A cigarette has 60 carcinogens that can damage or mutate DNA in the human body, and can cause 1,000 to 10,000 mutations per cell. (Representational Image)
Smoking can cause mutation in cells, leading to cancer. So, if a smoker quits, how far does his or her health recover? A study published in the journal Nature says quitting can help reverse the potential risk; it can replenish cells that can actually resemble those of a person who has never smoked.
How smoking causes cancer
A human body is structured to bear damage, but only for some time. A cigarette has 60 carcinogens that can damage or mutate DNA in the human body, and can cause 1,000 to 10,000 mutations per cell. “Smoking causes mutations pretty steadily over time, gradually building up. The more you smoke the more mutations you have,” Sam M Janes, professor of thoracic medicine at University College London Hospital and one of the 21 authors of the study, said by email.
The constant damage to the cells lining the airway and lungs can lead to cancers in the lungs, oesophagus, larynx and pharynx. Lung cancer is the most common, and 80-90% deaths due to it are attributed to tobacco, the study found. Chemicals in cigarette can also enter the bloodstream and affect multiple organs. It can also cause cancer in the liver, pancreas, stomach, kidney and blood, but these are rare. Smoking also causes emphysema, damage of air sacs in lungs, which is irreversible.
What the study found
It analysed bronchial epithelial cells (taken from airway near lungs) of 16 people in London: three children, four people who never smoked, six ex-smokers and three current smokers. From a sequencing of 632 bronchial cells, the researchers identified “driver mutations” as being more frequent in people smoking or with a history of smoking.
Driver mutation, in simple terms, is like a biochemical causing cell to mutate and lead to cancer. In current smokers, 25% cells were found carrying driver mutations. No such driver mutations were found in children, and in adult non-smokers only 4-14% cells had driver mutations due to various other factors. The key finding was that gradually the cell mutation burden in ex-smokers becomes similar to that of non-smokers.
Janes said: “Ex-smokers have many cells with lots of mutations that non-smokers don’t have, but importantly a large fraction of the cells (up to 40%) don’t have mutations — meaning healthy cells are gradually replacing the mutated or damaged cells.”
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The wider implications
“By stopping smoking in middle age or earlier, smokers avoid most of the risk of tobacco-associated lung cancer. This benefit begins to emerge almost immediately and accrues steadily with time,” the study observes.
“But it is not an overnight change,” said Nagpur-based oncologist Dr Abhishek Vaidya, who was not involved in the study. “Once a person quits, the cumulative risk of cancer keeps decreasing,” said Vaidya, who treats a number of oral cancers.
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Among other Indian doctors, Dr Pankaj Chaturvedi, head and neck oncosurgeon in Tata Memorial Hospital, said it can take a minimum 10 years for driver mutations to reduce in human cells. Dr P C Gupta, director a Healis-Sekhsaria Institute for Public Health, said the risk of lung cancer remains. Gupta said that within 24-48 hours of quitting smoking, the cells start repairing themselves, although the healing is slower if organs are damaged.
The key message from the research is that stopping smoking, at any age, is important and rapidly reduces risk of getting lung cancer. Emphysema if caused, however, is irreversible.