Animal experiments by Philip Morris laboratories have demonstrated that sidestream smoke is three to four times more toxic than mainstream smoke (Schick and Glantz, 2005).

This complex picture becomes even more complicated over time. The ambient emissions from cigarettes can undergo further chemical reactions and deposit at varying rates on surfaces (Jenkins et al., 2000). For example, chemical analyses of aging sidestream smoke have shown that the carcinogenic nitrosamine NNK can form from nicotine and increase over time (Schick and Glantz, 2007). The chemical and physical properties of PM in secondhand tobacco smoke also change rapidly due, for example, to diffusion and coagulation, particle setting and impaction, and chemical reactions (Benner et al., 1989; Eatough et al., 1989); however, measurements of concentrations in smoking environments averaged over a day to a week have demonstrated similar ratios of PM to nicotine (Daisey, 1999; Leaderer and Hammond, 1991).

The toxicity of sidestream smoke appears to increase over time. Schick and Glantz (2006), using data from a series of inhalation experiments in rats conducted at Philip Morris, compared freshly generated sidestream smoke to sidestream smoke that had been aged for 30–90 minutes in a 30 m³ chamber. When the smoke doses were equalized on the basis of particulate material concentration, aged sidestream smoke was four times more toxic in 21-day exposures and two times more toxic in 90-day exposures than the freshly generated sidestream smoke. Moreover, current methodologic limitations prevent estimation of concentrations of highly reactive compounds; this is particularly important for the more reactive constituents of tobacco smoke and for estimating their concentrations in secondhand smoke dispersed in an unspecified space. A partial list of cigarette smoke constituents in mainstream and sidestream smoke in amounts exceeding 10 μg/per cigarette is presented in Table 2-1.

Tobacco smoke is a complex mixture of thousands of compounds. The composition of secondhand smoke changes over time; substances emitted from cigarettes can undergo chemical reactions and deposit on surfaces at various rates (Singer et al., 2002). Several approaches to evaluating and comparing human exposures to secondhand smoke, including measurement of airborne tracers or biomarkers of exposure (see Table 2-2), are useful.

In a 1986 report (NRC, 1986) on secondhand smoke (or environmental tobacco smoke, ETS), the National Research Council stated that “a marker or tracer for quantifying ETS concentrations should be: