Justification of an Air Quality Standard for Particles

27. The Panel first considered the method by which any proposed standard should be measured. We concluded PM₁₀, rather than Black Smoke, to be the more appropriate method, since it represents most closely those particles of greatest potential toxicity and it has been used in many of the epidemiological studies on which our conclusions are based. We have also considered the time period over which PM₁₀ should be measured and reported. The evidence indicates that acute health effects occur after pollution episodes lasting at least 24 hours. No studies have investigated episodes of shorter duration. In the absence of such studies we have therefore concluded that PM₁₀ should be measured as a 24-hour running average.⁵
28. The Panel have concluded that the present evidence supports a causative link between exposure to particulate air pollution in the urban environment and certain indices of ill-health. In particular, we believe that public health benefits could accrue from further reduction in particle concentrations in our towns and cities, in terms of fewer episodes of doctor consultation and hospitalisation for respiratory and cardiovascular diseases. We are less sure of the benefits in terms of reduction in premature mortality, since if pollution-related deaths simply occur a few days early, the public health benefits are likely to be small, whereas if they occur years early they would be larger. On this important matter, as on the question as to whether current levels of air pollution actually contribute to the causation of heart and lung disease, we believe there is as yet insufficient evidence.
29. In recommending a concentration of PM₁₀ at which a standard should be set, the Panel have differentiated between the concentration that in their judgement would be regarded as safe for individuals, including those with illness that makes them susceptible, and the concentration that would need to be achieved in order to produce significant benefit to the public health. Our task has first, therefore, been to decide on a concentration at which health effects on individuals are likely to be small and the very large majority of individuals will be unaffected. A rise from a daily average level of 20 to 50 µg/m³, a concentration which was exceeded on average one day in ten in the Birmingham study mentioned above, would be expected to be associated with just over 1 extra patient on average being admitted to hospital with respiratory disease daily in a population of 1 million. We have argued above that such admissions may represent the effects of exposure of susceptible people to concentrations at the upper extreme of the range represented by the figure recorded centrally. In terms of individuals, therefore, we have concluded that daily average concentration of 50 µg/m³ (equivalent to the inhalation of not more than 1 mg in 24 hours) would be unlikely to affect the health of the very great majority of people. In the best judgement of the Panel, it is considered that very few individuals in the population will react adversely to this concentration of particles, to which all urban dwellers are exposed on a frequent basis.
30. In contrast, on a population basis, the Panel acknowledge that epidemiological studies have shown evidence of effects on health when local area measurements record concentrations of PM₁₀ below 50 µg/m³. If an Air Quality Standard set at 50 µg/m³ were adhered to, as presently monitored, it is certain that some members of the local population would nevertheless be exposed to higher concentrations when living close to the major sources of the pollution, and it is likely that health consequences on a population scale would still be detectable. The Panel are of the view that the most effective means of ensuring a reduction in the adverse health effects of particulate pollution on the population is by a progressive lowering of the average concentrations of particles in our cities throughout the year, rather than simply by action aimed at limiting the number of peak concentrations exceeding 50 µg/m³
31. In the judgment of the Panel, 50 µg/m³ is likely to be a safe concentration for exposure of the very large majority of individuals, and we recommend this figure as an ambient Air Quality Standard for PM₁₀ in the United Kingdom, measured as a 24-hour running average. This figure is close to the 90th percentile of measurements of 24-hour averages made to date in the United Kingdom, that is one out of 10 measurements have exceeded it. It is between 20 to 30 µg/m³ higher than the annual averages so far recorded, and has been exceeded regularly in winter in most cities in which it has been measured. In coming to this conclusion, the Panel are aware that we have had to make a number of judgements unguided by hard scientific evidence, and that we are recommending a Standard at which adverse effects on the health of populations may still be measurable. Since the greatest health benefits are likely to accrue from a reduction in the annual average concentrations of particles, we recommend that the Government implement a strategy which will reduce progressively both the numbers of 24-hour exceedences of 50 µg/m³ and the annual average concentrations of PM₁₀. Such reductions could also be expected to ensure that there would be a decrease in peak concentrations.
32. Because of the many uncertainties surrounding the evidence upon which our recommendations are based, and since the Government has recently commissioned further research in this area, we believe that the recommended Air Quality Standard should be reviewed, in the light of United Kingdom experience and of any new data, within the next five years.