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1. We have been asked to advise on the use of statistical associations between daily concentrations of particles and admissions to hospital for treatment of cardiovascular diseases in a cost benefit analysis for the review of the Air Quality Strategy objective for particles. It is recognised that the evidence for such an association has developed since we last advised on this question in 1998. Not only has the number of relevant epidemiological studies, some from the UK, increased, but a number of mechanistic hypotheses to explain the associations have been advanced. Some support for these hypotheses have been provided by both epidemiological and experimental studies.

2. It is our view that the evidence relating to an association between daily concentrations of particles and numbers of admissions to hospital for treatment of cardiovascular diseases is sufficiently strong for us to regard this association as more likely than not to be causal. In reaching this view we have been influenced by the development of mechanistic evidence which provides some plausibility for a causal association. We accept and point out that certainty regarding the causality of the association is unattainable, being based on a judgement of the overall mechanistic and epidemiological evidence. However, because cardiovascular diseases are common and ambient air pollution affects the whole population, even a small increased risk due to air pollution could have a substantial impact on public health.

3. Having accepted that the association is more likely than not to be causal we addressed a further point: can we provide an estimate of the strength of the association?

4. It is our view, based on a meta-analysis of studies currently available [^1], that a 10 µg/m³ reduction in 24 hour average concentrations of particles (measured as PM₁₀) is likely to be associated with a 0.8% reduction in all age, all cause cardiovascular admissions [^2]. The meta-analysis has provided a confidence interval around the central estimate of 0.6-0.9% [^3]. This is the range within which the true value is likely to lie, with 95% confidence. We regard this quantitative estimate of the size of the association as provisional. We are aware of a major European, multi-centre study and a similar US study that will soon provide further data: when such data are available we may need to refine our estimate. This estimate of potential benefits of a reduction in airborne particles on cardiovascular admissions may misrepresent the overall benefits of pollution control because part of the observed association with PM₁₀ may be due to correlations with other pollutants, or other confounding factors.

5. We stress that there is a clear need for more UK research into the mechanisms and epidemiology of air pollution and cardiovascular disease.

² Particle concentrations may be measured in a number of ways. PM10 refers to the mass of particles generally less than 10 µm in diameter per cubic metre of air. 1 µm = 1 thousandth of a millimetre, 1 µg = 1 millionth of a gram. PM10 itself can be measured using different techniques. The summary estimate is based mainly on studies using high volume samplers (gravimetric) to measure PM10. The inclusion of a few TEOM studies may mean that the summary estimate is slightly overestimated. The current annual average concentration of particles, measured as PM10, in London is about 25 µg/m3.

COMEAP Secretariat
December 2001

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