The exposure assessment was based on self-report, which may have caused some misclassification. However, this was the best assessment method available, as we hypothesized that long-term exposure is of biological relevance in our study.37 Currently there are no biomarkers for SHS exposure that would measure long-term exposure. In a study of Coultas et al.38 repeated self-reports of the duration (years) of exposure to SHS were reliable, while the reports on the amount of cigarettes smoked were somewhat less reliable. Another study showed a good agreement between the subjects’ own questionnaire reports on SHS exposure and surrogate reports of most exposure measures.39 The questionnaire allows the exposure assessment on several periods of life retrospectively when it is no longer possible to monitor the exposure with other methods.