According to the European Respitory Journal, it is now known that smoking the substance known as cannabis increases the risk of lung cancer much more than previously believed. Inhaling the smoke of one joint, is the equivalent of 20 cigarettes - the cannabis smoke contained twice the level of carcinogens, such as polyaromatic hydrocarbons, compared with tobacco cigarettes, and the "smokers end up with five times more carbon monoxide in their bloodstream (than tobacco smokers)." They say the method of smoking has something to do with it, being that it is more deeply and fully inhaled causing a "deposition of carcinogens in the airways". It's stated that generally a joint is also unfiltered and smoked to the end.
To read more on this study go here : http://news.yahoo.com/s/nm/20080129/hl_nm/cancer_cannabis_dc