Background Cross-sectional reports have suggested that, among active smokers, previous exposure to parental smoking may increase susceptibility to development of chronic obstructive pulmonary disease. We assessed prospectively whether parental smoking enhances the effects of active smoking on early deficits of lung function in young adults.

Methods We used data from the prospective birth cohort, the Tucson Children's Respiratory Study. Maternal and paternal smoking was assessed via questionnaires completed by the parents at the time of the participant's birth. Active smoking by participants was assessed via personal questionnaires completed at ages 16 (YR16), 22 and 26 years. Four groups were generated based on the combination of parental and active smoking. Lung function parameters, including forced expiratory volume in 1 s (FEV₁)/forced vital capacity (FVC) ratio, were assessed by spirometry before and after inhalation of 180 μg of albuterol at YR11, YR16, YR22 and YR26.

Results Complete data were available for 519 participants. Pre-bronchodilator FEV₁/FVC values did not differ at YR11, YR16 or YR22 by parental or active smoking. However, at YR26 participants with exposure to parental and active smoking had pre-bronchodilator FEV₁/FVC levels that were, on average, 2.8% (0.9% to 4.8%; p=0.003) lower than participants who were not exposed to parental or active smoking. In contrast, subjects who were only exposed to active smoking or only exposed to parental smoking did not differ from those who were not exposed to either. Between YR11 and YR26, participants with exposure to parental and active smoking had the steepest decline in sex, age and height adjusted residuals of FEV₁/FVC, FEV₁, forced expiratory flow between 25% and 75% of the FVC (FEF_25–75) and FEF_25–75/FVC (all p values between 0.03 and <0.001).

Conclusions Parental and active smoking act synergistically to affect early lung function deficits in young adulthood.