MSG,
I don't want to come across as in anyway defending smoking, but I don't believe your assumption that 20% of smokers develop lung cancer is anywhere near accurate.
Research shows that even among long-term, heavy smokers, the risk of getting lung cancer can vary dramatically -- from less than 1 percent to 15 percent.
To be in Sloan-Kettering's long-term, heavy smokers, high risk category, the person has to have smoked at least a pack a day for the last 20 years and be over 50 years old.
SOURCES: Peter Bach, M.D., epidemiologist and pulmonary physician, Memorial Sloan-Kettering Cancer Center, New York
http://www.lifeclinic.com/fullpage.aspx?prid=512291&type=1