Genes:
Smoking, in the teenage years, causes permanent genetic changes in the lungs, and forever increases the risk of lung cancer, even if the smoker quits, a study finds. And the younger the smoking starts, the more damage is done.  "The research, at a time when more than a third of teens take up the smoking habit, shows "there is something uniquely bad about starting young," said John K. Wiencke, a genetics expert at the University of California, San Francisco, School of Medicine.
The research, gives powerful laboratory evidence of why starting smoking before the age of 18, can be particularly harmful to long term health.
"Youthful smoking on a daily basis apparently causes lung damage that lasts a lifetime," Wiencke said. Such damage is less likely among smokers who start in their 20's. "It looks like it is the age when smoking starts that is important," Wiencke said. "It didn't matter if they were heavy or light smokers, what mattered was, that they started young. Earlier studies, have indicated that young smoking, 'stunts the lungs'full development, and increases the risk of breathing problems later in life. Studies have also shown that smoking in the teen years, is more addictive, and that smokers who begin young, are less likely to break the habit.
Wiencke's study for the first time, shows dramatic and enduring DNA damage caused by youthful smoking. This reinforces the idea that we need to stop young people from smoking, not only from the addiction standpoint, but also from the cancer risk standpoint.
Surveys by the Centers for Disease Control and Prevention, indicate that 34.8% of high school students, were regular smokers in 1995. That number rose to 36.4% in 1997 Of smokers ages 30 to 39, 62% had tried smoking by the age of 16, and 24.9% had taken up the smoking habit permanently by that age, the CDC found.
About 3 million teen-agers now smoke, the government estimates. And about a third of all smokers will die of smoking related illnesses, including lung and other types of cancers, heart disease, stroke, emphysema, and chronic pulmonary obstruction In their study, Wiencke, and colleagues, tested for DNA alterations, in the non tumor lung tissue of patients being treated for lung cancer. The group included 57 people who were current smokers, 79 who were former smokers and seven who had never smoked.
The healthy lung tissue, was tested for the number of DNA alterations per 10 billion cells. Some alterations occur with age, but the number of gene changes was much higher among smokers, -and highest of all, among those who started smoking at a young age.
For nonsmokers, there were 32 DNA alterations per 10 billion cells. For current smokers, the alterations were about eight times higher. The findings were adjusted statistically for the number of years smoked, and for the amount smoked. The startling discovery, was that for former smokers, the important factor determining DNA damage, was when they started smoking, not how long or how much. Former smokers who started at age 7 through their 15th birthdays, had an average of 164 genetic alterations. Exsmokers who started from ages 15 through 17, had an average 115 alterations. Among ex-smokers who didn't start smoking until after they were 20, however, the DNA alternations averaged 81, fewer than half that of people who started smoking earlier.
Such alterations occur when, chemicals in tobacco smoke, fuse with genes in the DNA of lung cells. These chemical complexes, called adducts, cause mutations, and significantly increase cancer risk.
The study shows that genetic alterations, from tobacco carcinogen exposure, may persist in former smokers, said an editorial in the journal, by three cancer researchers at the University of Texas, M.D. Anderson Cancer Center in Houston. The editorial said, "the finding fits one more piece into the lung cancer risk assessment puzzle."  
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