use a bowl, stem, and water to produce smoke. Most glass bongs are made from heat resistant borosilicate glass, allowing the bong to withstand repeated use and heat exposure without breaking. After the bowl has been packed and water has been inserted into the bong, the substance is lit and the smoke is drawn through water to produce a smoother smoke than other methods of smoking do. To smoke a bong, the smoker must inhale in the bong so bubbles containing smoke begin to come from the stem. Once the bong has a fair amount of smoke built up, either the carb is uncovered or the stem is separated from the bong, allowing the remaining smoke to be inhaled. However, a 2000 NORML-MAPS cannabis study found that "water pipes filter out more psychoactive THC than they do other tars, thereby requiring users to smoke more to reach their desired effect". In the study, smoke from cannabis supplied by the NIDA was drawn through a number of smoking devices and analyzed. This study looked at the tar to cannabinoid ratio in the gas in output by various bongs, as well as unfiltered and filtered joints, and vaporizers. The results showed that only vaporizers produced a better tar to cannabinoid ratio than unfiltered joints, but that within the cannabinoids produced, even vaporizers warped the ratio of THC (the psychoactive component of the smoke) to CBN (capable of producing medical benefits but is not psychoactive) in favor of CBN. This showed an unfiltered joint had the best tar to THC ratio of all, and bongs were actually seriously detrimental in this respect. MAPS also reviewed a study that examined the effects and composition of water-filtered and non-filtered cannabis and tobacco smoke. It found that when alveolar macrophages were exposed to unfiltered smoke, their ability to fight bacteria was reduced, unlike exposure to water-filtered smoke. It also found substantial epidemiological evidence of a lower incidence of carcinoma among tobacco smokers who used water-pipes, as opposed to cigarettes, cigars, and regular pipes. "It appears that water filtration can be effective in removing components from cannabis smoke that are known toxicants... The effectiveness of toxicant removal is related to the smoke's water contact area." Specially designed water pipes, incorporating particulate filters and gas-dispersion frits, would likely be most effective in this regard; the gas-dispersion frit serves to break up the smoke into very fine bubbles, thereby