June 3 (UPI) -- Occasional smokers are nearly four times as likely to die from smoking-related cancers than non-smokers, a study published Wednesday by JAMA Network Open has found.
Daily smokers, meanwhile, are nearly five times as likely to die from the diseases as non-smokers, the researchers also found.
"Even smoking just a few cigarettes per month increases risk of death from cancer and other causes," co-author Maki Inoue-Choi, a staff scientist a the National Cancer Institute, told UPI.
"While reducing smoking from daily to non-daily lowers risk, non-daily smokers still experience higher risk of death than former and never smokers," she said. "Our study provides further evidence that there is no safe level of smoking and that all smokers of any level should quit."
Fewer than 14 percent of all adults in the United States smoke cigarettes, according to the Centers for Disease Control and Prevention. Still, smoking causes nearly 500,000 "premature" deaths -- or deaths occurring before typical life expectancy -- across the country each year, the agency estimates.
For their study, researchers from the National Cancer Institute reviewed data on tobacco use and health outcomes for more than 505,000 U.S. adults over a 20-year period. Daily smokers in the study smoked roughly 20 cigarettes a day, while non-daily smokers smoked roughly 40 per month, the authors said.
Researchers found that non-daily smokers were at lower risk for smoking-related health problems than daily smokers, according to the authors. However, they were still significantly more likely to die from lung cancer and other lung illnesses, heart disease or stroke than non-smokers.
Life-long occasional smokers who never used cigarettes daily, for example, are nearly 3.5 times as likely to die from a lung illness other than cancer than life-long non-smokers, the authors found. Similarly, these non-daily smokers were nearly twice as likely to die from heart disease than life-long non-smokers.
Risk for lung cancer is 10 times higher among occasional tobacco users -- and 14 times higher among daily users -- than in non-smokers, researchers also found.
Daily cigarette smokers, meanwhile, were nearly six times as likely to die from a respiratory illness other than lung cancer than non-smokers, and had nearly twice the risk for death from heart disease.
"There has been a common perception that non-daily and occasional smoking poses little or no risk, [but] our study showed that lifelong non-daily smokers had ... higher mortality risk than never smokers," Inoue-Choi said. "Our study provides further evidence that there is no safe level of smoking."