Demonstration of a dose‐response relationship for environmental tobacco smoke (ETS) is an important indication of causality. Central to the analysis and interpretation of dose‐response relations as described in epidemiological studies is the relationship between dose and exposure. It must be recognized that in studies of ETS we have only surrogate measures of dose, and these surrogate measures (based on exposure) are imperfect. The question‐based measures of ETS exposure generally have not been standardized, may have limited validity and reliability, and cannot comprehensively describe total ETS exposure, exposure to individual ETS components, nor doses of biologically relevant agents at target sites. Nevertheless, useful data have been yielded in epidemiologic studies linking ETS exposure to increased respiratory infection and symptoms, reduced lung growth in children, and increased lung cancer in nonsmoking adults. The more consistent exposure‐response data for studies on acute health in children may reflect the greater difficulty in measuring exposure in studies of chronic health in adults.SUMMARYFor children, adverse effects have been causally associated with exposure to ETS. The epidemiological evidence and the supporting toxicological data link ETS to increased lower respiratory illness, increased respiratory symptoms, and reduced lung growth. Exposure‐response relationships have been described for these effect; errors in exposure estimates would tend to lessen rather than to exaggerate these relationships. By contrast, in adults, the evidence for adverse cardiopulmonary effects of ETS exposures, other than lung cancer, it's presently less conclusive. Misclassification of exposure may be more severe for adults than for children, and the resulting bias toward the null may obscure effects of respiratory symptoms and lung function. Research on adults is hindered by the difficulty of estimating exposures received in diverse environments over a lengthy period. Continued controversy related to the effects of ETS exposure in the workplace and in public locations provides a rationale for further research. However, more valid methods for assessing the exposures of adults to ETS are needed for such