Population-based studies of adults have generally shown a strong dose-response relationship between FEV1 with dose measured either in terms of years smoked, the number of cigarettes per day, or the integrated dose, i.e., pack-years (U.S. Public Health Service, 1984). It is worthwhile noting, however, that in two major studies (Burrows et al., 1977a; Beck et al., 1981) the active smoking dose accounted for only about 15% of the variation of FEV1 even after age and height adjustment. Most of the variance could be attributed to the naturally occurring large variability in pulmonary function. Another reason the active smoking dose did not explain much of the variance is that the number of cigarettes an individual smokes cannot readily be translated into the dose of smoke that is delivered into the airways and parenchyma. Many factors, such as puff volume and lung volume at which inhalation starts, clearance rates, and airway geometry of the lungs of exposed individuals, will influence the dose and the distribution of the smoke within the lungs. Variability in individual susceptibility to the effects of chemicals deposited in the lung has been demonstrated in studies of animals (Evans et al. 1971, 1975, 1978).
The dose of cigarette smoke delivered to the lungs of nonsmokers exposed to ETS is both qualitatively and quantitatively different from mainstream smoke, being a small fraction of that delivered to the lungs of an active smoker (see discussions in Chapter 7). Exposure to constituents of tobacco smoke may begin in utero and continue throughout childhood through ETS exposure. During these periods, the lung is undergoing both growth and remodeling. Therefore, the lung of the fetus and young child may be particularly susceptible to environmental insults.
Despite qualitative differences between mainstream smoke, sidestream smoke, and ETS, it has been customary to assume that exposure to ETS approximates a low-dose exposure to tobacco smoke. The ability to measure responses to low doses depends on the shape of the dose-response curves, the sensitivity and specificity of the measurement tools available, and whether there is a threshold of exposure below which there is no response in any individual.