“The common perception is that marijuana smoke is less toxic and that smoking a few marijuana joints per day has far fewer consequences than smoking a pack of tobacco cigarettes (5). However, the lack of filtering and differences in the smoking technique associated with marijuana use result in an approximately fourfold greater deposition of tar particulates in the lung than occurs from smoking similar amounts of tobacco (6). In addition, the concentration of pro-carcinogens such as benz-[α]-anthracene and benzo-[α]-pyrene are up to twofold higher in marijuana tar (3,7). The presence of irritants and pro-carcinogens in marijuana smoke and the enhanced deposition of these in the lung during smoking suggest that habitual smoking of marijuana might result in a spectrum of respiratory consequences similar to those described for tobacco smoking. Moreover, THC has recently been shown to exert potent biological effects on lung epithelial cells and on the immune system (8-10). Consequently, it is possible that regular exposure to marijuana smoke, a large proportion of which is THC, might predispose to lung injury, pulmonary infections, and/or tumor growth.”

Donald P. Tashkin and Michael D. Roth, “Effects of Marijuana on the Lung and Immune Defenses“, Forensic Science And Medicine, 2007, pg. 253-275.