Casual smokers may think that smoking a few cigarettes a week is no big deal, but new research suggests that having an infrequent smoke, or being exposed to secondhand smoke, might be doing more harm than people realize.

The findings provide further evidence supporting public smoking bans, say the authors, who are physician-scientists at New York-Presbyterian Hospital/Weill Cornell Medical Center, in New York City.

The study, published earlier this month in the American Journal of Respiratory and Critical Care Medicine, found that being exposed to even low-levels of cigarette smoke may put people at risk for future lung disease, such as lung cancer and chronic obstructive pulmonary disease (COPD).

Epidemiological studies long have shown that secondhand smoke is dangerous, but until now there have never been conclusive biological tests dem-onstrating what it does to the body at a gene function level, the study authors claim.

"Even at the lowest detectable levels of exposure, we found direct effects on the functioning of genes within the cells lining the airways," says Dr. Ronald Crystal, senior author of the study. Crystal is chief of the division of pulmonary and critical care medicine at New York-Presbyterian/Weill Cornell and chair of the department of genetic medicine at Weill Cornell Medical College, in New York City.

Genes that commonly are activated in the cells of heavy smokers also are turned on or off in those with very low-level exposure, Crystal says.

"The genetic effect is much lower than those who are regular smokers, but this does not mean that there are no health consequences," he says. "Certain genes within the cells lining the airways are very sensitive to tobacco smoke, and changes in the function of these genes are the first evidence of 'biological disease' in the lungs or individuals."

To make their findings, Dr. Crystal and his collaborators tested 121 people from three different categories: nonsmokers, "active smokers," and "low exposure smokers." The researchers tested urine levels of nicotine and cotinine—markers of cigarette smoking within the body—to determine each participant's category.

The research team then scanned each person's entire genome to determine which genes either were activated or deactivated in cells lining the airways. They found that there was no level of nicotine or cotinine that didn't also correlate with genetic abnormalities.

"This means that no level of smoking, or exposure to secondhand smoke, is safe," Crystal says. He describes the genetic changes as being akin to a "canary in a coal mine," warning of potential life-threatening disease, "but the canary is chirping for low-level exposure patients, and screaming for active smokers."

Funding for the study came from grants from the National Institutes of Health, the Flight Attendants Medical Research Institute, and the Cornell Center for Comparative and Population Genomics.