Here’s another reason to “keep off the grass.” Researchers report that marijuana smoke contains significantly higher levels of several toxic compounds — including ammonia and hydrogen cyanide — than tobacco smoke and may therefore pose similar health risks. The scientists found that ammonia levels were 20 times higher in the marijuana smoke than in the tobacco smoke, while hydrogen cyanide, nitric oxide and certain aromatic amines occurred at levels 3-5 times higher in the marijuana smoke, they say.