After quitting it took me several years for the effects to totally go away. I think your on to something with the lung cancer, pot is packed full of carcinogens.
As is any carbon based smoke, be it from a camp fire, a banana peel, whatever. Its the repeated use that gets you. I still have issues with short term memory loss, which frustrates Mrs. Cport to no end. She knew me from before I engaged in daily use, and having never used, she does not understand my inability to recall conversations and appointments. I am heavily dependent on my cell phone calendar to remind me of the most mundane things, such as remembering that tomorrow is water day for the kids at summer camp.
All that being said, I am also of the belief that occasional use, as with drinking, is not societally detrimental, and that valid medical benefits, as with any drug, should be explored and not rejected reflexively due to some anti-intoxicant bias. People should be able to self medicate in a free society, unless it has been proven to infringe on another's rights.