This is takn from the article "Drug Prohibition in the United States:
Costs, Consequences and Alternatives" by E.A. Nadelmann (Science, Sept 1,
1989).

"Certainly it is not healthy to inhale marijuana smoke into one's lungs;
Indeed, the National Institute on Drug Abuse (NIDA) has declared that
'marijuana smoke contains mor cancer causing agents than tobacco smoke'.
On the other hand, the number of joints smoked by all but a very small
percentage of marijuana smokers is a tiny fraction of the 20 cigarettes a
day smoked by the average cigarette smoker; indeed the average may be one
or two per week than one or two per day. Note that NIDA defines a heavy MJ
smoker as one who consumes at least 2 joints 'daily'. A heavy tobacco
smoker, by contrast, smokes about 40 cigarettes a day"