San Francisco, CA: Individuals who regularly smoke cigarettes and marijuana experience a greater likelihood of developing symptoms of chronic obstructive pulmonary disease (COPD) than do individuals who smoke cigarettes only, according to clinical data presented this week at the 2007 International Conference of the American Thoracic Society.

Investigators at St. Paul's Hospital in Vancouver, British Columbia, reported that the combined use of tobacco and marijuana was associated with an increased risk of COPD among individuals age 40 and older. Researchers found that cigarette-only smokers were two-and-a-half times more likely than nonsmokers to develop COPD, while those subjects who reported smoking both substances were three-and-a-half times more likely.

Investigators did not report whether individuals who consumed only cannabis were at a higher risk for developing COPD compared to nonusers.

A recent meta-analysis published in the Archives of Internal Medicine reports that although chronic cannabis smoking is associated with an elevated risk of respiratory complications – including an increase in cough, sputum production, and wheezing – it is not associated with a decline in pulmonary function.

Authors of the Canadian study did not suggest whether vaporizing cannabis would reduce its adverse interaction with tobacco smoke.

Last month, investigators at San Francisco General Hospital reported that use of the Volcano vaporizing device significantly reduces cannabis consumers' intake of gaseous combustion toxins, including carbon monoxide. A separate study conducted by investigators at the University of Albany reports that marijuana consumers who vaporize pot are less likely to suffer from respiratory symptoms than are those who do not vaporize, after controlling for subjects' age, sex, and cigarette use.

Vaporization heats cannabis to a temperature where active cannabinoid vapors form (typically around 180-190 degrees Celsius), but below the point of combustion where noxious smoke and associated toxins (i.e., carcinogenic hydrocarbons) are produced (above 230 degrees Celsius).