Abstract
Particulate matter (PM) refers to the mixture of solid and liquid particles suspended in the air. The particles vary greatly in size and composition. Ambient air consists basically of two classes, or modes, of particles. The fine mode consists of particles with diameters less than about 2 µm, formed by gas-to-particle conversion processes (for instance in an internal combustion engine), while the coarse mode particles are formed through mechanical processes (such as windblown dust or sea salt).
The deposition of particles in the human lungs and conducting airways depends on the particle size. Around 10 to 60% of the inhaled particles with aerodynamic diameters less than 10 µm may deposit in the lung. Particles formed in combustion processes belong to the fine mode and can thus reach deep into the human respiratory system.
In recent years, several epidemiologic studies have indicated an association between particulate matter (often measured as PM10 or PM2.5) and increased daily mortality (death), as well as an increase in factors such as daily rates of hospitalization and visits to emergency rooms because of asthma or heart symptoms. These adverse health effects seem to occur even at the relatively low ambient concentrations prevalent in Europe and the USA. The European APHEA2 study in which 29 cities were included, reported a 0,6 % increase in mortality to be associated with a 10 g/m3 increase in PM10 and nearly identical figures have been reported from the USA.
Pope and coworkers recently reported the results from a study on the relationship between long-term exposure to fine particulate air pollution and mortality. It was found that each 10 g/m3 elevation in fine particulate air pollution
(measured as PM2.5) was associated with approximately a four percent increased risk of all-cause mortality. The risk estimates were higher if one looked at deaths due to lung cancer (8 %) or heart or lung disease (6 %). Measures of coarse particle fraction and total suspended particles, on the other hand, were not consistently associated with mortality.
The mechanism by which low PM levels cause health effects is not known and a number of laboratory studies have been performed to back up the epidemiologic findings with data from animal or human exposure experiments. It has not, however, been possible to convincingly identify either which physical and chemical properties of PM are relevant from a toxicological point of view or by which biological mechanism the particles may cause disease or death.
There are some indications that ultrafine particles (< 100 nm) have toxicologic properties different from that of larger particles. So far, there are only a few human studies that show that ultrafine particles are harmful. Ultrafine particles in ambient air could come from a range of other sources than internal combustion engines.
Diesel particulate matter is believed to be more hazardous upon inhalation than particulate matter from spark-ignition engines. One reason for this is that diesel particles carry more toxic compounds on the surface (such as mutagenic PAHs and nitro-PAHs).
Inventories from regions or nations usually indicate that combustion particles from road traffic make up less than 10 % of ambient PM10 or PM2.5. The percentage may be considerably higher in urban centers, however. PM10 emission inventories for the European Union provided by the Auto Oil II program show that road traffic accounts for less than 10 % of the total and that the emissions will decrease in both absolute and relative terms during the coming decade.
Many uncertainties exist regarding the potential adverse health effects caused by low ambient PM concentrations and it is even more difficult to assess, with a high degree of certainty, the risk attributable to road-traffic. Furthermore, other pollutants present in ambient air could, at least in part, cause effects believed to be caused by PM. Nevertheless, it appears obvious that exposure to PM from road traffic (just as with PM from any other combustion process) poses a health risk. It has not, however, been possible to describe this risk numerically with a reasonable degree of certainty.
The health effects are small compared to the normal frequency of disease and deaths in the population and thus difficult to study.
Future studies may well show that there are size fractions of exhaust particles that are more critical than others from a toxicologic point of view.
The debate on PM will continue and more stringent regulations on engine emissions and air quality are expected in the future. Regulators will probably motivate their decisions by referring to the precautionary principle rather than costbenefit analysis based on firm medical evidence (since the amount of risk reduction associated with a decrease in PM levels is currently not quantifiable).