One of the main reasons we can't pinpoint what part of smoking causes cancer is that cigarette companies have a big secret. An incredible loophole in the law allows them not to disclose about 500 of the ingredients in cigarettes. Maybe the burning of one of those ingredients causes lung cancer. But we can't test it, because we don't know what it is.
http://www.healthcentral.com/drdean/408/14275.html