The SS generated between puffs originates from a strongly reducing atmosphere. Therefore, undiluted SS contains more combustion products that result from oxygen deficiency and thermal cracking of molecules than does MS. In addition, SS formation involves generation of higher amounts of compounds from nitrosation reactions. Consequently, SS differs substantially from MS.

Table 2–1 compares MS and SS from nonfilter cigarettes. During the consumption of one whole cigarette under standard smoking conditions, the formation of cigarette MS generated during 10 puffs (each 2 seconds) of a blended nonfilter cigarette requires 20 s and consumes 347 mg of tobacco. The formation of SS from the same cigarette smoldering requires 550 seconds and consumes 411 mg of tobacco. However, as shown with experimental cigarettes, the amounts of tobacco consumed during and between puffs depend greatly on the type of tobacco (Johnson et al., 1973a). In addition, MS and SS are generated at different temperatures. For example, under laminar atmospheric conditions, the SS of a smoldering cigarette enters the surrounding atmosphere about 3 mm in front of the paper burn line, at about 350°C (Baker, 1984).

The pH of the MS of a blended American cigarette ranges from 6.0 to 6.5, whereas the pH of SS is 6.7 to 7.5. Above a pH of 6.0, the proportion of unprotonated nicotine in undiluted smoke increases; therefore, SS contains more free nicotine in the gas phase than MS. The pH of SS of cigars is 7.5 to 8.7; pH values for pipe smoke have not been reported (Brunnemann and Hoffmann, 1974). Under conditions prevailing in MS, SS, and ETS, unprotonated nicotine is primarily present in the vapor phase; its absorption through the mucous membranes is faster; thus, its pharmacologic effect is different from that of unprotonated nicotine in the particulate matter (Armitage and Turner, 1970).

About 300–400 of the more than 3,800 compounds identified in tobacco smoke have been measured in MS and SS. Table 2–2 lists the amounts of selected substances reported to occur in the MS and in SS from the burning of a whole nonfilter cigarette and the range of the ratio of their amounts in SS/MS. A ratio greater than unity means that more of a substance is released in SS than in MS. The separation of the compounds in Table 2–2 into vapor phase and particulate phase constituents reflects the conditions

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