Pretty interesting findings here:
"After 20 years of testing, researchers found some buzzworthy results: regular marijuana smokers (defined by up to a joint a day for seven years) had no discernable impairment in lung activity from non-smokers. In fact, researchers were surprised to find marijuana smokers performed slightly better than both smokers and non-smokers on the lung performance test. Why? The most likely explanation seems to be that the act of inhaling marijuana—holding each puff in for as long as possible—is a lot like a pulmonary function test, giving marijuana smokers an edge over their cigarette smoking counterparts."