Table 1.

US approaches to account for human variability in derivation of reference values for risk assessment

Approach	Intended Use	Method
Default adjustment factor (AF)	Adjust for total intra-species variability.	AFH = 10.
Default subfactor approach	Adjust for intra-species variability in metabolism (TK) and tissue response (TD).	AFH ≤ 10 (usually 3X each for AFH-TK and AFH-TD).
Chemical-specific adjustment factor (CSAF) or data-derived extrapolation factor (DDEF)	Adjust for chemical-specific intra-species differences in metabolism (TK) and tissue response (TD).	AFH-TK and AFH-TD based on case-by-case TK/TD data.

AF_H Intra-species human variability adjustment factor, AF_H-TK Intra-species human variability adjustment subfactor for variation in kinetic chemical metabolism, AF_H-TD Intra-species human variability adjustment subfactor for variation in target tissue response