Cannabis has been used as a drug for thousands of years, but marijuana smoking has become prevalent in Western society only during the last 40 years (1,2). An annual survey conducted in the United States from 1975 to 2002 documented that marijuana is now the second most commonly smoked substance after tobacco (1,2). Marijuana smoke, like tobacco smoke, is generated by the pyrolysis of dried plant leaves. As a result, it shares thousands of chemical features in common with tobacco smoke, including qualitatively similar amounts of carbon monoxide, cyanide, acrolein, benzene, vinyl chlorides, aldehydes, phenols, nitrosamines, reactive oxygen species (ROS), and a variety of polycyclic aromatic hydrocarbons (3,4). The primary distinction between marijuana and tobacco is the presence of A9-tetrahydrocannabinol (THC) and other cannabinoids in Cannabis vs the presence of nicotine in tobacco (3,4). Although the hazardous effects of tobacco smoking have been extensively documented and include emphysema, chronic obstructive pulmonary disease (COPD), heart disease, and risk for developing several different types of cancer, studies on the health effects of marijuana smoking are less abundant. The common perception is that marijuana smoke is less toxic and that smoking a few marijuana joints per day has far fewer consequences than smoking a pack of tobacco cigarettes (5). However, the lack of filtering and differences in the smoking technique associated with marijuana use result in an approximately fourfold greater deposition of tar particulates in the lung than occurs from smoking similar amounts of tobacco (6). In addition, the concentration of pro-carcinogens such as benz-[a]-anthracene and benzo-[a]-pyrene are up to twofold higher in marijuana tar (3,7). The presence of irritants and pro-carcinogens in mari-

From: Forensic Science and Medicine: Marijuana and the Cannabinoids Edited by: M. A. ElSohly © Humana Press Inc., Totowa, New Jersey

Marijuana Introduction
Fig. 1. Habitual marijuana smoking delivers toxic smoke components and high concentrations of tetrahydrocannabinol to the lung with subsequent effects on the lung, respiratory cell function, and host immune defenses.

juana smoke and the enhanced deposition of these in the lung during smoking suggest that habitual smoking of marijuana might result in a spectrum of respiratory consequences similar to those described for tobacco smoking. Moreover, THC has recently been shown to exert potent biological effects on lung epithelial cells and on the immune system (8-10). Consequently, it is possible that regular exposure to marijuana smoke, a large proportion of which is THC, might predispose to lung injury, pulmonary infections, and/or tumor growth. This chapter reviews the current knowledge concerning the pulmonary and immune consequences of marijuana smoking and THC, as briefly outlined in Fig. 1.

Continue reading here: Acute Effects of Marijuana on Airway Physiology

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