Smoke is truly not the only reason for a lung cancer diagnosis. There are many cases diagnosed which are unrelated to tobacco, even second hand. Several doctors have proposed theories including a viral cause for mine somewhere down in the family tree (related to a sheep virus), a genetic anomoly, and scar tissue combining with other problems leading to BAC. Other lung cancers have been related to asbestos, agent orange, viral exposure, scarring, radon, and of course tobacco exposure, first hand or otherwise.