I've read that even though there is heavy tar in marijuana smoke, it doesn't create a significant decline in respiratory function, even with regular use. I can vouche for this in that I cannot run for jack shit if I'm currently smoking cigarettes..but in the times that I have quit smoking cigarettes and remained smoking marijuana, I had no problems whatsoever with breathing difficulty after running/physical activity. On a separate note, I've read several different things about the way marijuana's beneficial for asthma patients because of the small tiny tiny airways inside your lungs that something from it opens..and in fact smoke before I go skateboarding because of the way that it enables me to not tire quickly and keep my breath up without having to breathe heavily and all crazy. Marijuana is one of the most mysterious things..in the way that it affects us without bringing risks to our health and without adverse side effects. :stoned: