On 23 May 2006, Donald Tashkin, M.D., Professor of Medicine at the David Geffen School of Medicine at UCLA in Los Angeles announced that the use of cannabis does not appear to increase the risk of developing lung cancer, or increase the risk of head and neck cancers, such as cancer of the tongue, mouth, throat, or esophagus.[144] The study involved 2252 participants, with some of the most chronic marijuana smokers having smoked over 22,000 marijuana cigarettes.[144][145][146][147] The finding of Donald Tashkin, M.D., and his team of researchers in 2006 refined their earlier studies published in a Dec. 17th 2000 edition of the peer-reviewed journal Cancer Epidemiology Biomarker and Prevention.[148] Many opponents of marijuana incorrectly cite the original finding of UCLA Medical Center from 2000 as "proof" that marijuana leaves the users at higher risk for cancer of the lung, and cancerous tumors,[140] even though the researchers at the UCLA Medical Center have revised their finding with a more in-depth study on the effects of the use of marijuana. This seemed to contradict assumptions made after some studies, like those from Dale Geirringer et al., which found that 118 carcinogens were produced when marijuana underwent combustion, and two carcinogens {2-Methyl-2, 4(2H-1-benzopyran-5-ol) & 5-[Acetyl benz[e]azulene-3,8-dione} formed when marijuana underwent vaporization with the Volcano Vaporizer.[51]