3.0 Background

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3.0 Background

3.1 Tobacco—a leading preventable cause of death and disease

3.2 Cardiovascular diseases

3.3 Lung cancer

3.4 Respiratory diseases

3.5 Other cancers caused by or associated with smoking

3.6 Reproductive health and smoking

3.7 Pregnancy and smoking

3.8 Infant health and smoking

3.9 Increased susceptibility to infection in smokers

3.10 Eye diseases

3.11 Dental diseases

3.12 Gastrointestinal diseases

3.13 Bone density and risk of fractures

3.14 Effects of smoking on the skin

3.15 Smoking and complications in medical treatment

3.16 Smoking and diabetes

3.17 Poorer levels of general health

3.18 Smoking, motor vehicle crashes and other injuries

3.19 Burns and fires caused by tobacco use

3.20 Tobacco poisoning

3.21 Health effects for younger smokers

3.22 Genetic influences on tobacco-caused disease

3.23 Smoking, dementia and cognitive decline

3.24 Lung disease, smoking and occupational exposures

3.25 Air pollution, cigarette smoking and ill health

3.26 Health effects of smoking brands that claim to deliver lower levels of tar, nicotine and carbon monoxide

3.27 Health effects of smoking tobacco in other forms

3.28 Health 'benefits' of smoking?

3.29 Morbidity (ill health) attributable to tobacco-caused disease

3.30 Deaths attributable to tobacco by disease category

3.31 Morbidity and mortality due to tobacco-caused disease and socioeconomic disadvantage

3.32 Health effects of smoking other substances

3.33 Health effects of chewing tobacco, and of other smokeless tobacco products

3.34 Public perceptions of tobacco as a drug, and knowledge and beliefs about the health consequences of smoking

3.35 Health benefits of cessation

3.36 The global tobacco pandemic

Acknowledgments

References

Smoking of tobacco as we know it today, in the form of manufactured or 'tailor-made' cigarettes, became common in Australia in the late 1800s. Pipe and cigar smoking was already widespread among men, but the convenience and ready availability of the cigarette soon made it a popular alternative.1 Manufactured cigarettes were supplied to Australians and their allies in the trenches of World War I,1 and by the end of World War II, nearly three quarters of Australian men and one quarter of women were smokers, the majority using cigarettes (see also Chapter 1, Section 1.1).

Similar changes in smoking behaviour had occurred in Western Europe and North America, and with them, a marked escalation in lung cancer death rates and the growing suspicion that cigarette use was implicated in this trend. By 1950 several studies had been published in the medical literature2 and the finding that cigarette smoking and lung cancer appeared to be causally linked was reported.3, 4

Several series of authoritative, landmark reports have since been published by national and international agencies,[1] documenting the damaging effects of smoking and calling for action to help halt the smoking epidemic. Of these, the most regular series has been that issued by the Office of the US Surgeon General. Since 1964, 28 comprehensive and rigorous reports on various aspects of tobacco and health have been issued by the US Surgeon General, repeating the conclusion that smoking is 'the single greatest cause of avoidable morbidity and mortality in the United States,' and never finding reason to reverse any earlier conclusions of causality.5

The 2004 report of the US Surgeon General, The Health Consequences of Smoking, published the following four major conclusions:5

1. Smoking harms nearly every organ of the body, causing many diseases and reducing the health of smokers in general.
2. Quitting smoking has immediate as well as long-term benefits, reducing risks for diseases caused by smoking and improving health in general.
3. Smoking cigarettes with lower machine-measured yields of tar and nicotine provides no clear benefit to health.
4. The list of diseases caused by smoking has been expanded to include abdominal aortic aneurysm, acute myeloid leukaemia, cataract, cervical cancer, kidney cancer, pancreatic cancer, pneumonia, periodontitis and stomach cancer.

3.0.1 Defining causality

The US Surgeon General's reports have provided a detailed review of definitions of causality of disease, and how measures of causality may be applied. Causality is determined by evaluating the range of available evidence and considering it against well-established criteria. The more that an observed association fulfils the criteria, the more likely it is that a causal relationship can be inferred. These criteria are outlined in the US Surgeon General's Report for 2004:5

Consistency: This refers to the persistence of the finding of an association between exposure and outcome in a number of methodologically valid studies undertaken in a range of settings. This helps ensure that possible confounding effects are eliminated, and also increases the statistical validity of the finding through the accumulation of additional evidence.

Strength of association: Strength refers both to magnitude of the association, and to its statistical strength. The greater the measured association and the more sound its statistical basis, the less likely it is that the findings are influenced by chance, bias, or unmeasured or poorly controlled confounding factors. However the observed association must also have a plausible basis in understood biological processes.

Specificity: Specificity refers to 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. For example, not all smokers develop lung cancer, and not all cases of lung cancer are caused by smoking. However the extremely high relative risk for lung cancer in smokers, and the high percentage of lung cancers attributable to smoking, gives the association between smoking and lung cancer 'a high degree of specificity'.

Temporality: Exposure to the causative factor must precede the onset of the disease. Considered alone, temporality is a poor predictor of causality, but no association can be considered to fulfil the criteria for causality if temporality is not satisfied.

Coherence, Plausibility and Analogy: Taken together, these three criteria require that the proposed causal relationship must not defy known scientific principles, and that it must be biologically plausible and consistent with experimentally demonstrated biological mechanisms and other relevant patterns.

Biologic gradient (dose-sesponse): This criterion refers to the observation of increased effect (for example incidence of disease) in response to increased dose (heavier and/or longer duration of smoking). Meeting this criterion forms a strong support for causality, except in the unlikely event that there is an unidentified confounder, which happens to be varying in the same manner as the observed dose and which could account for the measured association. Virtually all health outcomes causally linked to smoking have demonstrated a dose-response relationship of some description.

Experiment: This criterion refers to naturally occurring 'experiments' which might be considered to imitate the conditions of a properly conducted experiment in a scientific environment, and whose outcomes might have the force of a true experiment. An example of a 'natural experiment' in the smoking arena is assessing the health consequences of quitting smoking. To attribute observed improvements in health outcomes to factors other than smoking cessation would necessitate identifying alternative influences and demonstrating that those who continued smoking had also attained a health benefit where that alternative influence was present.

The more closely an association fulfils the above criteria, the stronger its claim to causality. Not all inferences of causality will necessarily satisfy all criteria. For example, where biological mechanisms may not be completely understood, causality may still be justified by satisfaction of other criteria, such as consistency and strength of association. Those applying the criteria must weigh all of the scientific evidence and make a multidisciplinary judgement.5