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doses from the various routes of entry discussed in the last slide should be summed to
yield an aggregate dose. Unless localized effects are of concern, certain route-specific
absorption factors are required in order to account for the intake (absorbed) portions of
these applied doses.
The typical default value for inhalation uptake is 50% or 100%, depending on the particle size involved. Inhalation uptake is referred to as the inhaled portion that is eventually brought to the lungs from the upper respiratory passage; and, as such, it is a factor more for quantifying the applied dose. The typical default for inhalation intake (i.e., that for the amount brought into the circulation from the lungs) is almost always 100% where absorption data are not available. So is the typical default for oral absorption. Both inhalation intake and oral absorption are assumed to occur (almost) instantaneously, partly due to the fact that the intestine and the lungs each have a larger surface for absorption compared to the skin (see Ross et al., 2000).
For dermal absorption, the default is either 50% or 100% of the applied dose whenever chemical-specific data are not available to support the contrary. Dermal penetration and dermal acquisition are known to be more protracted when compared to the oral route or inhalation. There is evidence showing that dermal absorption varies with anatomic regions. Dermal absorption is also shown to be dose-dependent, with lower applied dose apparently having higher absorption rate. Ross et al. noticed from literature review that the rat tends to overestimate human dermal absorption by 2- to 10-fold.