"Marijuana smoke contains about 50% more benzopyrene and nearly 75% more benzanthracene, both known carcinogens, than a comparable quantity of unfiltered tobacco smoke (Tashkin, 2013). Moreover, the deeper inhalations and longer breath-holding of marijuana smokers result in greater exposure of the lung to the tar and carcinogens in the smoke. Lung biopsies from habitual marijuana-only users have revealed widespread alterations to the tissue, some of which are recognized as precursors to the subsequent development of cancer (Tashkin, 2013)."