FASE II: PLANIFICACIÓN

Several major classes of air pollutants of varying toxicity originate from road transport. These contaminants emerge from the tailpipes of vehicles with internal combustion engines, from other vehicle components (such as brake and clutch linings, tyres and fuel tanks) and from road surface wear and treatment materials (WHO, 2005). Vehicle emissions can be labelled as one of the most important source for some pollutants of great concern such as carbon monoxide, nitrogen dioxide, volatile organic compounds

(VOCs) and particulate matter. Before delving further into the nature of these contaminants, it is important to understand that the composition of motor vehicle exhaust depends on the fuel used as well as on the type and operating condition of the engine (Romieu, 1999). Generally, the major differences between diesel and petrol engines are in the quantity of carbon monoxide, particulate and nitrogen dioxide produced (Chow and Chan, 2002). While the major concerns of diesel engine emission are nitrogen dioxide, particulate matter and sulphur dioxide, petrol engines emissions are known to have much higher levels of carbon monoxide (Chan et al., 1999). As Wohrnschimmel et al. (2008) contend, the air people breathe while in transportation is particularly unsafe due to the high concentrations of carbon monoxide (CO), suspended particles (PM10 and PM2.5) and

volatile organic compounds. Furthermore, with respect to CO, transport

microenvironments have been identified as the most polluted spaces in comparison with other microenvironments (Georgoulis et al., 2002). With regard to VOCs, transport microenvironments were also shown to be a significant contributor to personal exposure (Edwards et al., 2006). A study conducted by Behrentz (2005) showed that such microenvironments are responsible for 15% of total PM2.5 personal exposure. For every

hour that was spent in transport, commuters are exposed to higher than average levels of air pollution. This has been shown for a wide variety of cars, buses, subways and cycles.

As the scientific literature demonstrates, there are a considerable number of pollutants resulting from vehicular emissions. They include many types of particulates, sulphur oxides, carbon monoxide, lead, nitrogen dioxide and a variety of VOCs (Murray and McGranahan, 2003). A thorough examination of all the pollutants is required to better understand the impacts that vehicular emissions have on human exposure and health. However, due to time and research constraints, only five categories of traffic air pollutants will be discussed. The five pollutants central to this thesis include carbon monoxide, PM10, PM2.5, PM1 and ultra fine particles (UFPs).

2.3.2

Carbon Monoxide (CO)

Carbon monoxide (CO) is a gas produced by the incomplete combustion of carbon-based fuels, and by some industrial and natural processes. The most important outdoor source

of CO can be attributed to emissions from petrol-powered vehicles. Although it is always present in the ambient air of cities, maximum concentrations are often common in major highways during peak traffic conditions. Poor ventilation near unvented combustion appliances can lead to very high CO levels indoors (Murray and McGranahan, 2003). Short or long-term exposure to CO can lead to severe health complications (Romieu, 1999). CO is rapidly absorbed in the lungs and is taken up the blood, greatly reducing the oxygen carrying capacity of blood. Organs which are dependent on a large oxygen supply are the most at risk, particularly the heart, the central nervous system and foetus. Research has also confirmed that subjects with previous cardiovascular disease seem to be the group most sensitive to CO exposure.

The first air pollutant to be studied in vehicles, CO continues to be used as a marker of exhaust emissions (Wiesel, 2001). When studying the personal exposure to carbon monoxide, declining CO emissions over time have to be taken into consideration, especially in North America and Western Europe (Kaur et al., 2007). While in the 1970s CO levels were tens of ppm, in the 1990, this decreased to a few ppms. Duci et al. (2003) examined CO levels experienced by pedestrians along heavy traffic routes in the urban areas of Athens, and mean exposure concentrations were found to be similar in winter and summer-11.5 and 10.1 ppm respectively. The study identified the mode of transport commuters choose to travel in as one of the main factors that had a significant influence on CO concentrations. In another study conducted along Champs Elysees Avenue in Paris, France measured the average CO exposure concentration for pedestrians to be 5ppm (Dor et al., 1995). Even lower pedestrian exposures have been recorded in the studies undertaken in the United Kingdom (UK) with little variation in exposure levels experienced across the country. A study carried out by Kaur et al. (2005) found the mean personal exposure concentration to be 0.9 ppm. The same study reported no difference in CO personal exposure levels based upon the timing, position on pavement and walking direction of the travel. As has been established already, the mode of transport can influence the exposure experienced (Kaur et al., 2007). With regards to CO exposure, it has been generally noted that pedestrians and cyclists often experience exposure

concentrations that are lower than those experienced within vehicles (See Boogaard et al., 2009).

2.3.3

Particulate Matter (PM

PM

PM

)

Particulate matter (PM) is a complex mixture typically divided in fractions based on particle size. Coarse particles with diameters less than 10 microns correspond to particles defined as PM10. Fine particles, on the other hand, with diameters less than 2.5 microns

are collectively referred to as PM2.5 (Tsai et al., 2008). These particulate matters can be

attributed to two major sources. While the first is a natural aerosolisation of crustal matter, which includes re-suspended dust from roadways, sea salt, and biological material such as pollen and fungi, the second source is combustion of fossil fuels (Koenig, 2000). Exposure to airborne particulate matter has become a serious public health issue (Cheung et al., 2008). Both PM10 and PM2.5 are known as major traffic- related air pollutants in

urban environments and recent epidemiological studies have demonstrated that exposure to airborne PM is responsible for a wide range of adverse health effects (Pope et al., 2002). A study done in 2002 (Pope et al.) discovered that a 10 µg3 increase in fine particulate pollution was associated with approximately a 4%, 6% and 8% increased risk of all cause, cardiopulmonary and lung cancer mortality respectively. Several studies indicate the PM2.5 particulates are more directly linked to negative health effects than are

PM10 particulates, as the smaller particles can penetrate further into the lungs than PM10

particulates and can reach the alveoli of the lungs (Ministry for the Environment, 2007). ‘There is an abundance of mass concentration, distribution, and chemical component measurements for ambient PM2.5 and PM10 in many urban and industrialised areas.

However, much less is known, and even less done about PM1 (Lin and Lee, 2004; p.

469). These fine particles in urban areas originate primarily from the gas-to-particle conversion processes within the atmosphere. Secondary anthropogenic combustion products from vehicular traffic and energy production are also known sources of PM1

(Hildemann et al., 1991; Schauer et al., 1996). The mass of the sub-micronic fraction is mainly composed of anthropogenic components such as heavy metals, organics and sulphates, thus enhancing PM1 toxicity (Vecchi et al., 2004).

2.3.4

Ultrafine Particles

Among the numerous components of vehicle-produced pollution, ultrafine particles (UFPs) have generated considerable interest in recent years (Tsai et al., 2008; Morawska et al., 2008; Hagler et al., 2009; Berghmans et al., 2008). Defined as those particles with diameters smaller than 0.1 um, ultrafine particles are abundant in number but contribute little to the mass (Penttinen et al., 2000). The effect of ultrafine particles on adverse health effects is clearly established in scientific literature; studies have shown that ultrafine particles are more toxic than larger particles (Wahlin et al., 2001). Given their small size, UFPs have been shown to efficiently penetrate the respiratory system and even affect extrapulmonary organs (Elder et al., 2006). UFP exposure is also detrimental to respiratory and cardiovascular health (McCreanor et al., 2007).

In urban areas, ultrafine particles are primarily sourced from emissions from motor vehicles, and most UFP emissions can be attributed to diesel vehicles (Fine et al., 2004; Int Panis et al., 2006). Investigations on human exposure to UFPs have discovered that different modes of transport resulted in different exposures (Kaur et al., 2006). Considerable variability was seen in UF particle exposure within a few seconds and over a few meters as commuters moved through polluted microenvironments (Morawaska et al., 2008). For example, a study carried out by Gourioi et al. (2004) showed that car passengers are exposed to high peaks of up to 106 particles cm3. As these results indicate, it is important to realise that the influence of time-activity and movement can be easily missed by using averaged results, leading to underestimation of exposures (Morawaska et al., 2008).

2.4

Comparison to Inter-modal Exposure to Pollutants