Smokers who go through much less than a pack of cigarettes a day still have a higher risk of an early death than non-smokers, a new study suggests.

"There is no safe level of cigarette smoking," said lead study author Maki Inoue-Choi, a researcher at the National Cancer Institute in Rockville, Maryland.

"Even smokers who consistently smoked less than one cigarette per day were more likely to die in our study than never smokers," Inoue-Choi said by email.

Tobacco smoking poses a major public health challenge and claims about five million lives each year worldwide, researchers note in JAMA Internal Medicine.

A growing number of smokers tend to be "light" smokers, going through less than half a pack of cigarettes a day, the authors write. This used to be how people cut back gradually on the path to quitting, but it's increasingly a pattern that smokers follow for years at a time.

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To get a better picture of the health effects of light smoking, researchers tracked more than 290,000 adults aged 59 to 82, including more than 22,000 current smokers and more than 156,000 former smokers, who completed surveys in 2004 and 2005.

By 2011, compared to people who never smoked, adults who consistently smoked at least part of one cigarette a day were 64 percent more likely to have died of any cause, researchers report in JAMA Internal Medicine.

Smoking one to 10 cigarettes a day was associated with 87 percent higher odds of dying from all causes during the study than not smoking at all.

Lung cancer deaths in particular were much more likely among light smokers than non-smokers. The odds of death from lung cancer were more than nine times higher with a habit of even one cigarette a day, while smoking up to 10 cigarettes a day was associated with almost 12 times the risk of death from lung cancer.

Former smokers fared better when they quit at younger ages. For example, ex-smokers of one to 10 cigarettes a day who kicked the habit after age 50 had a 42 percent higher risk of death from all causes during the study period, compared to those who kicked the habit at younger ages. One limitation of the study is that researchers relied on participants to accurately recall and report on how often they smoked even may years in the past, the authors note.

Even so, the findings should reinforce that even light smokers can face serious health risks from the habit, the authors note.

"The take home message is that all smokers should stop smoking, even if they smoke only occasionally, or if they smoke very few cigarettes a day," Jean-Francois Etter, a researcher at the University of Geneva in Switzerland who wasn't involved in the study, said in an email.

The study also showed very little benefit from cutting back from two packs a day to half a pack a day, said Judith Prochaska, a researcher at Stanford University in California who wasn't involved in the study.

"Low intensity smokers often downplay their use of tobacco - may even identify as nonsmokers - and may rationalize their behavior as low risk," Prochaska said by email.

"The findings ought to compel physicians to intervene with patients who report any level of current tobacco use," Prochaska added. "As a motivating message, the sooner individuals quit smoking, the greater the health benefits in extending years of life."