The FACT that marijuana increases the diameter of bronchi in a manner unlike that of standard anti asthmatic agents, made its further investigation desirable. If you search for studies about the correlation between cannibals use and lung cancer, the studies show that its not harmful to the human body. That being said, there is an extremely high correlation between smoking and cancer, but tobacco only, not marijuana. So no, we cabt 'prove' that smoking causes lung cancer, but we can observe that you are much more likely to get lung cancer if you are a tobacco smoker.