A bong, also commonly known as a water pipe, is a smoking device, generally used to smoke cannabis, tobacco, or other substances. The construction of a bong and its principle of action is similar to that of the hookah, which is also called "water pipe." Smoking a bong contrasts with smoking a pipe or cigarette in two major ways: bongs cool the smoke before it enters the user’s lungs (making it easier to smoke), and a large amount of smoke is inhaled quickly as opposed to the smaller, more frequent, inhalations of pipe and cigarette smoking.
To use a bong, the base of the bong pipe is filled with water. The substance to be smoked is packed into the cone piece and ignited. The user places his/her lips inside the mouthpiece, forming a seal, and inhales, causing the flame to be drawn toward the substance. An inhalation is known as a "hit", "pull", "rip" or "toke." As the user inhales, the flame is drawn towards the substance, igniting it, and the smoke which is produced travels through a hollow pipe that is attached to the bottom of the bowl. The pipe enters via an airtight stem into a vessel containing water (or whatever other liquid is used). The smoke rises through the water, which cools the smoke, and then the smoke is trapped in the air chamber above the water. At the side or back of the bong, above the water level, there is usually a small air hole called a "carburetor", "carb", "choke", "shotgun", "shottie" or "rush hole". The user of the bong covers the carb with a finger until the material in the bowl has burnt away, then uncovers and pulls all the smoke from the bong into their lungs (called "clearing" the bong). Not all bongs use a carb, however. Many high end models have a removable bowl piece which works the same as a carb. These are usually known as "pull-stem" or "slide" bongs, and usually glass bongs that sometimes have glass-on-glass connections have this arrangement.
The rationale behind the use of a bong is the claim that the cooling effect of the water helps to reduce the chance of burning the mouth, airways, and lungs. The water can trap some heavier particles and water-soluble molecules, preventing them from entering the smoker's airways. This "filtration" can lead to the belief that bongs are less damaging than other smoking methods. However, a 2000 NORML-MAPS 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. An inhalation machine, adjusted to mimic the puff length of cannabis smokers, drew smoke through a standard bong, a small portable bong with a folding stem, a bong with a motorized paddle that thoroughly mixes the smoke with the water, and two different types of vaporizers. Comparisons to traditional non-filtered smoking methods were not included in these experiment.
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 increasing its water-contact area. This study suggests that a bong's smoke is less harmful than unfiltered smoke.
A bong may be constructed from any air- and water-tight vessel by adding a bowl and stem apparatus (a slide); there are reports of such things as videogame controllers or coffee tables being used as bongs.