I still think a large part of the reason the two smoke differently can be extrapolated from the different effects the two have:
“The study also found that marijuana smoke and that of tobacco affected the lungs in different ways. Tobacco smoke predominantly caused degeneration of the peripheral airways and alveolated regions of the lung, while marijuana smoke affected mainly the large pathways. Because these two drugs affect different parts of the pulmonary system, their damaging effects can be additive.”
–The Physiological Effects of Cannabis Consumption, citing National Institute of Drug Abuse Research Findings on Smoking of Abused Substances. Editors: C. Nora Chiang, Ph.D. and Richard L. Hawks, Ph.D. 1990. page 72
As for the comparison between weight and volume, etc.,his page says this:
“Marijuana users typically do not smoke anywhere near the volume of marijuana as tobacco smokers smoke tobacco. One joint usually has about half the smokable substance as one tobacco cigarette. But because marijuana is not usually filtered and users tend to hold the marijuana in their lungs for longer periods of time, up to four times as much tar can be deposited in the lungs per ounce of smoked marijuana as an ounce of tobacco.”
–citing Wu TC, Tashkin DP, Djahed B, Rose JE. 1988. Pulmonary hazards of smoking marijuana as compared with tobacco. New England Journal of Medicine 318:347-351.
Tashkin is also cited on this page:
“Anti-marijuana propagandists like to say that one joint per day is equivalent to one pack a day of cigarettes. This myth misrepresents a study by Dr. Tashkin, which found that one-joint-per-day marijuana smokers experienced a “mild but significant” increase in airflow resistance in the large airways greater than that seen in persons smoking 16 cigarettes today. However, the same study found that marijuana smokers did much better in other measures of respiratory health. A more accurate comparison based on studies by Dr. Tashkin’s group is that marijuana smokers absorb four times as much tar in their lungs than cigarette smokers per weight smoked.13 Given that a typical joint weighs about .4 - .5 grams, one-half as much as a tobacco cigarette, a rough equivalence is 2 cigarettes = 1 joint.”
I am reasonably convinced by the above comparison, which would put a cigarette at about one gram of smokeable substance. Note also that the above cited study is misquoted by anti-dopers because of the effects on the large airways of potsmokers’ lungs, allowing anti-dopers to make a 16 to 1 comparison. Dope smoke doesn’t go to the more sensitive parts of the lungs as much as cigarette smoke does. That’s my story, and I’m stickin’ to it.