indoor environments, and the application of modeling to assessing ETS exposures.

It is difficult to assess the ETS contribution to exposures because it usually exists in a complex mix of air contaminants from other sources. It is not practical, or possible, to monitor the full range of air contaminants associated with ETS, even under laboratory conditions. Chamber and field studies of ETS have monitored proxy contaminants as indicators of ETS. Most studies to date have been less than ideal because the component that was measured did not meet all the following criteria for an ETS tracer. A marker or tracer for quantifying ETS concentrations should be:

• unique or nearly unique to the tobacco smoke so that other sources are minor in comparison,

• a constituent of the tobacco smoke present in sufficient quantity such that concentrations of it can be easily detected in air, even at low smoking rates,

• similar in emission rates for a variety of tobacco products, and

• in a fairly consistent ratio to the individual contaminant of interest or category of contaminants of interest (e.g., suspended particulates) under a range of environmental conditions encountered and for a variety of tobacco products.

While a variety of measures have been used as proxies or tracers of ETS, no single measure has met all the criteria outlined above, nor has any measure been universally accepted or recognized as representing ETS exposure.

Carbon monoxide (CO) has been measured extensively both in chamber studies (Bridge and Corn, 1972; Hoegg, 1972; Penkala and De Oliveira, 1975; Weber et al., 1976, 1979a,b; Weber and Fisher 1980; Weber, 1984; Muramatsu et al., 1983; Leaderer et al., 1984; Winneke et al., 1984; Clausen, et al., 1985) and in occupied public and nonindustrial occupational indoor spaces (see Table 2–4) to represent ETS levels. Under steady-state conditions in chamber studies, where outdoor CO levels are known and the tobacco brands and smoking protocols constant, CO can be a reasonably reproducible indicator of ETS exposure. The variability