That's fair, I suppose. Still, even in a clean-air environment, studies from the UK suggest the maximum potential risk of COPD from heavy cannabis use to be no greater than 4.5%, when used alone. Your patients may be non-smokers, but do they live with smokers? Did they frequent night clubs in their youth? Have they attended a bonfire and stood downwind? Have they driven a car older than 1980? Have they been stuck in a traffic jam? Any of these things will cause greater exposure to more harmful substances than the inhalation of cannabis.