 Numerous studies since the 1970s have documented links between B"a"P and cancers.It has been more difficult to link cancers to specific B"a"P sources, especially in humans, and difficult to quantify risks posed by various methods of exposure (inhalation or ingestion).A link between vitamin A deficiency and emphysema in smokers was described in 2005 to be due to B"a"P, which induces vitamin A deficiency in rats.