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Reference population represents a minority

Distribution of smoking habits in general populationOnly a limited percentage of a population satisfies all criteria of ‘pulmonary health’, leaving only a small reference group. The populations of Vlaardingen and Vlagtwedde (the Netherlands), who took part in a longitudinal epidemiological study of lung health carried out by the TNO Working group Epidemiology of CNSLD in the 1960-1980s may serve as an example. Among adult men investigated by staff of the Physiology Department of Leiden University fewer than 13% of men and than 48% of women were lifelong non-smokers.
Distribution of respiratory symptoms in general populationAmong women, 70% had never experienced chronic respiratory symptoms, in men the corresponding percentage was 65%.
Distribution of smoking habits and respiratory symptoms in general populatioApplying both criteria, about 1 in 10 males were assigned to the reference group on the basis of respiratory symptoms and smoking habits; in women only one third fulfilled the combined set of criteria.
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