image courtsey of URMC Rochster
Heavy, long-term cigarette smoking is often said to cause cancer, most prominently lung cancer and cancer of the larynx. And, the evidence is very strong, amounting to near certainty. But, interestingly enough, exactly how it does so is not fully known. It remains an active area of research.
Normal cells may be damaged, but they have the ability to repair themselves. In other cases, the cells are sloughed off and eliminated by the lymph system, then replaced by new ones. But this process can go awry. Cells can grow abnormally, taking on inappropriate shapes and performing incorrectly. When they do, and that growth reaches a certain level that the body can't cope with, the result is cancer.
Tar, for example, is present in cigarette smoke chiefly from the burning paper that holds the tobacco, about 10-14 mg per cigarette. It gradually builds up in the alveoli, the small sacs in the lung that make possible absorption of oxygen into the blood stream. It's believed that their presence is a continual irritant to the cells. That irritation eventually leads to uncontrolled growth of abnormal cells.
Other compounds, called nitrosamines, are present in varying amounts. They're known to be carcinogenic from hundreds of clinical studies on small mammals. NNK is present in a very low concentration: 56.53 nanograms per cigarette. Other nitrosamines, like NNN and NAT, are present in roughly similar amounts.
A few dozen nanograms (one billionth of a gram - 1 g = 0.0353 oz) may sound like a small amount. But sometimes small amounts can have a large effect. Dog's noses, for example, are so sensitive they can detect a few molecules of certain substances. Some systems in humans are equally sensitive to certain chemicals. Add to that the fact that many of the compounds and their effects are cumulative and the case begins to look very strong.
No study has found any link between cancer and consuming one or two cigarettes per day. But such smokers are extremely rare and the odds of them catching some other serious disease are so much higher it may be masked. A smoker who consumes a pack a day for 20 years has 2-4 times the chances of getting lung cancer than a non-smoker.
Non-smokers do, in fact get it. But that doesn't show that smoking isn't a cause, only that other causes can lead to the same effect. One reason scientists have good cause to believe that smoking increases the odds of getting lung cancer is just the odds cited above. Studies also show that lung cancer was much more rare prior to WWI when smoking rates were much lower. As the number of people smoking cigarettes rose, so did the cancer rate. Similarly, as people smoke more, the rates go up.
No single fact or study proves the case. But put enough of them together, over a long enough period, and eventually the case becomes very strong. So strong that saying 'long-term, heavy smoking greatly increases the odds of acquiring lung cancer' becomes a very reasonable statement indeed. It's estimated that 87% of lung cancers are attributable to that habit.
Don't let the odds get you. Start a stop-smoking program now.