People have been smoking marijuana for thousands of years, there have been no deaths attributed to it and if it caused any significant lung impairment you would think it would have been documented. Instead all the anti-marijuana propaganda claims that smoking marijuana may cause lung damage to the same if not worse extent than tobacco. If you read the links I posted on the subject you would see why this is not the case.

I believe one article I read about a marijuana/lung function study stated that marijuana smokers appeared to have 10-16% greater risk of developing lung infections such as pneumonia.
If you're a frequent or heavy smoker and you believe you have harmed your lungs or experience shortness of breath don't be so quick to attribute this soley to cannabis. Do you smoke tobacco products which would include blunt wrap, use soda cans as smoking devices, tin foil or plastics, have moderate-high exposure to second hand smoke, smoke low quality or uncured/otherwise harsh weed etc.
I have been smoking multiple times a day for the past six months or so, and still frequently about a year prior, I experience no pain in my lungs, am able to run without shortness of breath and have had no lung infections.
It should also be considered that legalization of cannabis means easy access to more potent bud and hash, this means a user would not be required to inhale much smoke to get high, less smoke means less potential for damage.