may also aggregate with pollutants already in the environment and thereby change character. The composition of this complex mixture, known as ETS, has different physicochemical characteristics than the MS.

There are various terms in the literature that refer to the inhalation of ETS by nonsmokers, e.g., “passive smoking,” “involuntary smoking,” and “breathing other people’s smoke.” We will refer to the inhalation of ETS by using the terms “passive smoking” and “exposure to ETS by nonsmokers” interchangeably.

Exposure of nonsmokers to ETS is a function of several variables, one of which is the number of active smokers with whom the nonsmoker comes into contact throughout some period of time. The percent of the population who smoke steadily increased over the first two-thirds of this century but has declined more recently. In 1980, 32% of the adult population considered themselves to be cigarette smokers (U.S. Department of Commerce, 1984). This percentage, now roughly equal for men and for women, reflects a reduction of almost one-third in men since the publication of the first Surgeon General’s Report on Smoking and Health in 1964 (U.S. Public Health Service, 1964). Figure 1–1 shows the trends in cigarette usage between 1955 and 1985 for males and females. Table 1–1 gives cigarette consumption since 1900. Table 1–2 illustrates an overall increase in cigar and pipe smoking, followed by a decline during the past decade. The actual probability of exposure to ETS is complex, affected by ventilation rates, size of houses, restrictions on where tobacco products may be smoked, and changes in the cigarette itself. The consequence of Figure 1–1 is that the general probability of being exposed to some ETS for the nonsmoker has increased until quite recently.

The magnitude of exposure to ETS will depend upon the number of cigarettes and/or cigars and pipes smoked in a given environment, as well as other factors such as ventilation. Light smokers are more likely to stop smoking than heavy smokers, which might explain why over the past 30 years the number of cigarettes per smoker and the total consumption (Figure 1–2) have not declined as rapidly as the percentage of people who smoke (see also cigar and loose tobacco consumption in Table 1–2). From a peak consumption in the early 1960s, there has been a decline of