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Over the past 10 years there has been increasing evidence that particles generated by the combustion of fossil fuels adversely affect health. To what extent should paediatricians be concerned about particle pollution? This review assesses what we know, and what we still need to know about the health effects of fossil fuel particles.
Since the early 1990s there has been increasing evidence that particles generated by the combustion of fossil fuels adversely affect health. In January 2001. the UK government was sufficiently concerned to issue a warning when air pollution levels for particulates were forecast to be high across most of the UK. (1) Hourly updates on particle levels across the country were, and still are available on the internet (www.environment.detr.gov.uk; accessed 1/12/01), or on a freephone number. The minister for the environment at the time, Michael Meacher, commented: "I have taken the precautionary step of making sure vulnerable people, such as those with heart and lung disease, are aware and can take appropriate measures to reduce their exposure" (1). children were not specifically identified in this warning, but to what extent should paediatricians be concerned about particle pollution?
SOURCES OF PARTICLES
Polluted air is a complex mixture of vaporous, liquid, and solid components (fig 1). There are major qualitative and quantitative differences in this mixture, depending on emission sources, geographical location, weather conditions, time of day, and season. Irrespective of these variations, particles from internal combustion engines are a consistent contributor to the particle fraction of the pollutant mix. Although petrol engines emit particles, diesel engines, especially heavy duty engines, are a major emission source as they emit 100 times more particles than do petrol engines of corresponding performance. Diesel exhaust particles (DEP) are defined as "primary" particles-- that is, they are emitted directly and are not formed in the atmosphere. DEP consist of a carbonaceous core (fig 2) onto which over 18 000 different high molecular weight compounds are adsorbed. (2) DEP are regarded as the prototypic pollutant particle, but there are other types of particles, such as natural wind blown dusts, and sulphat e from gas-to-particle interactions in the atmosphere (secondary particles). The health effects of the combustion of biofuels (wood, agricultural residues, and dung) are not discussed in this review, but they are a major source of particles in rural underdeveloped countries.
Older monitoring devices for primary particles suck in a volume of air, and measure the total mass of suspended particles, irrespective of size. current samplers determine the mass of particulate matter (PM) in diameters under 10 [micro]m ([PM.sub.10]), and under 2.5 % [micro]m [micro]m (fine particles, [PM.sub.2.5]). These measurements are more relevant to health effects as [PM.sub.10] corresponds to the "inhalable" fraction, and [PM.sub.2.5] is the size that is most likely to penetrate deepest into the airways. Another important component of [PM.sub.10] in terms of number but not mass, are ultrafine (UF) particles of less than 0.1 [micro]m in diameter (3); 10 (9) UF particles have the same mass as one "coarse" particle of 10 [micro]m diameter, and readily penetrate into houses through walls. (4) In general, the "naturally" generated particles such as wind blown soils fall into the coarse fraction of [PM.sub.10] (less than 10 [micro]m and greater than 2.5 [micro]m), whereas fossil fuel combustion is the majo r source of smaller particles ([PM.sub.2.5] and UP). (5)
TOXICITY AND PARTICLE SIZE
Although particle concentrations in ambient air may change in parallel to gaseous pollutants such as nitrogen dioxide, it is possible to identify the independent effect of [PM.sub.10] in large studies. (6) By contrast, there is a high degree of intercorrelation between the coarse, fine (0.1--2.5 [micro]m), and UF particles, and it is difficult to pinpoint which fraction of [PM.sub.10] is the most toxic. From a public health perspective this may not be a problem, as long as the emission source can be identified. However, vehicle manufacturers need to know the size and composition of the most toxic fraction. …