Check out this article by the NATIONAL INSTITUTE OF HEALTH. (NIH)
Like tobacco smoke, marijuana smoke is an irritant to the throat and lungs and can cause a heavy cough during use. It also contains levels of volatile chemicals and tar that are similar to tobacco smoke, raising concerns about risk for cancer and lung disease.68
Marijuana smoking is associated with large airway inflammation, increased airway resistance, and lung hyperinflation, and those who smoke marijuana regularly report more symptoms of chronic bronchitis than those who do not smoke.68,69 One study found that people who frequently smoke marijuana had more outpatient medical visits for respiratory problems than those who do not smoke.70 Some case studies have suggested that, because of THC’s immune-suppressing effects, smoking marijuana might increase susceptibility to lung infections, such as pneumonia, in people with immune deficiencies; however, a large AIDS cohort study did not confirm such an association.68 Smoking marijuana may also reduce the respiratory system’s immune response, increasing the likelihood of the person acquiring respiratory infections, including pneumonia.69 Animal and human studies have not found that marijuana increases risk for emphysema.68
Whether smoking marijuana causes lung cancer, as cigarette smoking does, remains an open question.68,71 Marijuana smoke contains carcinogenic combustion products, including about 50 percent more benzoprene and 75 percent more benzanthracene (and more phenols, vinyl chlorides, nitrosamines, reactive oxygen species) than cigarette smoke.68 Because of how it is typically smoked (deeper inhale, held for longer), marijuana smoking leads to four times the deposition of tar compared to cigarette smoking.72 However, while a few small, uncontrolled studies have suggested that heavy, regular marijuana smoking could increase risk for respiratory cancers, well-designed population studies have failed to find an increased risk of lung cancer associated with marijuana use.68
One complexity in comparing the lung-health risks of marijuana and tobacco concerns the very different ways the two substances are used. While people who smoke marijuana often inhale more deeply and hold the smoke in their lungs for a longer duration than is typical with cigarettes, marijuana’s effects last longer, so people who use marijuana may smoke less frequently than those who smoke cigarettes.
Additionally, the fact that many people use both marijuana and tobacco makes determining marijuana’s precise contribution to lung cancer risk, if any, difficult to establish. Cell culture and animal studies have also suggested THC and CBD may have antitumor effects, and this has been proposed as one reason why stronger expected associations are not seen between marijuana use and lung cancer, but more research is needed on this question.68