FRIDAY, Aug. 20 (HealthDay News) -- A drag from a cigarette now and then can't hurt, right?

Wrong, according to a new study that finds even low levels of smoke exposure can cause irreparable damage to cells essential to breathing.

The damage occurred among "casual" smokers and even after exposure to secondhand smoke. The initial damage, while not usually severe, can be cumulative and prolonged exposure to tobacco smoke could lead to chronic obstructive pulmonary disease (COPD) and even lung cancer, the researchers reported.

"It has been known for a long time that secondhand smoke or smoking occasionally can be risky for your health," said study author Dr. Ronald Crystal, chief of the division of pulmonary and critical care medicine at NewYork-Presbyterian Hospital/Weill Cornell, in New York City.

Just how much a little exposure might damage airway cells hasn't been clear, however.

"We found that if we could detect nicotine in the urine we could also detect changes in the genes in the cells lining the airways," said Crystal, who is also chair of the department of genetic medicine at Weill Cornell Medical College.

The bottom line: "There is no level of cigarette smoking or exposure to cigarette smoke that does not make the cells in your lungs sick," he said. "If you are an occasional smoker you are still at risk. Don't think that smoking one or two cigarettes a week means you are home free."

As for secondhand smoke, "if you are working in a place where people smoke, either get them to stop or go get another job," Crystal advised. "If you have somebody at home who smokes, send them outside to smoke. Don't be exposed to secondhand smoke."

The report is published in the Aug. 20 issue of American Journal of Respiratory and Critical Care Medicine.

For the study, Crystal's team recruited 121 people who were nonsmokers, active smokers or low-exposure smokers. To determine who belonged in which group, all participants had their urine tested for levels of nicotine.

Crystal's group next scanned each person's entire genome to determine whether genes governing airway cells were turned on or off.

They found that there was no level of nicotine or cotinine, no matter how small, that did not produce genetic abnormalities.

"These cells are like canaries in the mine, they're crying out for help -- this gene is being turned on, this gene is being turned off," Crystal said. "This now gives us clues to what are the earliest events in terms of what makes our cells go wrong and is the start of these lung diseases, like COPD and lung cancer."

Knowing which genes are damaged could provide targets for new drugs that could protect the lungs, Crystal said.

Dr. Norman H. Edelman, chief medical officer at the American Lung Association and a professor of preventive medicine, internal medicine and physiology & biophysics at Stony Brook University in New York, applauded the study. "I like this one because it cleverly uses molecular biology to answer a very important question, one that I get asked very often ... 'Is there a threshold below which inhalation of tobacco smoke is safe?'" he said. The question is usually asked as, " Is it safe to smoke a few cigarettes a week?" or, "Is it safe to hang out with smoking friends a few times a week?" Edelman added.

"Within the limits of their detection methods, the answer is 'no,'" Edelman said. "Whether the changes they see in folks with minor exposure will eventually lead to disease is unclear, but it is getting more and more clear to me that there really is no totally safe level of tobacco exposure."

More information

For more information on secondhand smoke, visit the U.S. National Library of Medicine.