Score another victory for the tokers in the audience. A study into the long-term effects of marijuana smoking on lung function has shown that it's not nearly as bad as many have assumed. Despite having many of the same constituents as tobacco smoke, the long-term effects of marijuana smoking on breathing is poorly understood, which this study has at least partly rectified.
The researchers analyzed information from the The Coronary Artery Risk Development in Young Adults (CARDIA) study, which gathered data over a period of 20 years. What they discovered is that low to moderate smoking lead to either no or an insignificant decline in the amount of air the patients could blow out in the first second of breathing. In this case, there was no significant change in anything less than 10 joint-years of use over a lifetime, some 3,650 spliffs. Perhaps unsurprisingly heavy users, those that imbibe more than 20 times a month, did suffer a significant decline.
So puff in moderation with impunity. Whatever it's doing to your body, it isn't destroying your ability to breathe.