ABSTRACT
For each individual, there is a dose or dosage 1 that is just suffi cient to produce a specifi ed biological response. These just-suffi cient dosages are called effective dosages to distinguish them from administered dosages. The statistical properties of this response distribution have been reviewed extensively (Finney, 1971, 1978; Neumann and Kimmel, 1998) and summarized (Salem, 1987; Crosier and Sommerville, 2002). One of the more important aspects of the effective dosages is that the distribution of effective dosages for a homogeneous population is usually lognormal. The severity and probability of effects experienced depend on several factors but primarily on the amount of toxic
material received. For inhalation (IH) and percutaneous (PC) vapor toxicology, when the just-suffi cient dosage for an effect is relatively insensitive to exposure duration, the calculation of the probability of effect is straightforward (as explained in greater detail later in this chapter). However, when the effective vapor/aerosol IH or PC dosages are a function of exposure duration, a different approach is required. Instead of quantifying the amount of toxic material in terms of dosage, a new term, toxic load (TL), has been developed and extensively used to predict the probability and severity of the response (Fairhurst and Turner, 1993; Ride, 1995; Yee, 1996a, 1996b; Yee and Ye, 1996; Mannan, 2005). TL is normally expressed as some function of vapor concentration ( C ) and exposure duration ( T ), with TL equaling C n T being a typical form. Unfortunately, the introduction of the toxic load to solve the problem of duration-dependent effective dosages has led to other complications and issues.
