There are always a few die-hards who smoke a pack a day from age eighteen and live to be ninety. That's because a very few people are physiologically less susceptible to the arterial aging and carcinogenic effects of cigarette smoke than the rest of us. These people have higher levels of specific enzymes that activate the carcinogens contained in smoke. Nitrosamines, by-products of cigarette smoking, interact with the body's own enzymes to create a new chemical that is highly inflammatory and highly carcinogenic, or damaging to deoxyribonucleic acid (DNA). Some people have much more or much less of the human acetylator enzyme that helps the body remove certain carcinogens from the body. The people who produce less of this enzyme than others, called "slow acetylators," are predisposed to breast cancer, as well as other kinds of cancer.
Don't assume you're one of the rare people who have better genes for preventing inflammation and for fighting cancer. The ingestion of any tobacco products, whether through smoking, chewing, or inhaling secondhand smoke, increases aging of the immune system (and arteries). The single best way to safeguard yourself against the carcinogenic effects of tobacco is not to use it at all.