Combustion products of cigarettes are the main contributors of ETS. Therefore, comparisons of concentrations of specific toxins and carcinogens in ETS (Tables 2–4 through 2–9) with corresponding concentrations in MS are relevant.

However, comparisons of MS and ETS can be appropriate only if one considers the important differences in chemical composition (including pH) and physicochemical nature (e.g., particle size, air dilution factors, and distribution of agents between vapor and particulate phases) between the two aerosols. Furthermore, ETS in indoor environments is often accompanied by pollutants in the work environment or derived from other sources, such as cooking stoves and space heaters. There are also important differences between inhaling ambient air and inhaling a concentrated smoke aerosol during puff-drawing. Finally, chemical and physicochemical characteristics based on analysis of smoke generated by machine smoking are not fully comparable with those of compounds generated when a smoker inhales cigarette smoke. Especially in the case of low-yield cigarettes, the yields of constituents appear to be different between machine smoking and human smoking (Herning et al., 1981).

Table 2–10 compares concentrations of some smoke constituents in the MS generated in the laboratory from one cigarette to those inhaled by a nonsmoker exposed to ETS for 1 hour.* The physical and chemical changes that occur in reactive smoke constituents during aging of the compounds after their emission into the environment must also be considered. For example, nitric oxide is generated in a cigarette during smoking and is chemically

*	The computations for exposures to nonsmokers for 1 hour in Table 2–10 are made using the equation: assuming an average respiratory rate of 10 L/minute. To convert from ppm (or ppb) to mg/m3, the following equation is used: where RT at 20°C is 24.45.