"3-4 Cannabis cigarettes a day are associated with the same evidence of acute and chronic bronchitis and the same degree of damage to the bronchial mucosa as 20 or more tobacco cigarettes a day.
Cannabis smoking is likely to weaken the immune system. Infections of the lung are due to a combination of smoking-related damage to the cells lining the bronchial passage and impairment of the principal immune cells in the small air sacs caused by cannabis."

-- British Lung Foundation
"Smoking Gun: The Impact of Cannabis Smoking on Respiratory Health,"
a publicly disseminated report
Nov. 2002