I am suprised no one has posted this here yet so here it is. http://www.sciencedaily.com/releases/2006/05/060526083353.htm The article was also featured in the Washington Post. Basically it says that they couldn't link lung, head, or neck cancer to marijuana, and that THC may actually prevent lung cancer.
I don't doubt that THC may work as a cancer preventative in some cases. But it seems that if you smoke it to get the THC you are shooting yourself inthe foot somehow. You should see some of the crap I have coughed up from my lungs on many occasions. Just my opinion, your milage may vary....
Smoking MJ has a few benefits, i have also hard it helps to prevent some diseases like parkansins (not sure on spelling) and alzheimers. but it is also a proven fact that smoking ANY dried plant is harmful for your lungs. there are still significant amounts of tar and other things in it, they may not cause cancer, but it also doesnt mean its healthy.
reefer dilates your bronchial passages - i smoke cigs, too, and i find that if i only smoke cigs without indulging in my reefer ritual my lungs can get really tight...but as soon as i do a few bong hits they loosen up and i cough up the cigarette crud i'm sure the deeper breathing involved in hitting a bong is part of it, but it still dilates your bronchii either way
Smoking any burning plant matter will not have a positive effect on your lungs. Vapourising, however, is thought to be far safer. As for the report; bear in mind it is now a year old, and no conclusive evidence has yet been found... Maybe it's true, maybe it isn't, but personally, I think it's far too early to be complacent.
Smoke is ash. Plain and simple. The only safer way is a vape. Other than that, you'll always be doing damage to your lungs.