Mortality by smoking habit and cause of death
shows mortality by smoking habit for 11 major categories of cause of death, and for all causes combined. In these and all other analyses, current cigarette smokers are restricted to those who in all their previous replies (including the first, in 1951) reported smoking only cigarettes. Likewise, former cigarette smokers are restricted to those who, before stopping, had reported in all their previous replies smoking only cigarettes (or who had stopped before 1951, having last smoked only cigarettes).
Cause specific mortality by smoking habit, standardised indirectly for age and study year, for all 34 439 men born in 19th or 20th century (1851-1930) and observed 1951-2001
lung cancer and chronic obstructive lung disease are closely related to continued cigarette smoking and to the daily number of cigarettes smoked. For each of the other nine categories of cause of death there are more moderate, but again highly significant (each P < 0.0001), positive relations with the continuation of cigarette smoking and with the daily number smoked.
Effects on overall mortality
How far, in this particular population, such relations between smoking and mortality reflect cause and effect has been discussed previously.17,23
Midway through the study, the results from the 1978 questionnaire confirmed the well known association between smoking and drinking,24
but showed little or no relation between smoking and either obesity or blood pressure (), so these particular factors cannot help to account for the excess vascular mortality among smokers. The excess mortality from “external” causes—accidents, injury, and poisoning—among smokers is unlikely to be due chiefly to smoking (although two men did die from fire because of smoking in bed) but, rather, is likely to be due to other behavioural factors with which smoking is associated, such as the heavy consumption of alcohol or a willingness to take risks. Such external causes, however, account for less than 3% of the overall excess mortality among cigarette smokers. A quarter of the excess mortality among smokers is accounted for by lung cancer and chronic obstructive lung disease and another quarter by ischaemic heart disease; most of the rest involves other neoplastic, respiratory, or vascular diseases that could well be made more probable (among the survivors at a given age) by smoking.
Characteristics in 1978 of smokers, ex-smokers, and smokers born in 20th century (aged 48-78 at 1978 survey). Means and prevalences are standardised to age distribution of all 12 669 respondents to 1978 questionnaire
Some of the 11 relations in have been increased by confounding, most notably with alcohol (which can increase the risk of developing cirrhosis of the liver and cancers of the mouth, pharynx, larynx, and oesophagus) and with personality (in the case of accidents, injury, and poisoning). Confounding, however, can act in two directions, as alcohol consumption—which is higher among smokers than among non-smokers ()—can also decrease the risk of ischaemic heart disease and perhaps of some other conditions.24
Another important factor, not previously much emphasised, is the possibility of “reverse causality”—that is, some reduction in the apparent risk of death among current smokers because of a tendency for people to give up smoking after they begin to be affected by some life threatening condition (), whether or not their illness was caused by smoking.
When all 11 categories in are added together, however, yielding overall mortality, the combined effects of all these non-causal factors—acting to increase or decrease the apparent hazards among smokers—are unlikely to have influenced greatly the absolute difference between the overall mortality rates of cigarette smokers and lifelong non-smokers. This difference, we conclude, provides a reasonably quantitative estimate of the extent to which, at particular ages, cigarette smoking caused death in this population.
Trends over time: successive birth cohorts
19th and 20th century births
In , the relations between smoking and mortality at ages 60 and over are shown separately for the cohort of men born late in the 19th century and for the cohort born early in the 20th century. (Restriction to ages 60 and over is necessary because the study began in 1951, so for those born in the 19th century it provides little information on mortality at earlier ages.)
Relative risks of smokers versus non-smokers by century of birth: overall mortality among men aged 60 and over. In each century of birth (19th or 20th), relative risks are standardised indirectly for age and for study year (1951-2001)
Cigarette smoking had become common among young men in Britain by the end of the first world war (1914-1918) and remained so for half a century,18
reinforced by the issue of low cost cigarettes to young military conscripts from 1939. As a result, men who were born in the first, second, and, particularly, the third decade of the 20th century and were still smoking cigarettes after the age of 60 had been smoking substantial numbers throughout adult life. This would not have been as much the case for men who were born in the last decades of the 19th century, even if they too were smoking cigarettes after the age of 60.
The smoker versus non-smoker relative risks in are, therefore, much more adverse for smokers born in the 20th century than for those born earlier, particularly for cigarette smokers. For the cohort born in the 19th century the cigarette smoker versus non-smoker relative risk when they were over 60 years of age was only 1.46 (excess mortality 46%), but for the cohort born in the 20th century it was 2.19 (excess mortality 119%). For heavy cigarette smokers (25 or more a day at their last reply) the smoker versus non-smoker relative risk was 1.83 for the cohort born in the 19th century and 2.61 for the cohort born in the 20th century, corresponding, respectively, to 83% and 161% excess mortality among the smokers.
The patterns of survival from age 60 in these two birth cohorts among lifelong non-smokers and among those who smoked only cigarettes since 1951 are shown in , which gives the proportions of 60 year olds who would be expected to be still alive at ages 70, 80, 90, and 100. The substantial improvement among non-smokers is apparent, as is the lack of substantial improvement among smokers, corresponding to the increasing effects, from one century of birth to the next, of cigarette smoking on overall mortality. The comparisons in and involve only about a 20 year difference between the times when the mortality rates at a given age are being compared—for, although the median years of birth for all men in the two cohorts differed by 26 years, the median years of birth of those who died differed less.
Survival from age 60 for continuing cigarette smokers and lifelong non-smokers among UK male doctors born 1851-1899 (median 1889) and 1900-1930 (median 1915), with percentages alive at each decade of age
Subdivision of the younger participants by decade of birth (1900-1909, 1910-1919, or 1920-1929; only one was born in 1930) shows that, even among those born in the 20th century, the hazards associated with cigarette smoking differ substantially from one birth cohort to another (). For those born in the first decade of the century the difference between cigarette smokers and non-smokers in the probability of surviving from age 35 to age 70 was only 18% (58% ν 76% (corresponding to a twofold death rate ratio)), but it was 28% (57% ν 85% (a threefold death rate ratio)) for those born in the third decade. This comparison again involves only a 20 year time difference.
Survival from age 35 for continuing cigarette smokers and lifelong non-smokers among UK male doctors born 1900-1909, 1910-1919, and 1920-1929, with percentages alive at each decade of age
Trends over time: successive study decades
If we compare mortality at ages 70-89 during the five separate decades of the study (1951-2001) then the effects of a 40 year time difference (1950s ν 1990s) can be studied, as the ages of the original respondents varied from only 21 to 100. Some 2000 men were already aged 70-89 at the start of the study, and some 4000 of the younger respondents eventually survived to reach their 70th birthday during the fifth decade of the study. Thus, in each study decade we can observe the death rates of men in their 70s and 80s and can calculate the probability that, at the age specific death rates prevailing in that particular decade, a 70 year old man would survive to age 90 ().
Trends during 1951-2001 in overall mortality at ages 70-89 among lifelong non-smokers and continuing cigarette smokers (men born November 1861-1930 who survived to age 70)
Over the five decades there is both a progressive reduction in the mortality of elderly never smokers and, counterbalancing this, a progressive maturing of the epidemic of the effects of cigarette smoking on mortality in old age. shows a halving of the standardised mortality rate at ages 70-89 among non-smokers and almost a tripling of their probability of surviving from age 70 to age 90, which was 12% at the non-smoker death rates of the 1950s and 33% at those of the 1990s. (The mean years of birth for those who died at ages 70-89 in the 1950s and the 1990s were, respectively, 1875 and 1915.)
These reductions in mortality of the lifelong non-smokers were presumably due both to prevention of, and, particularly, to improvements in the treatment of, various diseases in elderly people. (For example, one of the present authors who is a participant in the study acquired a pacemaker 10 years ago.) But, among successive birth cohorts of cigarette smokers the increasing effects of cigarette smoking completely eliminated the great reductions in overall mortality at ages 70-89 that were occurring among non-smokers (). Among those born in about the 1870s and observed at ages 70-89 during the 1950s, the cigarette smoker versus non-smoker death rate ratio was only 1.16 (92.9/80.1), whereas for those born in about the 1910s and observed at ages 70-89 during the 1990s the death rate ratio was 2.83 (113.1/39.9).
Hazards among cigarette smokers born 1900-1930
By decade of birth
For those born in 1900-1909, annual mortality among non-smokers was, both in middle and in old age, about half that among cigarette smokers. (These twofold death rate ratios are calculated from the logarithms of the probabilities of surviving from 35-70, 70-80, and 80-90 in (top graph).) Taking, as before, the excess overall mortality among these smokers as an approximate measure of the excess mortality actually caused by smoking, this twofold ratio indicates that about half of the persistent cigarette smokers born in 1900-1909 would eventually be killed by their habit.
For those born in 1920-1929 the probability of death in middle age (35-69) was 15% in non-smokers and 43% in cigarette smokers, corresponding to a threefold death rate ratio (calculated from the logarithms of the survival probabilities in (bottom graph)). Extrapolation of the trends in suggests that these men will also have about a threefold smoker versus non-smoker death rate ratio in old age (70-89). This indicates that about two thirds of the persistent cigarette smokers born in the 1920s would eventually be killed by their habit.
Over whole 30 year period
averages the findings in for all men born in 1900-1930, distinguishing between lifelong non-smokers and continuing cigarette smokers. (Among the latter, the median age when they began smoking was 18, and at the start of the study their median age was 36 and their mean self reported cigarette consumption was 18 a day.) The results suggest a shift of about 10 years between the overall survival patterns of the continuing cigarette smokers and the lifelong non-smokers in this particular generation. That is not to say that all such smokers died about 10 years earlier than they would otherwise have done: some were not killed by their habit, but about half were, thereby losing on average more than 10 years of non-smoker life expectancy. Indeed, some of those killed by tobacco must have lost a few decades of life.
Survival from age 35 for continuing cigarette smokers and lifelong non-smokers among UK male doctors born 1900-1930, with percentages alive at each decade of age
Mortality on stopping smoking
A high proportion of the doctors who had been smoking in 1951 stopped during the first decades of this study, after it had become generally accepted by the British medical profession that cigarette smoking was a cause of most of the UK mortality from lung cancer.25,26
We can, therefore, examine mortality for some decades after stopping smoking (). As the benefits of stopping cigarette smoking can be assessed directly only in a population where the hazards of continuing to smoke cigarettes are already substantial, our main analyses of cessation are again restricted to the men born in 1900-1930. Among them, those who stopped and those who continued smoking differed little in obesity and blood pressure and differed only moderately in mean alcohol consumption ().
Overall mortality among never smokers, ex-smokers, and continuing cigarette smokers in relation to stopping smoking at ages 35-64 (men born 1900-1930 and observed during 1951-2001)
But, although many stopped when still relatively young and healthy during the 1950s and '60s, some who stopped in later middle age did so because they were already ill (). This removal of some imminent deaths of smokers from the current to the ex-smoker category reduces the apparent mortality among current smokers and may substantially inflate the apparent mortality of recent ex-smokers. For example, mortality at ages 55-64 among those who stopped smoking at ages 55-64 was spuriously somewhat greater than mortality among continuing smokers in that age group. Although the death rates for ex-smokers in are given only for the age ranges after the range in which smoking stopped, they too may still be somewhat affected by such reverse causality (see above for definition).
Mortality by age stopped smoking
Nevertheless, shows a steady trend in mortality at older ages (65-74 or 75-84) between lifelong non-smokers, ex-cigarette smokers who stopped at ages 35-44, 45-54, or 55-64, and continuing cigarette smokers. Thus, stopping earlier is associated with greater benefit.
Mortality for ex-smokers is shown in , which (bearing in mind the possible relevance of reverse causality) indicates that even a 60 year old cigarette smoker could gain at least three years of life expectancy by stopping ( (bottom graph)). In , the mortality rates that would be seen at ages 60-64 among those who stop at about age 60 had to be interpolated (half way between the smoker and non-smoker rates), as they could not be assessed directly (partly because of reverse causality and partly because in this study there was often a delay of a few years before the next questionnaire arrived and cessation could be reported). This, however, makes little difference to the long term survival differences between continuing and ex-cigarette smokers.
Effects on survival of stopping smoking cigarettes at age 25-34 (effect from age 35), age 35-44 (effect from age 40), age 45-54 (effect from age 50), and age 55-64 (effect from age 60)
Those who stopped at about age 50 gained about six years of life expectancy; those who stopped at about age 40 gained about nine years; and those who stopped before middle age gained about 10 years and had a pattern of survival similar to that of men who had never smoked ( (top graph)).
The mean cigarette consumption in 1951 (or when last smoked, if this was before 1951) was similar to that in continuing smokers for those who stopped at 45-64 and was only slightly lower for those who stopped at 25-34 or 35-44. Thus, the latter had, on average, had substantial exposure to cigarette smoking for about 20 years before giving up the habit, yet they still avoided most of the excess mortality that they would have suffered if they had continued to smoke.
Lung cancer mortality
describes the age specific mortality just from lung cancer among the same non-smokers, ex-smokers, and current cigarette smokers, comparing the observed numbers with the numbers that would have been expected if they had had the age specific lung cancer death rates of lifelong non-smokers. For statistical stability, these lung cancer death rates for non-smokers are taken from a much larger prospective study, which began in 1981 in the United States (see footnote to ).19
At these US rates, 19.5 lung cancer deaths at ages 35-84 would have been expected among the lifelong non-smokers in the present study, and 18 were observed, which is a reasonably good match (mortality ratio 0.9). For the current cigarette smokers the corresponding mortality ratio was 15.9 (13.7 expected and 218 observed).
Table 6 Mortality from lung cancer among never smokers, ex-smokers, and continuing cigarette smokers, in relation to stopping smoking at ages 35-64 (men born 1900-1930 and observed 1951-2001), compared with that expected at death rates for US male non-smokers (more ...)
There was a steady trend in this lung cancer mortality ratio between lifelong non-smokers, ex-smokers who had stopped at 25-34 (see footnote), 35-44, 45-54, or 55-64, and continuing smokers. This trend confirms the findings from case-control studies27
that there is substantial protection even for those who stop at 55-64, and progressively greater protection for those who stop earlier. None the less, those who had smoked until about 40 years of age before they stopped had some excess risk of lung cancer at older ages.
By combining the penultimate columns of tables and , we calculated the annual lung cancer mortality rates per 1000 continuing cigarette smokers at ages 45-54 through to 75-84 to be 0.6 (that is, 8.5×33/487), 1.8, 6.2 and 8.7 respectively. At ages 45-64 these rates are somewhat lower than in the general UK population born around 1915, which includes a mixture of non-smokers, ex-smokers, and current smokers, and at ages 65-84 they are similar. Hence, the lung cancer death rates among male cigarette smokers must have been even greater in the general UK population than in this study.