[PAHs]), nitrogen oxides, pyridine, ammonia, nitrosamines, and hydrogen cyanide (Cal EPA, 2005b). The particulate phase, “tar,” consists of thousands more chemicals, including alkaloids, larger PAHs, tobacco-specific nitrosamines, polonium-210, nickel, cadmium, arsenic, and lead. Some compounds, such as cresols and PAHs, are partitioned between vapor and particulate phases.

About 85% of secondhand smoke is composed of sidestream smoke emerging from the burning tip of the cigarette and the remainder is exhaled in mainstream smoke (the smoke inhaled by a smoker when puffing on a cigarette) (Kritz et al., 1995). The measured sidestream emissions of chemicals are quite similar among a wide range of cigarette brands and styles, including regular, unfiltered, filtered, and “low tar, low nicotine” cigarettes.3 Although the composition of sidestream and mainstream smoke are qualitatively similar, there are substantial quantitative differences in composition between mainstream and sidestream smoke because the chemicals emitted in tobacco smoke change with temperature, oxygen concentration, pH, and the extent of combustion.4 Those factors are different in mainstream and sidestream smoke (Jenkins et al., 2000). As summarized elsewhere, most compounds from cigarettes are emitted in sidestream smoke in much higher amounts than in mainstream smoke (Cal EPA, 2005a; Jenkins et al., 2000; NRC, 1986). For instance, the ratio of the mass of benzene emitted into sidestream smoke compared to that emitted into mainstream smoke is approximately 10, while the corresponding ratio for the 4-aminobiphenyl is 30, and that, for nicotine is approximately 2. More recently, Lodovici et al. (2004) reported that the amount of total PAH in sidestream smoke “was about tenfold higher compared with mainstream smoke.” Nicotine is primarily in the particulate phase of mainstream smoke but predominantly in the vapor phase in secondhand smoke (Cal EPA, 2005a). This variable ratio from compound to compound between sidestream and mainstream smoke makes it impossible to characterize a passive smoking exposure as a simple fraction of the dose a smoker receives; such a comparison must be chemical specific (Hammond et al., 1993). Thus, while on average nonsmokers exposed to secondhand smoke have about 1% the cotinine (a metabolite of nicotine) as smokers, they have 14% as much 4-aminobiphenyl (a potent human carcinogen) adducted to their hemoglobin (Hammond et al., 1993).


The variability in mainstream smoke among these designs is due to the ventilation holes in some cigarettes; the ventilation dilutes the mainstream smoke when tested on cigarette machines, but not when smoked by smokers. The resultant variability in reported mainstream emissions among these cigarettes results in wide ranges in reported ratios of sidestream to mainstream smoke emissions, despite the consistency in the sidestream emissions.


Inhaling through the cigarette draws air to the burning end of the cigarette so that it burns hotter (just as embers in a wood stove burn hotter and turn red when air is blown on them) as it has more oxygen than when the burning tip is smoldering.

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