The number of published studies on passive smoking as a risk factor for adult onset asthma and chronic obstructive pulmonary disease (COPD) is small compared with the number on the adverse health effects of passive smoking on childhood respiratory symptoms and diseases. The paucity of research among adults may partly be due to a number of factors that make it difficult to design studies of these non-malignant respiratory diseases. The potential for misclassification of smoking status, with former or current smokers categorised as passive smokers, has been a longstanding concern in studies that rely on self reports of past smoking habits. Measuring past passive smoke exposure presents a major challenge in studies of chronic diseases that may become clinically apparent only after 20 or more years of exposure. Over-reporting of symptoms that subjects attribute to passive smoking is increasingly likely as public awareness of passive smoking increases. Also, it is difficult to measure lifetime exposure to the number of other confounding agents that are risk factors, which must be controlled for in studies of passive smoking.

While causation of asthma and COPD from passive smoking may not be directly demonstrable, it is possible to infer causal relationships from the concordance of scientific evidence, and Hill’s nine criteria for causal association provide a useful guide for evaluating available evidence.1 The nine criteria include strength of association, consistency, specificity, temporality, dose-response, plausibility, coherence, experimental evidence, and analogy. Of these criteria, plausibility, coherence, and analogy are fulfilled in relation to COPD by the established association of active cigarette smoking with chronic airflow obstruction. However, the criteria of specificity and experimental evidence have little relevance for human diseases associated with cigarette smoking.2 Of the remaining criteria, strength of association, consistency, temporality, and dose-response have the greatest relevance for evaluating the evidence on …