 There is unequivocal evidence that habitual or regular marijuana smoking is not harmless and causes respiratory symptoms and airway inflammation. If you take biopsies from the airways of those who smoke crack, cannabis, or tobacco, compared to non-smokers, there was significantly more damage in the lungs of crack smokers, marijuana smokers, and tobacco smokers. And the levels of damage seems comparable, especially between the marijuana smokers and tobacco smokers, which is remarkable, since the tobacco smokers were smoking about a pack a day, whereas the marijuana smokers were only smoking about 20 joints a week rather than 25 cigarettes a day. And those smoking crack were just doing it like a grammar to a week. So to see similar rates of damage between marijuana smokers and cigarette smokers suggests each joint is way worse than each cigarette. And indeed, we've known for 30 years that smoking three or four joints is the equivalent of smoking a pack a day of cigarettes in terms of bronchitis symptoms and acute lung damage. How is that possible? Well, it may be the way they're smoked. Pot smokers inhale more deeply and then hold the smoke in four times longer, resulting in more tarred deposition in the lungs. And joints are more loosely packed and unfiltered, resulting in both hotter smoke and smokier smoke. And so even though in many ways smoke is smoke, the different method of smoking may explain how a few joints a day appear to cause as much inflammation as an entire pack a day of cigarettes. The visual evidence of airway injury was at times striking. This is what your airways are supposed to look like. The tubes inside your lungs. This is your lung. This is your lung on tobacco. See how your airways get all inflamed? And this is your lung on pot. You get the same kind of inflammation. And what's crazy is that's just five joints a day compared to 26 cigarettes a day in the tobacco smokers. If you compare the respiratory symptoms associated with marijuana versus tobacco, compared to non-smokers, both marijuana smokers and tobacco smokers have elevated rates of chronic cough and excess sputum production, acute episodes of bronchitis, and wheezing. Now when you quit tobacco, these respiratory symptoms eventually go away. Does the same thing happen with marijuana? What are the effects of quitting cannabis on respiratory symptoms? About 30 to 40% of regular cannabis users suffer from cough, excess sputum, wheezing, and shortness of breath. A thousand young adults were followed for years, and those who kept smoking their respiratory symptoms got worse or remained the same, but those that quit tended to get better. If we don't quit, what are the long-term lung consequences? What about COPD, chronic obstructive pulmonary diseases like emphysema? Even if smoking a single joint compromises lung function as much as sub-to-5 cigarettes, you're still smoking 15 times less overall, and so should end up with less long-term lung damage, right? That's indeed what's been found. Even long-term pot smokers don't appear to suffer lasting lung damage. Followed people for 20 years. An occasional joint appears to have no discernible effect on long-term lung function, though maybe some accelerated decline in function among those smoking joints every day for decades, and so marijuana moderation is suggested. In other words, a caution against regular heavy marijuana usage is prudent, but even regular heavy use of marijuana is nothing compared with the grave pulmonary consequences of tobacco. Any toxicity of marijuana pales when compared with the greatest legalized killer in the world today. In fact, the greatest risk to our lungs from marijuana may be that it can be a gateway drug to cigarettes.