“…habitual use of marijuana alone does not appear to lead to significant abnormalities in lung function, except for possible increases in lung volumes… no clear link to chronic obstructive pulmonary disease has been established… findings from a limited number of well-designed epidemiological studies do not suggest an increased risk for the development of either lung or upper airway cancer from light or moderate use… In summary, the accumulated weight of evidence implies far lower risks for pulmonary complications of even regular heavy use of marijuana compared with the grave pulmonary consequences of tobacco.” http://www.ncbi.nlm.nih.gov/pubmed/23802821