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Clearing the Smoke: Assessing the Science Base for Tobacco Harm Reduction
quickly from the lungs of cigarette smokers as well, although this has not yet been demonstrated experimentally in smokers.
The nitrate content influences the carcinogenic potential of smoke. Nitrogen oxides formed during pyrolysis are free-radical precursors of the polycyclic aromatic hydrocarbons (PAHs). As nitrate concentration in tobacco increases, the synthesis of benzo[a]pyrene (BaP), a carcinogen, is inhibited. Air-cured tobaccos have higher content of nitrates than sun-cured or flue-cured tobaccos and, therefore, lower BaP content. However, the higher the nitrate concentration, the higher are the levels of tobacco specific-nitrosamines (TSNAs), also a known carcinogen.
Hundreds of additives are used in the manufacturing of cigarettes. Several additives are known to have toxic properties. For example, glycerol is a humectant used in cigarettes. Glycerol may lead to the formation of acrolein, a ciliotoxic agent, and diethylene glycol can be converted to ethylene oxide, a carcinogenic compound (Hoffmann and Hoffmann, 1997). Eclipse, that heats but does not burn tobacco, uses glycerol particles as the carrier for nicotine. The glycerol level in smoke from this product is much higher than in conventional low-yield products. As described above, some additives can influence other tobacco constituents (e.g., the role of ammonia in nicotine protonation and TSNA formation).
Although some additives have toxic potential, the concentration of these compounds is low (other than those listed above) and their relative contribution to overall toxicity compared to compounds such as TSNA, BaP, and carbon monoxide (CO) is not definitively known. The toxicity of individual ingredients is sometimes well described, but little is known about how toxicants affect the body when smoked in combination (U.S. DHHS, 2000). The current emphasis on additive disclosure focuses on the consumer’s right to know and on understanding better which additives are used to increase the acceptability (e.g., by improving taste and smoothness) of the product to the consumer.
Menthol is a common additive used for flavor and customer acceptability. Early advertisements for menthol products claimed a “soothing” effect on irritated throats. Menthol can be added to cigarettes in several ways including addition to the tobacco shred through an ethanol spray and addition to the filter or packaging. Approximately 3 mg of menthol is added per cigarette. The rest is lost through the filter, sidestream smoke, and in packaging (Browne, 1990). Because menthol is an anesthetizing agent, it has been hypothesized that it may be easier to inhale deeper when smoking a mentholated cigarette. This might also help explain why there are higher rates of lung cancer among blacks despite their lower daily cigarette consumption than whites, who tend to not smoke mentholated cigarettes. Various studies, however, have produced mixed results on the subject (Carpenter et al., 1999; Clark et al., 1996; Gaworski et al.,