This article is a good summery of all my (scientific, knowledgable, from the viewpoint of a molecular biologist) observations on the matter of marijuana consumption for the past 20 years or so.
So, in short (for non medical students and doctors) heavy smoking of marijuana might lead to chronic bronchitis (as any other smoke, including urban smoke caused by cars), but moderate or light smoking is as good as safe. In contrast, any tobacco consumption (light or heavy) is bad for your health.
Now comes the $100,000 question - is the anti-marijuana and pro-tobacco lobby more powerful that the pro-marijuana? And a second question - does the pro-tobacco lobby gives a s**t about your health?
Disclaimer: In my entire life I neither tried tobacco or marijuana (and I hope I never will), although living in Amsterdam for over 10 years might count as a marijuana passive consumer.