The study comes out of Northwestern University, where assistant professor of medicine and preventive medicine Ravi Kalhan studied surveys from 5,115 men and women over a 20 year period. The people involved answered questions relating to their health habits and also occasionally took tests to measure pulmonary function.
According to the analysis, exposure to marijuana is nearly as common as exposure to tobacco, except that most of the exposure was very light, with a median lighting up of two to three times per month. And at those low exposure levels, the research shoed absolutely no negative effects regarding to lung capacity. This stands in stark contrast to cigarettes, where studies have shown even light amounts of smoking (a few per day) greatly decreases one’s pulmonary health.
Note, however, that I couldn’t find any studies about just a few cigarettes per month. This may be because cigarettes have a lower mental effect and it’s possible to smoke 20 per day, or it could be because tobacco is (debatably) more addictive than marijuana. Also, the study didn’t look at any other health affects other than respiration.
But still, the occasional joint won’t hurt your lungs.