Two thirds of all cancers are a result of normal cell replication. Environmental factors or an inherited predisposition for cancer are minor factors relatively speaking. Whenever a cell divides, hundreds of replication errors occur. Occasionally those errors result in a mutation which can cause cancer. Even if you eliminate all carcinogens in the environment the incidence of cancer would not drop dramatically.
Link… http://science.sciencemag.org/content/355/6331/1330