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EHA : Yearkbook 2009
EHA YEARBOOK 2009 of any indoor sources, indoor air will still contain 50-100% of the particulate matter found outdoors. Airborne Particulate Matter The last 10 years or so have seen an unprecedented increase in the interest in, and attention to, airborne particles, with a special focus on their finer size fractions. A clear turning point for this interest was the publication of findings from the Harvard epidemiological studies, pointing out a more distinct correlation between health effects and exposure to fine particles (smaller then 2.5 um) than to coarse particles (Dockery et al. 1993). This finding led to the introduction in the USA in 19 97 of a standard for PM2.5. However, it is now considered that even smaller particles, those in the submicrometre or even ultrafine (< 0.1 um) ranges, should be targeted by control measures, as they are associated with even higher risks than the larger particles (WHO 2002). In addition to ultrafine particles, their subset, nano particles (< 0.05 um), has also been widely discussed in the last few years. Both of these terms constitute a somewhat arbitrary classification of particles in terms of their size, indicating the significant role of this physical characteristic on particle fate in the air. It should be kept in mind, however, that the divisions between ultrafine and larger particles, similar to the other divisions between different particle size classes, are also somewhat arbitrary. On the one hand, there are no rigid boundaries created by nature between these size classes, but on the other hand, all natural sources (as opposed to laboratory generators) generate particles with a certain range of diameters, polydisperse particles, therefore, there is no sharp boundary delineating the contribution of particles from a given particle source. These small particles (ultrafine and nano particles) are generated mainly from combustion, gas to particle conversion, nucleation processes or photochemical processes, with some of them being primary (emitted directly by the source) and some being secondary in nature (formed in the air from the precursors emitted by the source/s). Particles generated from anthropogenic activities are mainly primary and secondary combustion products, with 89 % of the ultrafine and 65 % of the nano particulate matter emitted in the urban environment originating from vehicle combustion. Since the mass of these particles is very small, they are generally measured in terms of number concentration. Because of the small mass of the particles, PM2.5 or PM10 measurements provide very little or no information on particle number concentration. Particle concentration levels in clean environments, meaning those which are not influenced by human activities, are usually in the order of a few hundred particles per cm3. In urban environments, background particle number concentrations range from a few thousand to about 20 thousand particles per cm3 (e.g. Harrison et al. 1999; Thomas & Morawska 2002). Background concentrations mean the concentrations measured at monitoring stations, which are not influenced by a nearby emission source. Near roads and in the tunnels, vehicular traffic constitutes the most significant urban source of particle number, and particle number concentrations can be 10 times higher or more, and can reach or exceed levels of 105 particles per cm3 (Morawska et al. 2003). This is in contrast to PM10 and PM2.5 mass concentrations, which have been shown to be no more than 25-30% above background level on major roads, calculated as the difference between the maximum on the road and the background levels (Morawska & Salthammer 2003). A practical implication from these findings is that the exposure to airborne particles, in terms of their number concentration, is significantly increased within the first 10 0 meters from the road, compared to the urban average exposure levels, and decreases to the urban background level usually at distances greater than 30 0 meters from the road. On this basis, it is reasonable to assume that people living and working in close proximity to an urban arterial road are likely to be exposed to levels of ultrafine and submicrometer particles beyond that which could be considered ‘normal’ ambient levels, but only to somewhat elevated levels of PM10 and PM2.5. Particles and Health: A Special Challenge Health effects due to inhalation of particles: PM10 and PM2.5 The relationship between airborne particle mass concentration (PM10 and PM2.5) and health outcomes has been extensively investigated through epidemiological research (e.g. Dominici 2002; Katsouyanni et al. 2001; Pope 2000; Pope & Dockery 2006; Samet et al. 2000). It has been shown that the most significant health end points due to inhalation of particulate matter include: decreased lung function, increased respiratory symptoms, increased chronic obstructive pulmonary disease, increased cardiovascular and cardiopulmonary diseases and increased mortality. A recent literature review of the state of knowledge in relation to particle mass concentrations demonstrated: that the complexity of the mechanisms by which inhalation of particulate matter causes health effects are better understood; that there is no threshold in response; that the response is linear; and that it is similar over different geographic settings (Pope & Dockery 2006). The review also concluded that, in Air Quality and its impact on Health: (continued)
Annual Review and Yearbook 2008
EHA Yearbook 2010