These findings do not support an association between regular marijuana smoking and chronic COPD

Whereas regular smoking of tobacco alone or with other substances increases alveolar epithelial permeability, habitual smoking of cocaine and/or marijuana has no measurable effect on alveolar permeability in the absence of tobacco nor any additive effect to that of tobacco alone.
We conclude that smoking marijuana (13 to 27 mg THC) has no acute effect on central or peripheral ventilatory drive or metabolic rate in habitual marijuana smokers.
Based on the results of this study, pulmonary alveolar macrophages of marijuana-only smokers do not produce increased amounts of oxidants when compared to macrophages of non-smoking subjects.
Because these effects were transient and remained within the range of reported normal values, these data indicate that long-term, experimental exposure to MJ smoke is feasible and does not compromise the general health of the rhesus monkey.
These observations suggest that customary social use of marijuana may not produce abnormalities in airway function detectable by spirometry or bronchoprovocation.
A 20-day study of daily delta 9-THC (20 mg), CBN (600 mg), and CBD (1200 mg) did not indicate tolerance or reverse tolerance to any drug. We conclude that delta 9-THC and, to a lesser extent, delta 8-THC, have acute bronchodilator activity but that CBN, CBD, and their combinations do not provide effective bronchodilation. The daily use of delta 9-THC was not associated with clinical tolerance.
A group of regular marijuana smokers was given expired air carbon monoxide (CO) tests before and after smoking low-dose, high-dose, and placebo marijuana cigarettes. Expired air CO doubled following smoking. There were no significant differences in CO levels in the different dose categories.
The existence of a sizeable number of marijuana smokers in this sample of adolescents did not alter the correlation between CO and self-reports of cigarette smoking. However, in adult samples, where marijuana and cigarette smoking are less highly correlated, marijuana smoking could affect the relationship between CO and self-reported cigarette smoking. Carbon monoxide predicted self-reported cigarette smoking better than did saliva thiocyanate. There was an interaction between grade and the CO/cigarette smoking correlation. The correlations were generally higher in upper grades.
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