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The article of Bhatt et al addresses an important topic (1). The authors assessed the relative contribution of intensity and duration of tobacco smoke exposure to the development of chronic obstructive pulmonary disease (COPD). They concluded that smoking duration alone provides stronger risk than the composite index of pack years. In other words, the effect of long and low intensity exposure has a stronger association with COPD than short exposures of high intensities. The article of Marks consents this finding, concluding that pack years are a suboptimal index of exposure (2).
A major limitation of the study of Bhatt, which surprisingly is not stated as such, is the use of a cross-sectional design that does not allow drawing causal conclusions. The conclusions drawn therefore might be flawed.
Selection bias due the healthy ‘survivor’ effect might have occurred. The duration of smoking could have been influenced by the deleterious effects a person experiences from the exposure to smoke. Those with a long smoke duration are more likely not to experience (or experience less) health issues due to smoking, and might therefore have less severe (or no) COPD than those with a short smoke duration. In line with this, selective ‘drop-out’ of the more diseased persons may have biased the results.
Furthermore, the authors use retrospective data, while this often leads to recall bias. Participants often do not precisely remember the numbers of cigarettes smoked...
Furthermore, the authors use retrospective data, while this often leads to recall bias. Participants often do not precisely remember the numbers of cigarettes smoked per day in a specific year or time period. Assuming that participants underestimate smoke intensity, it is likely that the association between smoke intensity and COPD is less strong than the association between smoke duration and COPD. This weaker association is then not due to the relative contribution of smoke intensity compared to that of duration, but to recall bias.
To conclude, the statements made in the paper seem to be very convincing, but are based on research with methodological limitations. Prospective studies assessing the relative contribution of smoke intensity and smoke duration are thus urgently needed.
1. Bhatt SP, Kim YI, Harrington KF, Hokanson JE, Lutz SM, Cho MH, et al. Smoking duration alone provides stronger risk estimates of chronic obstructive pulmonary disease than pack-years. Thorax. 2018.
2. Marks GB. Guiding policy to reduce the burden of COPD: the role of epidemiological research. Thorax. 2018.