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."