Charles Ksir, PhD, Professor Emeritus of Psychopharmacology and Drug Policy at the University of Wyoming, and Oakley Ray, PhD, Emeritus Professor of Psychology and Pharmacology at Vanderbilt University, wrote in their 2004 textbook Drugs, Society and Human Behavior:

“Experiments have shown that chronic, daily smoking of marijuana impairs air flow in and out of the lungs. It is hard to tell yet whether years of such an effect results in permanent, major obstructive lung disease in the same way that smoking tobacco cigarettes does.

Also, there is no direct evidence linking marijuana smoking to lung cancer in humans. Remember that it took many years of cigarette smoking by millions of Americans before the links between tobacco and lung cancer and other lung diseases were shown…

Everyone suspects that marijuana smoking will eventually be shown to cause cancer, but how much of a problem this will be, compared with tobacco, is hard to say.”