Small but dangerous - new facts on particles
Fine particles cause 546,000 premature deaths each year in Europe, of which 39,000 are due to emissions from international shipping.
A large number of studies carried out in both the US and in Europe have shown that when the concentration of particulate matter (PM) in air rises, even from low levels, there is a rise in mortality from respiratory, cardiac and circulatory diseases, and more people seek hospital care for bronchitis and asthma.
It is the very smallest particles that are believed to be the most harmful, because when they are inhaled they can penetrate deep into the lungs. The focus of debate is turning to PM2.5 (see box). However, many researchers point out that coarse particles (PM2.5–PM10) also have considerable health effects and their levels also need to be reduced.
The shape and chemical compositionof the particles as well as their size are thought to influence their harmfulness, as do the substances that adhere to their surfaces.
So far it has not been possible to distinguish clearly between the health effects of particles with different origins, so particles in the same size range are usually regarded as equally harmful. Toxicology studies indicate, however, that so-called primary particles from combustion have a higher toxic potential than secondary particles (see box). These primary particles are often rich in metals and organic compounds, and also have a relatively high surface area.
In a new Swedish study1, an advanced computer model was used to estimate the population exposure to regional background concentrations of fine particles (PM2.5). The particles were split into two main categories – primary and secondary.
Most earlier studies have not differentiated between differences in relative risk between primary and secondary PM. In this study premature deaths due to secondary PM were estimated using the relative risk factor of a six-per-cent increase in mortality per 10 microgrammes per cubic metre (μg/m3) of PM2.5. For primary PM, a risk factor of 17 per cent per 10 μg/m3 of PM2.5 was used.
Based on emission data for the years 2001-2003, population exposure to PM was estimated by multiplying concentrations with the population density in the various parts of Europe. Secondary PM was found to contribute more to the population-weighted exposure than primary PM, but as a result of the larger relative risk factor assumed for the latter, primary PM was calculated to be responsible for a larger number of premature deaths.
The number of annual premature deaths in the EU27 due to regional background concentrations of PM2.5 was estimated at 316,000, of which 177,000 (56%) were linked to primary PM. For the whole of Europe, emissions of primary particles were responsible for more than half (55%) of a total of 546,000 PM-related premature deaths annually.
For comparison, the EU Clean Air For Europe (CAFE) programme estimated the number of premature deaths caused by PM2.5 in the year 2000 at 348,000 for EU25. It should be noted that the CAFE study included an “urban enhancement effect” to at least somewhat account for the local peaks in pollution and exposure in cities.
The contribution of international shipping to the total European emissions was about eight per cent for primary PM2.5, 11 per cent for sulphur dioxide, and 17 per cent for nitrogen oxides. These ship emissions resulted in about 39,000 PMrelated premature deaths each year. Due to the prevailing westerly wind directions, countries in western Europe contribute more than those in eastern Europe to overall European population exposure and PM-related premature deaths.
1 Population exposure and mortality due to regional background PM in Europe – long-term simulations of source-region and shipping contributions. By C. Andersson, R. Bergström and C. Johansson. Atmospheric Environment, March 2009.
What are particles?
Particulate matter (PM) is an air pollutant consisting of a mixture of particles that can be solid, liquid or both, are suspended in the air and represent a complex mixture of organic and inorganic substances. These particles vary in size, composition and origin.
Their properties are summarized according to their aerodynamic diameter, called particle size, which is measured in microns. A micron equals one thousandth of a millimetre and is written: μm. The measurements used at present are the weights of two particular fractions:
- PM10, particles with an aerodynamic diameter smaller than 10 μm, which may reach the upper part of the airways and lung.
- PM2.5, with an aerodynamic diameter smaller than 2.5 μm. These are regarded as more dangerous because they penetrate more deeply into the lungs and may reach the alveolar region.
Usually particles are divided into three groups: coarse, diameter 2.5–10 μm; fine, 0.1–2.5 μm; and ultrafine, less than 0.1 μm. The fine and ultrafine fractions are more strongly associated with anthropogenic activities than the coarse fraction, which may contain for example wind-blown dust.
The size of the particles determines the time they spend in the atmosphere. While sedimentation and precipitation remove PM10 from the atmosphere within a few hours of emission, PM2.5 may remain there for days or even a few weeks.
Consequently, PM2.5 can be transported over long distances. In most places only a small proportion of the background concentration is traceable to local emissions, and a large percentage, particularly of the finest fractions, consists of particles that were emitted in other locations or formed as secondary particles in the atmosphere. In urban environments, along major roads for example, the local contribution can be considerable, however.
Particles are classed as either primary or secondary:
- Primary particles are those that are formed during combustion, but may also consist of dust, small soot flakes, pollen, etc. Major sources of anthropogenic emissions are combustion processes (often small-scale appliances, e.g. domestic stoves or boilers) and internal combustion engines (primarily diesel engines).
- Secondary particles consist mainly of sulphate and nitrate salts that are formed in the air from sulphur dioxide and nitrogen oxides. Ammonia and volatile organic compounds (VOCs) are also of interest. Any source that emits these substances therefore contributes to their formation. Most fine particles in air are of secondary origin.