Over 57,000 reports have examined the connection between cigarette smoking and disease.(1) According to the US Surgeon General, 'It is safe to say that smoking represents the most extensively documented cause of disease ever investigated in the history of biomedical research.'(2)
The causal relationship between smoking and human disease cannot be tested directly in a laboratory setting: exposing human subjects to cancer and other disease producing agents in order to establish causality is morally and ethically unacceptable.(3) Instead, other criteria have been developed to establish causality with a very high degree of scientific probability.
These criteria are:(3)
ð Consistency of the association -- the findings being replicated under a variety of conditions, in differing settings and with different study designs. This helps eliminate the effects of any confounding factors.
ð Strength of the association -- measured by the difference in relative risk ratios, incidence ratios or mortality ratios for a given disease between those exposed to the suspected disease causing agent and those not.
ð Specificity of the association -- the degree to which exposure to the suspected disease causing agent can predict outcome. Other biological and epidemiological factors may need to be taken into account.
ð Temporal relationship of the association -- exposure to the causative factor must precede the onset of disease.
ð Coherence of the association -- disease progression is 'biologically plausible'. Development of disease should be plausibly explained, for example by biochemical and physiological means and measures of exposure to the suspected disease causing agent, such as the observation of a dose-response relationship (the increased occurrence of disease in increasingly heavily-exposed groups).
Smoking is now identified as a major cause of heart disease, stroke, several different forms of cancer, and a wide variety of other health problems.(1) The vast majority of deaths caused by smoking occur through development of heart disease and lung cancer, followed by chronic bronchitis, stroke, peripheral vascular disease and other circulatory diseases, and cancers other than lung (Figure 3.1).(4) Around one-third of all cancer deaths in Australia can be attributed to smoking.(5) Cancers were the leading cause of death in Australian men and women in 1993, representing 26.9% of all deaths. Ischaemic heart disease accounted for 24.5% of all deaths, and cerebrovascular disease (stroke) accounted for a further 10% of deaths.(6)
Recent research has shown that the hazards of prolonged cigarette smoking are even greater than used to be supposed.(7,8) The chief source of previous underestimations has been the very long delay between cause and full effect. If cigarette smokers commence smoking as teenagers and do not quit, then eventually about half of them will be killed by tobacco, in middle or old age.(9,10) This excess burden of death falls particularly heavily upon the 45-64 year old age group. Smokers between these ages are three times more likely to die prematurely than lifelong non-smokers of the same age, and in the 65-84 age group, smokers are around twice as likely to die compared to lifelong non-smokers.(9)
Globally, tobacco is a major cause of death. According to recent estimates, in developed countries alone, tobacco was responsible for 1.7 million deaths in 1985, and will cause a projected 2.1 million deaths in 1995. For the decade 1990-1999, 21 million deaths will be caused in developed countries; more than half of these deaths will be among those aged between 35 and 69, making tobacco the largest single cause of premature death in these countries.(11) There is wider uncertainty about the numbers of deaths caused by smoking in less developed countries, but it will probably be in the vicinity of around one million deaths per year in 1995, making a global total of tobacco deaths of around three million for that year. In many less developed countries the smoking epidemic is only just emerging, and current high smoking rates point to a heavy burden of death and disease in the coming century. Worldwide annual mortality is estimated to exceed ten million sometime during the 2020s.(12)