RESULTS: Repeated inhalation of tobacco smoke causes loss of airflow and lung volumes. Marijuana smoke, however, had the opposite effect, increasing both airflow rates and capacity.
CONCLUSION: Unlike cigarette smoke, use of marijuana does not result in lasting respiratory harm.
The exciting news of the day is that pot isn't bad for you, unless it makes you sneak weirdly out of your friends' homes, which is another good reason to turn once again to Caitlin Podiak's informative Cannabis 101. (The full study, published in the Journal of the American Medical Association, is also available here.) A frequent smoker, when asked just now how he feels about the study, said "Validated? I don't know."