Intuition may suggest that smoke is bad for the lungs, whether it comes from a campfire or from tobacco or marijuana. AÂ practical question is: how bad is an occasional joint, compared with some background level of air pollution and the lungsâ€™ ability to cope?
Since a few states have been loosening restrictions on marijuana, a group ofÂ Emory pulmonologists â€“ Jordan Kempker, Eric Honig, and Greg Martin — decided to look at the long-term effects of marijuana smoking on lung function. Their findings, published in the Annals of the American Thoracic Society (PDF), have already attracted some attention.
The Emory researchers mined data from the 2007-2010 NHANES survey, a standard source for public health studies involving more than 5000 adults.
As expected, recent marijuana usage was associated with an increase in symptoms of bronchitis such as coughing and wheezing. Less than 20 joint-years (one joint-year is a joint per day for a year) of cumulative exposure was not linked with significant changes inÂ lung function, but more than 20 joint-years was. Within the study, current marijuana smokers reported an average joint-year exposure of 15.8 joint-years.
With tobacco smokers, the risk of pulmonary disease tends to appear at 20 pack-years, representing a lot more smoke exposure than 20 joint-years. However, the pattern of marijuana’sÂ effects on lung function lookÂ different.Â Whether the marijuana-associated changes in lung functionÂ doÂ lead to lung disease is not known, the researchers emphasize.
Obstructive lung disease can be detected by looking at the ratio between two numbers: theÂ volume of air a person can force out of his or her lungs (forced vital capacity, or FVC) and the volume of air forced out in the first second of the maneuver (forced expiratory volume, or FEV1). With tobacco, researchersÂ see a decrease in FEV1, but withÂ marijuana, theyÂ actually see an increase in FVC; both decrease the ratio FEV1/FVC.
The results do generally agree with previously publishedÂ studies from UCLA. However, findings based on self-reported NHANES data have been criticized because of the problem of recall bias. For example, people inaccurately report how much food they eat, and the same uncertainties may plausibly apply to marijuana usage as well.