In a recent study published by the journal Chemical Research in Toxicology, Health Canada researchers found higher levels of certain toxins in marijuana smoke than in tobacco smoke. The researchers used a smoking machine to compare cigarettes made from Players brand fine-cut tobacco with cigarettes made from cannabis produced by Prairie Plant Systems of Saskatoon, which grows medical marijuana under contract with Health Canada. The marijuana smoke had 20 times as much ammonia and five times as much hydrogen cyanide and nitrogen oxides, possibly due to higher levels of nitrate fertilizer traces in the marijuana. Then again, only the tobacco smoke contained the potent carcinogens known as tobacco-specific nitrosamines, and it had "moderately higher levels" of potentially hazardous compounds such as formaldehyde, acetaldehyde, and polycyclic aromatic hydrocarbons. Guess which comparison was emphasized in the press coverage.
"Cannabis smoke 'has more toxins,'" BBC News reported, warning that ammonia is "linked to cancer," while hydrogen cyanide and nitrogen oxides "are linked to heart and lung damage respectively." The Guardian agreed: "Cannabis smoke more toxic than puffing tobacco." Even EarthTimes ("Cannabis smoke more harmful than tobacco smoke") and New Scientist ("Inhaled cannabis is more toxic than tobacco smoke") went along with this gloss. But it's not what the researchers reported (emphasis added):
The combustion of any plant material will result in a complex mixture of chemicals, the composition and percentages of which depend on a large number of variables. The present study supports previous research and found that marijuana smoke contains qualitatively many of the same chemicals as tobacco smoke. This qualitative similarity is more important when assessing the risks for adverse outcomes than are the differences in level of a particular substance, which can change from sample to sample or from one smoking condition to another.
Even more important in assessing the health risks of pot smoking is a point that BBC News halfway acknowledged: While a joint a day would count as fairly heavy consumption for a pot smoker (since most pot smokers light up occasionally), the current mean for cigarette smokers is about 14 cigarettes a day (down from about 20 in 1993). This huge difference in dose is presumably the main reason pot smoking has not been linked to cancer, heart disease, or emphysema in epidemiological studies, despite the similarity between marijuana and tobacco smoke. Those still concerned about possible respiratory effects, of course, can avoid combustion products by using vaporizers (as do many patients who use marijuana as a medicine).
Addendum: A commenter asked about the efficacy of water pipes in reducing the hazards of pot smoking. They do not work nearly as well as vaporizers, partly because they filter out THC, encouraging people to smoke more for the same effect. Here is a report on research comparing water pipes to vaporizers, which says, "We learned early on that waterpipes don't help filter out undesirable particulate matter, although waterpipes may help reduce certain water-soluble gases."