All the factors that cause cancer are not fully understood. But there are clear generally standard and well documented risk factors. Cigarette smoking is a major risk factor for cancer. It is also the most generally practiced act in the world that can cause cancer. There are roughly 1.2 billion smokers in the world today. Cigarette smoke contains as much as 50 carcinogens with 15 of them being Group 1 carcinogens. Group 1 carcinogens are those carcinogens which are confirmed to cause cancer in humans. Tobacco smoking alone causes as much as 2 million cancer death annually every year. With intense anti smoking campaign by World health Organizations and Governments in the west many are quitting cigarette smoking. In this context an foremost demand arises. If person has stopped smoking does he/she still runs the risk of developing cancer? The same demand can be extended to other types of carcinogen exposure, exposure to ionizing radiations etc.
It has to be standard that there is still less quantitative information about what regularly happens when the exposure to carcinogens ceases. Is there still a risk of developing cancer even after a person is no longer exposed to the carcinogens? In the case of cigarette smoking the rapidly rising yearly risk among those who continue to smoke stabilizes for one or two decades before expanding again slowly.
The person who had quit smoking consequently avoids the sizable progressive increase in risk suffered by the continuing smoker. These delayed effects accord with the idea that the appearance of clinical cancer is the end-result of a multistage process in which any mutations have to be produced in a singular stem cell to turn it into the seed of a growing cancer. From the practical point of view, the foremost conclusions are that cancer may be very much more likely to occur after continued exposure to a carcinogen than after short exposure. It means that cancer is seldom likely to appear within a decade after first exposure to the carcinogen or other triggering factors like exposure to ionizing radiations. There is an irregularity to this rule in the case of leukemia and the specific cancers of childhood. Cancer seems to occur only any decades after the first exposure, and that some excess risk may continue to occur for decades after exposure has ceased.
So it means that if person has been smoking for 10-20 years and then quits. He still runs the risk of developing cancer even after quitting smoking. The probability depends on how many cigarettes he been smoking and the type of cigarette. If cigars as against cigarettes are smoked regularly then the risk is increased as cigar smoke contains more toxic chemicals.
The exact connection may, however, differ for separate carcinogens and separate types of tumors. Bladder tumors, for example, began to appear within 5 years of laberious exposure to 2-naphthylamine in the dye industry, while mesotheliomas of the pleura have seldom, if ever, appeared within 10 years of exposure to asbestos, but they continue to increase in incidence for up to 50 years after first exposure, even if the exposure was relatively brief.
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