This snapshot taken on 16/07/2006, shows web content selected for preservation by The National Archives. External links, forms and search boxes may not work in archived websites.
Conclusions
135. The Panel has been asked to consider what is the most appropriate measurement on which to base a Standard for control of the population's exposure to ambient airborne particles. We have not been asked to consider the adequacy of the present Standard, which we recommended in 1995, and have not done so in this report. At present the Particle Standard is based on measurement of PM10, a metric that has stood the test of time and has allowed the demonstration of robust relationships to adverse health outcomes. In the absence of evidence that any alternative metric shows a closer and more reliable association with health outcomes, there is no pressing need to substitute one.
136. The main argument centres round the concept that the toxicological effects of particles are to be found in a finer fraction of PM10, measured, for example, as PM2.5. Moreover, it has been suggested that the effects of particles may be caused by the smallest, ultrafine, particles less than 100 nm in diameter, and that the numbers of these are poorly represented by PM10. This argument assumes that the fraction larger than 2.5 µm and smaller than 10 µm is essentially non-toxic in the concentrations found in ambient air, and that it is only necessary to control the finer fraction in order to limit effects on health.
137. The Panel has considered carefully the merits of this argument. First, while some epidemiological studies have suggested that the main toxic component is likely to be in the finer fraction, several have indicated that the toxic effects may not be confined to this fraction. Secondly, to date in the UK, PM2.5 and PM10 have been very closely correlated wherever they have been measured and their associations with the health effects have thus been inseparable. It is possible that a long time-series comparing the associations of these and other metrics with health outcomes will resolve these difficulties, and we recommend pursuing research in this area. Thirdly, while there is reason to suppose that metrics based on particle numbers or surface area may not correlate so closely with mass measurements and may better represent at least part of the toxic component, there are as yet insufficient data on which to base such a Standard. It is therefore premature to consider such metrics for control purposes, but the Panel recommends that they should be introduced into future epidemiological research projects in order that relevant data will be available when EPAQS next considers this issue.
138. The Panel concludes that, on the present evidence, measurement of particulate air pollution as the metric PM10, which includes essentially all respirable particles, provides the most appropriate basis for an Air Quality Standard in the United Kingdom. It may be that further research will lead us to additional or alternative metrics, for example, PM2.5 or counts of ultrafine particles, but the currently available data do not allow satisfactory derivation of a Standard based on these. In view of the likelihood of important advances in understanding of this area of science over the next few years, we recommend that this issue should be kept under active review.
Published 17 May 2001
Expert Panel on Air Quality Standards
Air and Envirnonmental Quality
Environmental Protection Index
Defra Home Page