The left has always told us that marijuana smoke does no damage to the lungs and contains no carcinogens.
The truth is that it contains carcinogens, but has a net positive effect regarding lung cancer.
https://www.scientificamerican.com/article/large-study-finds-no-link/
The smoke from burning marijuana leaves contains several known carcinogens and the tar it creates contains 50 percent more of some of the chemicals linked to lung cancer than tobacco smoke. A marijuana cigarette also deposits four times as much of that tar as an equivalent tobacco one. Scientists were therefore surprised to learn that a study of more than 2,000 people found no increase in the risk of developing lung cancer for marijuana smokers.(I say net positive effect since the study also found that those who smoked both pot and cigarettes had a lower lung cancer probability than those who only smoked cigarettes.)
Just like any smoke, it does some damage to the lungs. Evidently not too much if someone like Willy (who was also a heavy cigarette smoker) makes it to the ripe old age of 86 with no serious problems. Moderation also goes a long way, I'm sure.
Regardless, compared to cigarette smoke in particular, marijuana smoke appears relatively benign.