The research team, at the University of California Medical Center in San Francisco and the Harvard School of Public Health in Boston, announced the findings yesterday in the Journal of the National Cancer Institute. They studied so-called DNA-adducts, chemicals that have attached themselves to DNA.

"Our finding suggests that young smokers incur more severe or persistent DNA damage than adult smokers do," said epidemiologist John Wiencke. Perhaps, he said, the body's ability to repair genetic damage is weakened if smoking begins when lungs are more vulnerable.

"Smoking during adolescence may produce physiologic changes that lead to persistent or increased damage," he added. "Or young smokers may be markedly susceptible to DNA damage and have higher burdens of damage after they quit smoking, than those who started smoking later in life.