The Institute of Medicine concluded in its Mar. 1999 report titled "Marijuana and Medicine: Assessing the Science Base":

“A number of animal studies have revealed respiratory tract changes and diseases associated with marijuana smoking, but others have not. Extensive damage to the smaller airways, which are the major site of chronic obstructive pulmonary disease (COPD), and acute and chronic pneumonia have been observed in various species exposed to different doses of marijuana smoke. In contrast, rats exposed to increasing doses of marijuana smoke for one year did not show any signs of COPD, whereas rats exposed to tobacco did.”

Mar. 1999 - “Marijuana and Medicine: Assessing the Science Base”