Scientists have compared marijuana and tobacco smoking on the basis of many different factors but have failed to find consis tent evidence that either substance poses a greater health risk than the other. On the one hand, marijuana joints have been shown to deliver at least four times as much tar to the lungs as tobacco cigarettes of equivalent weight. This difference is due to the lack of filters on joints and because marijuana smokers typically inhale a larger volume of smoke and take it more deeply into the lungs than tobacco smokers do. Marijuana smokers also tend to hold smoke in for a time before exhaling, exposing the lungs to even greater levels of cancer-causing agents. Symptoms of marijuana inhalation are easy to notice. A person using marijuana may: Second-hand exposure to marijuana smoke can lead to cannabinoid metabolites in bodily fluids sufficient for positive results on testing of oral fluids, blood and urine, and can lead to psychoactive effects. There is evidence of a weak dose-response relation between THC content of cannabis and effects on those exposed to second-hand smoke, including metabolites found in blood and urine, and psychoactive effects. There is evidence that the relation is mediated by environmental factors, including whether the air space is cannabis board of ontario ventilated, volume of air, number of marijuana cigarettes lit at 1 time, potency of the marijuana and number of smokers.