Given the inconclusive science on the long-term effects of marijuana exposure on lung function, the increasing tetrahydrocannabinol composition of marijuana over time and the increasing legal accessibility of the substance, continued investigation is needed.

Objectives: To determine the association between recent and long-term marijuana smoke exposure with spirometric parameters of lung function and symptoms of respiratory health in a large cohort of U.S. adults.

Methods: This is a cross-sectional study of U.S. adults who participated in the 2007-2008 and 2009-2010 National Health and Nutrition Examination Survey cycles, using the data from the standardized spirometry and survey questions performed during these years.

Measurements and Main Results: In the combined 2007-2010 cohorts, 59.1% had used marijuana at least once in their lifetime and 12.2% had used in the past month. For each additional day of marijuana use in the prior month, there were no associated changes in mean percent predicted FEV1 (0.002% ± 0.04%, P=0.9) but there was an associated increase in mean percent predicted FVC (0.13% ± 0.03%, P<0.01) and decrease in mean FEV1/FVC (-0.1% ± 0.04%, P<0.01). In multivariable regressions, 1-5 and 6-20 joint-years of marijuana use were not associated with an FEV1/FVC < 70% (OR 1.1, 95% confidence interval (CI) 0.7-1.6, P=0.8 and OR 1.2, 95% CI 0.8-1.8, p=0.4, respectively) while > 20 joint-years were associated with a FEV1/FVC <70% (OR 2.1, 95% CI 1.1-3.9, P=0.02). For each additional marijuana joint-year smoked, there was no associated change in mean percent predicted FEV1 (0.02% ± 0.02%, P=1.0), but there was an increase in mean percent predicted FVC (0.07% ± 0.02%, P<0.01) and a decrease in mean FEV1/FVC (-0.03% ± 0.01%, P=0.02).

Conclusions: In a large cross-section of U.S. adults, lifetime marijuana use up to 20 joint-years is not associated with adverse changes in spirometric measures of lung health. While > 20 joint-years of marijuana exposure was associated with a two-fold increased odds of a FEV1/FVC < 70%, this was the result of an increase in FVC rather than the disproportional decrease in FEV1 seen with obstructive lung diseases.