It's long been thought that the damage caused by smoking was directly related to how long a person smoked. But Dr. John K. Wiencke, a genetics expert at the University of California, San Francisco, School of Medicine found that it's not how long they smoked, but when they started, reports CBS News Correspondent John Roberts.
"It would suggest that there probably are a lot of folks who have extensive damage in their lungs even after they quit smoking if they started to smoke when they were very young," said Wiencke, the study's first author.
Specifically, Wiencke looked at genetic evidence -- damage done to the DNA of ex-smokers who developed lung cancer. For patients who started smoking as teens, that damage was greater than those who started later in life.
"We think they may be more sensitive to the toxic and carcinogenic effects of tobacco smoke at that time," he said.
Wiencke said the research gives powerful laboratory evidence of why starting smoking before the age of 18 can be particularly harmful to long-term health. He said the damage is less likely among smokers who start in their 20s or later, although smoking at any age is unhealthy.
The study shows for the first time dramatic and enduring DNA damage caused by youthful smoking. 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, Wiencke said.
Wiencke and his colleagues tested for DNA alterations in the non-tumor lung tissue of patients being treated for lung cancer. The group included 57 current smokers, 79 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, Wiencke said.
Nonsmokers had 32 DNA alterations per 10 billion cells. Current smokers' alterations were about eight times higher. The findings were adjusted statistically for the number of years smoked and for the amount smoked.
Former smokers who started smoking before age 15 had an average of 164 genetic alterations. Ex-smokers who started between age 15 and 17 had an average of 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 earliest.
The government estimates that about 3 million teen-agers smoke and statistics also show that about one-third of all smokers will die of smoking-related illnesses. The results of the new study suggest that preventing teens frm even starting to smoke is more important than ever.
However, creative ad campaigns have dramatically reduced teen smoking in some states. The Centers for Disease Control and Prevention believes that nationally, we may be turning the corner on a decade of rising rates.
But researchers believe - among a huge group of Americans - that the damage has been done, even if they got the message about the dangers of tobacco.
"The majority of people who get lung cancer are ex-smokers, and we're really trying to understand that," said Dr. Karl Kelsey of the Harvard School of Public Health. "And it's clear that one of the things that keeps you at risk is if you started at an early age."
The study presents another challenge: to identify those ex-smokers most at risk and to find ways to aggressively intervene before they develop cancer.