First, it is unclear why the authors chose to examine two cohorts. The composition between the two cohorts differed greatly. The first cohort, which included a group of nonsmokers, passive smokers, and smokers, consisted of women with the average age ≈30 years. The second cohort, again including a group of nonsmokers, passive smokers, and smokers, consisted of both men and women with average ages of 35, 41, and 57 years, respectively. The question arises as to whether the observed difference between the cohorts (in passive smokers and smokers) is confounded by age and sex differences. Stone et al2 previously described a positive correlation between age and D/I excretion. To our best knowledge, there is no difference between men and women in this regard, although studies to date were conducted with small numbers of individuals and thus perhaps were underpowered.