cannabis does not negativey effect the lungs at all. tobacco bunches up your lungs and kills the air sacs in your lungs called aveoli. cannabis contains thc which open up the lungs, which is why its good for asthma, and cannabis smoke stays in your major air ways in your lungs unlike tobacco which goes deep into your lungs. ive been smoking for a while now and im more physically fit than i was before i started. i also know alot of jocks who smoke weed. if your worried about it, id invest in a vaporizer