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Executive summary 1.1 The Department of Health (DH) asked the Committee on the Medical Effects of Air Pollutants (COMEAP) to advise on the possible effects of outdoor airborne non-biological particles on health. The Committee formed a Sub-Group which reviewed the literature in detail and drafted this report. The report has been endorsed by the Committee. 1.2 The terms of reference of the Sub-Group were to advise on:
1.3 The Sub-Group has reviewed in detail the available information on levels of particulate air pollution in the United Kingdom (UK) and the evidence available from epidemiological studies conducted mainly in other countries regarding the likely effects of such pollution on health. The Sub-Group has also reviewed evidence regarding the likely mechanisms of effect of such particles. The report has not dealt with occupational exposure to particles or to tobacco smoke. Having considered the Sub-Group's work, the Committee's conclusions are summarised below. 1.4 There is clear evidence of associations between concentrations of particles similar to those encountered currently in the UK, and changes in a number of indicators of damage to health. These range from changes in lung function through increased symptoms and days of restricted activity to hospital admissions and mortality. 1.5 The evidence regarding these associations has been gathered from a range of well conducted epidemiological studies which have mainly been undertaken in the United States (US) and recently in Europe. The consistency of the associations demonstrated by these studies is notable especially as regards mortality, though the reported effects on health of day-to-day variations in concentrations of particles are small in comparison with other uncontrollable factors, eg, seasonal variations or variations in temperature. Considerable coherence of results across health endpoints has also been shown. The few studies which have been conducted in the UK have tended to confirm the findings of studies reported from other countries. 1.6 There is no clear evidence that associations with effects on health are restricted to specific types of particles. Epidemiological studies have demonstrated associations between effects on health and particles from a wide range of sources. These include primary emissions from motor vehicles, industrial sources or coal fires and secondary aerosols derived from gaseous emissions, including sulphur dioxide and oxides of nitrogen, from industrial and vehicular sources. In the absence of strong evidence on the relative effects of different particles within the respirable range, it seems reasonable, at present, to base policy on PM10 measurements. 1.7 The principal question to consider in reviewing the rapidly expanding literature on health effects of suspended particulate matter is whether the statistical associations demonstrated indicate a causal role. There is certainly a remarkable degree of consistency and coherence in the direction and magnitude of findings from a diversity of studies, carried out in different localities in the US and elsewhere, with a range of different health indicators and varying sources of pollution. We consider that the reported associations between levels of particles and effects on health principally reflect a real relationship and not some artefact of technique or the effect of some confounding factor. The indications that the association is likely to be causal are certainly strong. 1.8 We conclude, that in terms of protecting public health it would be imprudent not to regard the associations as causal. We also believe that the findings of the epidemiological studies of the acute effects of particles, which have been conducted in the US and elsewhere, can be transferred to the UK, at least in a qualitative sense. However, we consider that there are insufficient UK data available to allow direct extrapolation and reliable estimation of the size of the effects in the UK. 1.9 It would be possible, for any health effect of interest, to take a weighted average of the results of well-conducted published studies and apply this to conditions in the UK. [This would usually imply conversion across different measures of particles.] Thus, the relative risk calculated by Schwartz (Schwartz J. Air pollution and daily mortality: a review and meta-analysis. Environ Res 1994;64:36-52.) with regard to effects of particles on mortality was 1.06 (confidence interval [CI]:1.05-1.07) for a 100 µg/m3 change in total suspended particles, equivalent to some shortening of life in approximately 1% of daily deaths per 10 µg/m3 increase in PM10. Application to the UK of the results even of such structured meta-analyses does not formally take account of uncertainties in extrapolating to different air pollution mixtures (with generally lower concentrations of suspended particles), climate patterns and at-risk populations. Because of these uncertainties, we think it would be unwise to offer a single coefficient with regard to effects on mortality or any other index of ill health. The reader is referred to the tables in Annex 8A to Chapter 8 with the warning that the estimates based on studies reported in these tables are likely to provide only a first approximation to the actual effect. Studies should be undertaken urgently to allow better quantitative predictions to be made. 1.10 The only major difficulty in reaching any firmer conclusion about causality is the lack of any established mechanism of action. The mass of suspended particulate matter associated with adverse effects is very small, and while there is evidence relating to acute effects of some components, the fact that in epidemiological studies similar effects have been reported in localities with different types of suspended particulate matter suggests that particles may have a non-specific action. Reported studies indicate a range of effects, from small changes in ventilatory function or exacerbations of asthma through to increases in deaths among the elderly or chronic sick; it does not necessarily follow that the same components would be involved in each effect. The effects have not been explained in terms of the results of conventional inhalation toxicology studies, though few appropriate studies have been reported. It has been suggested, but by no means proven, that ultrafine particles (< 0.05 µm diameter) may play a role. These particles have been shown in recent animal studies to be unexpectedly capable of producing inflammatory reactions in the lungs. Concentrations of such particles would be higher close to sources in the environment because, with time, they would coalesce into larger and more stable forms. They would represent, therefore, only a small proportion of the mass of material measured as PM10, though they would represent a high proportion of the number of particles present. 1.11 It is well established from the reported studies that people with pre-existing respiratory and/or cardiac disorders are at most risk of acute effects from exposure to particles. It has been suggested that these effects occur when air pollution aggravates an acute condition such as a respiratory infection, an attack of asthma or a heart attack in people with pre-existing chronic disease. There is no evidence that healthy individuals are likely to experience acute effects on health as a result of exposure to concentrations of particles found in ambient air in the UK. 1.12 Evidence regarding the effects of long term exposure to particles on health is even less well developed than that regarding the acute effects. The possibility of confounding in such epidemiological studies is considerable and it is difficult to estimate the exposures of individuals over relevant time periods. Here again, the results of recent US studies reporting associations with mortality, respiratory symptoms and lung function are probably transferable to the UK in a qualitative sense, though the confidence in the accuracy of the predictions is lower than for the acute effects of particles. 1.13 Although the evidence is limited, the Committee advises that it would be prudent to consider these associations between long-term exposure to particles and chronic effects as causal. 1.14 There is little evidence to show that exposure to atmospheric particles contributes significantly to the burden of cancer in the UK. The presence of genotoxic carcinogens in particles means that such a contribution cannot be ruled out, although it is likely to be very small. 1.15 In terms of monitoring levels of particulate air pollution in the UK, we support the continued use of automatic measurement of PM10. Measurement of Black Smoke should also be continued. The need to determine the temporal and spatial mass and number distributions of particle sizes is stressed. 1.16 There is a need for research into the effects of particles on health. This research is needed both to improve the predictions of effects which can be made from currently available epidemiological studies and to investigate possible mechanisms of effect. A number of recommendations for research are provided in Appendix 1 of the report.
Conclusions 1.17 The Committee considers that the reported associations between daily concentrations of particles and acute effects on health principally reflect a real relationship and not some artefact of technique or the effect of some confounding factor. 1.18 In terms of protecting public health it would be imprudent not to regard the demonstrated associations between daily concentrations of particles and acute effects on health as causal. 1.19 We find it difficult to reach a firmer conclusion about causality due to the lack of any established mechanism of action. 1.20 We believe that the findings of epidemiological studies which have been conducted in the US and elsewhere, of the acute effects of particles, can be transferred to the UK, at least in a qualitative sense. 1.21 It is accepted that insufficient UK data are available to establish the reliability of quantitative predictions of the effects of particles upon health in the UK. 1.22 We consider that results of recent US studies of the effects of long-term exposure to particles are probably transferable to the UK though confidence in the accuracy of the predictions is lower than with regard to the acute effects. Although the evidence is limited, we advise that it would be prudent to consider these associations as causal. 1.23 There is no evidence that healthy individuals are likely to experience acute effects on health as a result of exposure to concentrations of particles found in ambient air in the UK.
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HMSO, 1995 Available from The Stationery Office |
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Executive summary