in teenagers who smoke, even if they quit later in life.

And the younger the smoking starts, the more damage is done. That's the conclusion of a study published April 8 in the journal of the National Cancer Institute.

"The research, coming at a time when more than a third of teens are taking 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, and first author of the study.

He said the research gives powerful laboratory evidence of why starting smoking before the age of 18 can be particularly harmful to long-term health.

Mr Wiencke said, "Such youthful smoking on a daily basis causes lung damage that the body cannot repair.'' He added that the damage is less likely among smokers who start in their 20s or later, although smoking at any age is unhealthy.

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 is 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.

But Mr 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 per cent of high school students were regular smokers in 1995. That number rose to 36.4P percent in 1997.

A CDC survey of smokers aged 30 to 39 showed that 62 per cent had tried smoking by the age of 16, and 24.9 per cent had taken up the habit permanently by that age.

About 3 million American teenagers 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, Mr Wiencke and his colleagues tested for DNA alterations in the non-tumour 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 discovery was that for former smokers, the most important factor affecting DNA damage was when they started smoking, not how long or how much, Mr Wiencke said.

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