you're right. all cells in the body can regenerate from damage, including lung cells. however, it's a limited regenerative capacity. they can only heal up to a specific point of damage, and not a very high one at that. if a person only takes a whiff of second hand smoke, the minute amount of free radicals they inhale likely won't harm them, but if they're exposed to it for a substantial amount of time, it will be harmful to them.
so, the fact that lung cells can regenerate doesn't debunk the "myth" of second-hand smoke diseases, nor does it mean that smoking isn't a problem, because the regenerative capability of lungs is limited.
here's two peer-reviewed journal article i found about marijuana smoke and effects, 1 review and 1 study:
review - "Marijuana: Medical Implications":
http://www.aafp.org/afp/991201ap/2583.html
abstract: Over 50 percent of people will use marijuana sometime in their life. While intoxication lasts two to three hours, the active ingredient in marijuana, delta-9-tetrahydrocannabinol, can accumulate in fatty tissues, including the brain and testes. Adverse effects from marijuana use include decreased coordination, epithelial damage to the lungs, increased risk of infection, cardiovascular effects and cognitive deficits. Unexplained behavior changes, altered social relationships and poor performance at school or work can signify a drug problem.
study - "Oxidative Stress Produced by Marijuana Smoke":
http://ajrcmb.atsjournals.org/cgi/conte ... /20/6/1286
bottom line of this study (because the abstract is too long): "We conclude that MJ smoke containing 9-THC is a potent source of cellular oxidative stress that could contribute significantly to cell injury and dysfunction in the lungs of smokers."
of course, these are only two studies, and ideally a good researcher would look at about 10 studies to make a definite conclusion, but i get the feeling that it wouldn't be too hard to find 10 studies about the adverse effects of marijuana use and smoke.
hmm, well, it's nice to know that all the research skills i've learned this year aren't going to waste...