Longitudinal analysis of the effects of smoking onset and cessation on pulmonary function

We examined the effects of smoking onset and cessation on FEV₁ in participants in the Tucson Epidemiological Study of Airways Obstructive Disease 18 yr of age or older who reported a change in smoking habits during 17 yr of follow-up. Subjects were classified as 'new quitters' (n = 288) and 'new starters' (n = 45) at each survey on the basis of questionnaire responses concerning current smoking habits. The pulmonary function data were analyzed using a mixed longitudinal or random effects model (REM). We compared FEV₁ before and after changing smoking habits in the same subjects while adjusting for important covariables such as height, pack-years, and respiratory symptoms and diseases. The results for smoking cessation showed a beneficial effect related to quitting that was largest for younger subjects and decreased linearly with age. For women, quitting resulted in an improvement in FEV₁ of 4.3% at 20 yr of age which decreased to 2.5% by 80 yr of age. Men had an improvement of only 1.2% at 20 yr of age and no improvement at 80 yr of age. After excluding subjects with low initial function (FEV₁/FVC in the lowest quartile) and those with only a single observation after quitting, men (n = 70) showed a significantly higher FEV₁ improvement of 4% at 20 yr of age that decreased linearly to zero by age 80 yr. In contrast, women in this subgroup (n = 89) had nearly the same degree of improvement at all ages. The onset results showed mean FEV₁ values after starting smoking that were higher at the younger ages (<23 yr) and that decreased more rapidly with age, compared with nonsmoking mean estimates, in both sexes.