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affected, there are an extra 320 deaths per 100000 from ischaemic heart disease and 312 from COPD and lung cancer together.

The crude relative increase in ischaemic heart disease from these figures is 1.55 compared to 12.7 for COPD and 14.9 for lung cancer, and the much higher relative risk suggests a much closer and more complete causal link to pulmonary disease (Fig. 1.1). For heart disease, it is recognized that smoking is only one factor amongst several involved in the causation of disease. Genetic susceptibility (as shown by the strong influence of a history of heart disease amongst close relatives) and lipid control are two other strong predictors that may be as important as smoking.

The increased risks attributable to smoking are dose-dependent (Fig. 1.2). Not only is the number of deaths per 100 000 much increased for COPD and lung cancer, but there is also a significant increase in deaths from pneumonia, and many of the patients concerned could well have had COPD too. The

COPD effect may therefore be underestimated. This leaves approximately 5-10% of cases of COPD that are not directly attributable to smoking.

The authors were able to go further in their estimations of risk to show that for the average smoker, there was a loss of 7.5 years of life, increasing to 10 years for a smoker of more than 25 cigarettes per day. Another way of describing the data is to state that only 21% of smokers will attain the age of 85 years, compared to 41% of non-smokers. If a person ceases smoking, then the risks of death are reduced and there are discernible benefits even for those quitting when over 65 years of age.

As was noted above, this was an observational study and there were a number of potential confounding factors. Death certification could have been wrong—it is known that the reliability of death certificates is not good, and there were many changes in the lifestyle, wealth and personality of the population over the period studied. While it is possible that some of these factors could have affected survival, it is unlikely that they could have altered the huge relative risks observed.

These data were derived from a relatively privileged sector of the population; while this has an advantage in that there was no social class disease gradient to be taken into account, it also raises the possibility that the relative risks could be different in other parts of society. More recent data for the UK based on survival data from life-insurance work [2] show a very similar effect on loss of life expectancy — 7years between the ages of 30 and 70 — suggesting that the Doll and Peto data can probably be extrapolated to the general population.

The size, completeness and length of this study make the links between smoking and both lung cancer and COPD irrefutable, and indeed many other studies since have confirmed and supported these conclusions.

How many non-smokers develop COPD? From the Doll and Peto figures, it would seem that of 285 deaths per 100000 due to COPD, there were 10 individuals, or about 3%, who had never smoked and were labelled as having COPD. Similar figures are reported from cross-sectional studies of living patients, e.g. that 5-7% of their cohort were non-smokers [3]. Assuming that these cases of COPD do not result from incorrect recording of diagnosis or smoking status, other aetiological factors may exist. This is discussed later in the chapter.

Many other examples from other countries have confirmed the dose-related relationship between the risk of developing COPD and cigarette smoking [4]. They also confirm the lower incidence of COPD in those who smoke a pipe or cigars rather than cigarettes [5]. The incidence of COPD is consistently reported to be significantly lower in women, reflecting the lower prevalence of cigarette smoking amongst females, but the pattern is changing. In the UK, death rates from COPD in men have fallen, reflecting a change from a 65% rate of current smokers in 1970 to less than 30% in 2000, but the death rate in women is still rising [6] following the surge in female smoking after the second world war. In Denmark, where a high proportion of women have been smokers for many years, the percentage of deaths in women attributable to tobacco already approaches that of men [7]. This trend is likely to be seen in other European countries in the coming years.

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