The definition of the respiratory input in experimental human exposure to volatile solvents was examined on theoretical grounds. The respiratory rate of input may be defined as the rate of uptake that equals the inhaled minus exhaled amount per minute. In the present paper the rate of respiratory input is defined as the rate of the functional intake (RFI) which equals the product of the inhaled concentration (CI) and a functional alveolar ventilation (Va). The functional Va is a virtual alveolar volume per minute which equilibrates completely with the mixed venous blood. Human subjects were exposed simultaneously to tetrachloroethene (PER, perchloroethylene) and trichloroethene (TRI) in order to study the consequences of the application of both definitions. It is shown that when using the uptake as the respiratory input some misleading conclusions may be drawn on (a) the dependence of the metabolised fraction on the duration of exposure, (b) the dependence of the kinetic characteristic on the duration and route of administration, and (c) the changes of the rate of metabolism during exposure due to physical exercise. The respiratory input defined as the rate of functional intake (RFI) rejects these misleading conclusions.
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