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1 Medical Research Institute of New Zealand, and 2 Wellington School of Medicine & Health Sciences, Wellington, New Zealand. 3 University of Southampton, Southampton, UK.
To the Editors:
Chronic obstructive pulmonary disease (COPD) is rapidly becoming a global public health crisis, with around three million people predicted to die from the disease in 2006 1, 2. There is growing awareness of the increasing global burden of COPD not only in terms of mortality, but also its prevalence, morbidity and economic costs 2. In response to these concerns, there has been a renewed research effort to understand the causes of COPD and to develop public health and pharmacological interventions that may reduce the burden.
Smoking is recognised as the most important causative factor for COPD, with an individual's susceptibility being a continuous, rather than a categorical characteristic that can interact synergistically with other risk factors. There is now evidence that most smokers develop some respiratory impairment due to COPD 3. One of the most informative recent studies addressing this issue is the Obstructive Lung Disease in North Sweden (OLIN) study 4. From this cohort study, it was reported that 
50% of smokers eventually develop COPD, as defined according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines 2. This finding is of major clinical significance, in that it provides a scientific basis for the advice that can now be given to smokers that if they continue smoking lifelong, they have at least a one in two chance of developing COPD. Importantly, the study also provides evidence that the risk of developing COPD falls by about half with smoking cessation.
The OLIN study also allowed determination of the proportion of COPD cases that is due to smoking 4. Utilising the GOLD criteria for the diagnosis of COPD, the population attributable risk of COPD for smoking was 45% (95% confidence interval (CI) 29–58%) in the 46–77-yr age group. Utilising the British Thoracic Society criteria 5, which are considerably more stringent than the GOLD criteria for the diagnosis of COPD, the population attributable risk was 50% (95% CI 28–67%). This population attributable risk is considerably less than current dogma suggests, with a figure of 80–90% commonly quoted 6–8. A comparable population attributable risk of 44% (95% CI 38–50%) can be calculated from the third National Health and Nutrition Examination Survey (NHANES III) 9. However, this figure is qualified by the lack of post-bronchodilator lung function measurements in the NHANES III, which means that the COPD criteria employed were more inclusive of cases with asthma.
If only about half of all cases of COPD are due to smoking, what factors are responsible for the other half and what can be done to reduce their impact? It is not widely recognised that up to 20% of cases of COPD worldwide can be attributed to indoor air pollution from exposure to smoke from cooking and heating with biomass fuels in poorly ventilated dwellings 10, 11. This is likely to explain the similar prevalence of COPD in females and males in developing countries, despite markedly lower rates of smoking in females 2. Greater recognition of the role of indoor air pollution is a necessary prerequisite for the development of strategies to lessen its impact, and to reduce the occurrence of COPD.
A comparable example, which is amenable to public health and government interventions, is workplace exposure to a wide variety of dusts, chemicals and fumes. Given that up to 20% of cases of COPD may be attributed to occupational exposures 12, the potential benefit of preventive measures are apparent. As with indoor air pollution, the greatest burden of COPD due to occupational exposures is in the developing world. Other risk factors that could potentially be modified at an individual or population level are outdoor air pollution, marijuana smoking, chronic respiratory infection, diet, childhood asthma and factors associated with lung growth in utero and infancy.
We suggest that a greater understanding of the causation of chronic obstructive pulmonary disease is a research priority to provide the basis for the development of primary and secondary intervention programmes. This recommendation does not lessen the significance of tobacco smoking as the most important cause of chronic obstructive pulmonary disease, or the public health importance of measures to reduce tobacco consumption internationally. Smoking remains an important cause of global mortality, being accountable for around five million premature deaths in the world every year, including about one million due to chronic obstructive pulmonary disease 13.
REFERENCES
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  J. Vestbo COPD, diagrams and traditions: time to move on? Thorax, September 1, 2008; 63(9): 755 - 756. [Full Text] [PDF]
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