Intra-species extrapolation of hazard characterisations

There is variation between individuals concerning their individual sensitivities to experience health effects. In some scenarios the aim is to perform assessments for the sensitive individuals instead of the average individuals for which the points of departure are derived. If this is the case, then extrapolation is required to translate hazard characterisations derived for the average individual to hazard characterisations for a sensitive individual. In traditional exposure assessments, a safety of 100 is commonly used as a margin of safety, that is assumed to be composed of a interspecies extrapolation factor (factor 10), and inter-individual extrapolation factor (factor 10). I.e., the hazard characterisation defined for the sensitive individual is defined as

$${\mathit{HC}}_{\mathtt{sens}}=\frac{1}{{\mathit{f}}_{\mathtt{intra}\mathtt{-}\mathtt{species}}}\cdot {\mathit{HC}}_{\mathtt{avg}}$$

Here ${\mathit{f}}_{\mathtt{inter}\mathtt{-}\mathtt{species}}$ denotes the intra-species factor. An alternative to using a fixed safety factor, is to define intra-species variability explicitly using a lognormal distribution, characterised by a geometric mean (GM) equal to 1 and a geometric standard deviation (GSD). For risks calculations, this distribution could be used to sample individual hazard characterisations. This effectively converts the description of hazard characterisations to include variability, with an unbiased central value.