Well, you do know that marijuana conains more tar than a non-filter cigarette.
Tar from cannabis cigarettes contains up to 50 per cent higher concentrations of carcinogens benzathracenes and benzpyrenes than tobacco smoke
THC, the primary psychoactive ingredient of cannabis, decreases the function of immune system cells that help protect the lungs from infection.
Here's a good study on
marijuana and lung function. The cited references are really good on this study as well as it shows mixed results.