April 13 (HealthDay News) -- People who smoke cigarettes and marijuana increase their risk of chronic obstructive pulmonary disease almost threefold, but smoking pot alone doesn't seem to increase the risk of the deadly lung condition, researchers report.

Chronic obstructive pulmonary disease (COPD) is a progressive condition with no cure that's characterized by diseased lungs and narrowed airways. Most cases are caused by prolonged smoking, according to the U.S. National Heart, Lung, and Blood Institute.

"Anti-smoking campaigns should include a reduction in marijuana use among their goals, aiming especially at those who smoke both marijuana and tobacco," said study lead researcher Dr. Wan Tan, of the University of British Columbia and St. Paul's Hospital in Vancouver, Canada.

The findings were published in the April 14 issue of the Canadian Medical Association Journal.

For the study, Tan's team collected data on 878 people who participated in the Burden of Obstructive Lung Disease (BOLD) Initiative, which was designed to identify the prevalence of COPD in people over 40.

The researchers found that smokers who use both marijuana and tobacco are 2.5 times more likely than nonsmokers to have respiratory disease and almost three times likelier to have COPD.

This combined effect suggests that smoking marijuana -- at least in relatively low doses -- may act as a "primer," or sensitizer, in the airways to amplify the adverse effects of tobacco smoke on respiratory health, Tan said.

"Smoking marijuana and cigarettes -- even small amounts -- is very harmful for your lungs, increasing the risk of COPD by several fold," Tan said.

Dr. Norman H. Edelman, scientific consultant to the American Lung Association, said he thinks smoking marijuana probably does cause COPD, but this study did not include enough marijuana smokers to draw that conclusion.

"Canada was a good place to do it as they are more relaxed about pot smoking, and it was likely to be easier to get cooperative subjects," Edelman said. "The most impressive finding is the synergism between tobacco and marijuana smoking."

"A modest amount of pot smoking added to cigarette smoking really knocks your lungs for a loop," Edelman added. "One wonders if this is a chemical synergism or behavioral -- those who smoke both pot and tobacco inhale each more deeply than those who smoke only tobacco."

SOURCES: Wan Tan, M.D., University of British Columbia and St. Paul's Hospital, Vancouver, British Columbia, Canada; Norman H. Edelman, M.D., scientific consultant, American Lung Association, Washington, D.C.; April 14, 2009, Canadian Medical Association Journal