, bus stops/ stations, metro subways and train stations. Although they might only spend a fraction of their total journeys in these micro-environments, scientific evidence has demonstrated that there are very high levels of pollutants in these environments (Chau et al., 2000; Adams, 2001; Aarnio, 2005; Park et al., 2008; Tsai, 2008). These authors suggest that individuals, thus, gain a significant contribution of their daily exposure in a short period of time. The next three sections will discuss the pollution exposure in such confined spaces such as sheltered car parks, bus stations and subways which might elevate commuters’ level of personal pollution exposure.

2.5.2.1

Bus Stations

Bus stations are similar to car parks and train and subway stations in that they are all confined spaces. It would then be safe to assume that bus stops might have higher levels and concentrations of pollutants compared to non-confined open areas. Kingham et al. (1999) carried out an experiment in West Yorkshire where they measured pollution exposure of individuals while travelling on a bus and compared the results to time- activity data collected through the use of journey diaries. The result (Figure 2.2) shows that certain activities, such as getting on the bus and waiting at the bus stop greatly increases PM exposure levels.

Figure 2.1 Particulate Peaks on a bus journey in West Yorkshire Source: Kingham et al. (1999)

A potential explanation for such high elevations when entering the bus and while waiting at bus stops has been provided by a study comparing commuters’ exposures to particulate matters while using different modes of transport (Tsai et al., 2008). They report that the physical distance between commuters and traffic-related emission sources may explain why bus commuters have a relatively high particulate exposure: bus commuters are potentially exposed to PM2.5 and PM1 emitted from vehicles passing by when they are

waiting at roadside bus stops.

Fig 2: Personal particulate exposure - bus passenger (25/9/96)

0 200 400 600 800 1000 1200 8.20 8.23 8.26 8.29 8.32 8.35 8.38 8.41 8.44 8.47 8.50 8.53 8.56 8.59 9.02 9.05 9.08 9.11 Time PM Respirable PM10 In bus station Get on bus

2.5.2.2

Car Parks

Very high concentrations of CO and other pollutants have been recorded in poorly ventilated, confined spaces used by motor vehicles (Flacshbart, 1999). Studies done as early as the late 60s reported higher than average levels of CO concentrations in garages (Trompeo et al., 1964; Chovin et al., 1967). Goldsmith (1970) found that large numbers of cars queuing to leave parking buildings could elevate pollution levels inside garages to extremely high concentrations. A later study (Barker and Fox, 1976) conducted tests inside garages using indicator tubes. This showed instantaneous concentrations of up to 210 ppm. Further tests revealed even higher levels for short periods. More recent research reiterates these findings (Chau et al., 2002; Papakonstantinou et al., 2003). Duci et al. (2000) claim the garage micro-environment to be a very important determinant of exposure to CO. Experiment carried out in an urban section of Athens to measure CO levels in garages found that there were increases to short and long term exposure limits to CO (Chaloulakou et al., 2002). This can be attributed to poor or malfunctioning ventilation inside, which allows contaminated air to accumulate and pollutant concentrations to increase.

2.5.2.3

Subway Systems and Train Stations

‘Commuters tend to spend only a short fraction of their day in the metro, but if the levels of particulate matter and its elemental composition in the metro are high, even short durations can contribute a lot to the total exposure of a person, and any related health effect’ (Nieuwenhuijsen et al., 2007, p.8001). Subway systems serve millions of passengers annually worldwide. The underground portion of a subway system is a confined space with concentrations of pollutants influenced either by the outside atmosphere or generated internally (Cheng et al., 2008). A multitude of scientific studies worldwide corroborate these findings. High concentrations of particulate matter have been measured in subway systems in London (Pfeifer et al., 1999; Adams et al., 2001), Stockholm (Johansson and Johansson, 2003), Berlin (Fromme et al., 1998) and Beijing (Li et al., 2007). The studies conducted in London further reported that PM2.5 exposure

levels in subway were 3-10 times higher than in road transport modes in London (Adams et al., 2001). Similarly, Johansson and Johansson (2003) found that PM10 and PM2.5 levels

at an underground station were 5 and 10 times, respectively higher than those measured at the busiest streets in Central Stockholm. Ripanucci et al. (2006) also observed that average PM10 levels in the underground platforms were 3.5 times the average level above

ground.

It has been established that diesel locomotives emit considerable amounts of air pollutants in a short time, and emissions are usually confined to a small area (WHO, 2005). Measurements of the air quality in and around railway stations were published in a French study conducted in Paris (Keuken et al., 2005). It reported that, on average, within a 1000-m radius of the station, the diesel trains emit about 16% of total NO2 and 9% of

PM. These figures escalate to 50% of total NO2 and 33% of PM at peak times. Depending

on wind direction and speed, the train plumes can lead to peak nitrogen dioxide concentrations of 750-1200 µg3 at 200-400 meters from the rail tracks. Although the levels declined after the train departured, they remained above 25% of the total concentration for up to nine minutes. A study carried out in West Yorkshire (Kingham et al., 1999) repeated these findings. The project measured pollution exposure of individuals while travelling on different forms of transport and compared the results to time-activity data collected through the use of journey diaries.

Figure 2.1 below shows the result for a commuter train journey from Marsden to Huddersfield in West Yorkshire. Particulate peaks can be seen to relate to a variety of journey features such as the train stopping at stations and the subject walking through the station.

Figure 2.2 Particulate peaks on a train journey in West Yorkshire Source: Kingham et al. (1999)