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Cannabis smoking involves inhaling vapors released by heating the flowers and subtending leaves of the Cannabis plants, known as marijuana. Alternatively, the cannabis plant flowers may be finely sifted producing kief, a powder especially rich in the oil-glands or trichomes which contain the highest amounts of cannabinoids. In exporting countries the kief is usually pressed under heat to form solid cakes of hashish, easily stored and shipped, which is widely marketed for smoking use. Cannabis is consumed for its hallucinogenic and sedative effects for recreation, to produce a feeling of euphoria, medically to stimulate the appetite  or to suppress nausea, or by inventors and artists in pursuit of creativity.
Smoking releases the main psychoactive chemical in cannabis, Δ9-tetrahydrocannabinol (THC), which is absorbed into the bloodstream via the lungs. It then mostly targets the brain, where it binds to cannabinoid receptors. The immune system also contains cannabinoid receptors and may modulate its function. The cannabinoid receptors receive the THC and other cannabinoids, leading to the feeling of a mental "high," which varies strongly by person. Studies have also found that the heating of cannabis (which can be achieved without the health hazards of combustion by means of a vaporizer) results in the production of additional THC from the decarboxylation of the non-psychoactive Δ9-tetrahydrocanabinoid acid (THCa).
While cannabis can be consumed orally, the bioavailability characteristics and effects of this method are different from smoking. The effect takes longer to begin, is typically longer-lasting, and can result in a more powerful psychoactive effect than expected.
A bong, is similar to a pipe, only it has a water-chamber  through which cannabis smoke passes prior to inhalation and a wide "mouth" typically around 3.8–5.1 cm (1.5–2.0 in) in diameter. Users fill the bong with water in order to cool the smoke and filter out particulate matter, sometimes also adding ice or other substances in place of water.
Joint is a slang term for a cigarette rolled using cannabis. Rolling papers are the most common rolling medium among industrialized countries, however brown paper, cigarettes with the tobacco removed, and newspaper are commonly used in the third world. Modern papers are now made from a wide variety of materials including rice, hemp, and flax. A joint can range in size, typically containing between 250–750 mg net weight of cannabis and/or fillers.
Pipes made for smoking cannabis, sometimes called pieces or bowls, are made of a variety of materials, including blown glass, metal fittings (except aluminum), ceramic, borosilicate, stone, wood, bamboo and other materials. Pipes vary greatly in shape and materials, and most are handmade. The common thread between them is that all of the pipes have a receptacle of some sort, a "stem" (which may be a long flexible tube as on hookahs and vaporizers) between the receptacle and a "mouthpiece". The smoking material is placed in the receptacle and affected with a heat source while air is drawn through the bowl and stem to the user.
Some pipes have a "choke" or "carb", a small hole usually located on the side of the bowl. These are used to clear the pipe of smoke, and to conserve material by stopping burning when enough smoke has been created. Blown-glass pipes and bongs are often intricately and colorfully designed. Some sub-types of pipes include one hitters, tays, bubblers, and standard pipes.
In India and Jamaica, the most commonly used pipe is the chillum.
A bucket (also known as a gravity bong, gravity or submarine) is a hydropneumatic accumulator used for smoking cannabis. It is best suited to hashish smoking, but can be used for smoking herbal cannabis with some reduction in efficiency. It has the advantage that by careful operation the smoke from a large quantity of hashish can be concentrated into a small volume which is less than the inspiratory capacity of the lungs, thereby providing a powerful "hit".
It consists of a bucket of water in which is placed a bottle —typically a 2-litre PET soft drink bottle— with the bottom cut off. A perforated grille of a suitably refractory material such as aluminium foil is placed over the neck of the bottle, and the cannabis is placed on the grille. A flame is held over the cannabis and the bottle is slowly raised out of the water, creating a negative gauge pressure inside the bottle which draws the smoke from the smouldering cannabis down through the grille. The grille is then removed, the mouth is placed over the neck of the bottle and the bottle is pushed back down into the water, causing the pressure to rise and forcing the smoke into the lungs.
A similar system using a plastic bag sealed around the cut-off bottom in lieu of water, which is drawn out with an attached handle as the cannabis is heated, is called a "chute".
In 1999 a paper summarized that "There is little direct evidence that THC or other cannabinoids are carcinogenic" but that epidemiological studies indicate that smoking cannabis may cause the development of head and neck carcinomas and for carcinomas of the respiratory tract in humans and "some experimental results suggest that cannabinoids may cause chromosomal damage" and that "cannabinoids represent several risks in terms of chronic toxicity". "By contrast, cannabis smoke is carcinogenic in rodents and mutagenic in the Ames test."  In humans it has been more difficult to definitively prove.
A 2008 study (later quoted in a 2012 survey of the field by the British Lung Foundation ) argued that the way cannabis is smoked compared to tobacco (such as the tendency not to use filters; and deeper, longer inhaling) made the risk of developing lung cancer from smoking a single cannabis cigarette daily the equivalent of smoking a pack of 20 cigarettes a day "despite similar carbon monoxide concentrations in the smoke". The 2008 study did not investigate the increased lung cancer risk of smoking a single tobacco cigarette daily, or twenty cannabis cigarettes daily, so the subsequent the claim by the 2012 British Lung Cancer foundation review which extrapolated that "a typical cannabis cigarette increases the smoker’s risk of developing lung cancer by 20 times the amount of one tobacco cigarette." was not derived from the study it based its claim on.
In September 2011, the University of Colorado Cancer Center published a paper which summarized the current status of Marijuana in the United States.
As of 2012, there is conflicting data on the correlations between various forms of cancer and cannabis use as studied in members of a US health management organization (HMO). In a study of 60,855 insured patients by HMOs showed "no increased risk of HNSCC, lung, colorectal, melanoma, or breast cancers in current or former cannabis smokers versus never smokers or experimenters when controlled for tobacco use, alcohol intake, and socioeconomic status."  However, the study did find an increase of prostate and cervical cancers. In another study of 105,005 HMO members, "found an increased risk of malignant primary gliomas (RR 2.8, 95% CI 1.3–6.2) in people who smoked cannabis once per month or more.  Smaller studies have implicated cannabis use in the development of bladder cancer  and testicular germ cell tumors.
As of September 2011 are currently 3 studies which show an increased risk of HNSCC and 7 (3 Large) studies which show no association. Studies which support HNSCC show that users of cannabis "had a 2.6-fold (95% CI 1.1–6.6) increased risk of HNSCC compared with blood-bank controls when adjusted for cannabis dose, duration of use, and confounding variables such as alcohol or tobacco use.  Similarly, heavy cannabis smokers in Northern Africa had an odds ratio of 2.62."  In addition, "a recent study found that human papilloma virus (HPV)-16 positive HNSCC was associated with increased cannabis smoking intensity (joints per month, p = 0.007), duration (in years, p = 0.01), and cumulative joint-years (one joint year equals one joint per day per year, p = 0.003) when adjusted for alcohol and tobacco use." 
Other studies have not shown a correlation between cannabis smoking and HNSCC. "Two small, population-based case–control studies of oral cavity and oropharyngeal cancers in England did not find an association between cannabis use and HNSCC"  and "Another small case–control study from New Zealand found no association between cannabis use and HNSCC once adjusted for tobacco and alcohol intake."  In large population based case control studies by the INHANCE Consortium, "no link between cannabis use and HNSCC was found when controlled for alcohol and tobacco use."  In another study from Boston "found that, after adjusting for confounders, 10–20 years of cannabis use was actually associated with a significantly reduced risk of HNSCC." 
As of 2012, there is conflicting data on the correlation of an increase in the incidence of lung cancer and cannabis smoking. "A systematic review evaluating 19 studies from 1966 to 2006  found no significant tobacco-adjusted association between cannabis smoking and lung cancer development despite evidence of precancerous histopathologic changes of the respiratory mucosa."  An INHANCE Consortium study showed that after analyzing 1200 lung cancer cases, there were no correlations between lung cancer and cannabis use. However, a pooled analysis of three studies of male cannabis smokers in North Africa found that the odds ratio for developing lung cancer was 2.4 (95% CI, 1.6–3.8) for cannabis smokers.  A case control study of patients with lung cancer under 55 years of age in New Zealand found an 8% (95% CI, 2–15) increased risk for each joint-year (one joint/day/year) of cannabis use.  This effect persisted only in the highest tertile of cannabis use (>10.5 joint-years of exposure) when adjusted for tobacco use (RR 5.7, 95% CI 1.5–21.6).
A major 2006 study compared the effects of tobacco and cannabis smoke on the lungs. The outcome of the study showed that even very heavy cannabis smokers "do not appear to be at increased risk of developing lung cancer," while the same study showed a twenty-fold increase in lung cancer risk for tobacco smokers who smoked two or more packs of tobacco cigarettes a day. It is known that cannabis smoke, like all smoke, contains carcinogens and thus subjects exposed persons to some increase in the risk of lung cancer, but THC, unlike nicotine, is thought to "encourage aging cells to die earlier and therefore be less likely to undergo cancerous transformation." Cannabidiol (CBD), an isomer of THC and another major cannabinoid that is also present in cannabis, also has been reported elsewhere to have anti-tumor properties. In January of 2012 another paper was published supporting the 2006 study. In it, another 20 year study of pulmonary function and marijuana exposure also concluded that "Our findings suggest that occasional use (1 joint a day for 7 years or 1 joint/week for 49 years) of marijuana for these or other purposes may not be associated with adverse consequences on pulmonary function. It is more difficult to estimate the potential effects of regular heavy use, because this pattern of use is relatively rare in our study sample; however, our findings do suggest an accelerated decline in pulmonary function with heavy use and a resulting need for caution and moderation when marijuana use is considered."
A 2008 study found that asymmetrical bullous lung disease occurs in marijuana smokers approximately 20 years earlier than tobacco smokers.
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