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are presented to the test system as compared with that which occurs in the human situation. Despite these limitations, the use of cigarette smoke condensate (CSC) from MS has provided insight into the relative carcinogenic potential of various constituents in the MS of cigarettes. Similar studies using suitable condensates from SS and aged ETS could provide additional data on the effects of ETS.

IN VIVO ASSAYS ON ENVIRONMENTAL TOBACCO SMOKE

Exposure Methods in Laboratory Research

Several methods are available to evaluate the potential health effects of inhaled pollutants. Some common ones are whole-body exposure, head-only exposure, nose- or mouth-only exposure, lung-only exposure, or partial-lung exposure. Since the primary objective of an inhalation experiment is to determine the effects of the test substances or mixture on the respiratory system, it is preferable to eliminate or limit exposure through the skin or through ingestion (such as through contact with materials deposited on the fur or contaminated food and water).

Three methods have been used to determine the amount of material deposited in the respiratory tract (Phalen, 1984): direct measurement, calculations using airborne concentrations and uptake models, and calibration of the exposure apparatus using tracer substances. Direct measurement requires analysis of major components and their metabolites in tissues as well as in urine and feces or measurement of the amounts of material in the inspired and expired air. Aside from calculating dose based upon particle aerodynamic size and physiological data on lung function of experimental animals, tracers can provide reasonable estimates of exposure.

Inhalation exposure chambers are used for those studies in which whole-body exposure is desired. The ability to expose a large number of animals at one time and the absence of a need to restrain or anesthetize the animals are among the advantages in using this approach. There are, however, several major disadvantages. The animals are exposed through skin absorption and mouth ingestion and, in prolonged instances, by food and possibly water contamination. Animals tend to avoid exposure in such



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