EPA Region X approach

California DEP notes that these UFs are in the range of chronic and subchronic

NOAEL comparisons in studies of uncertainty factors currently in preparation by

USEPA and other discussions of uncertainty factors.

EPA Region X approach

EPA Region X provides an approach for calculating toxicity factors (from EPA

Region 10, 1996 and based on Sigal and Suter 1989).

The features of this approach follow.

a. Apply a UF of 10 to convert from an acute or subchronic LOAEL value to

a NOAEL value.

b. Apply a UF of 5 to convert from a chronic LOAEL to a chronic NOAEL

value.

c. Apply a UF of 2 for interspecies extrapolations among families within the

same order for nonprotected species.

d. Apply a UF of 2 for interspecies extrapolations among orders within the

same class for nonprotected species.

e. Apply a UF of 2 to convert a NOAEL for a nonprotected species to a

related protected species.

The investigator should determine which approach is most appropriate for a site.

Often, this is based on geography inasmuch as different states or regions may have

developed different approaches for accounting for uncertainty.

The use of toxicity models

There are currently several efforts to develop models which may aid in the

assessing the toxicity factors in a comprehensive and additive manner. Examples

include the summed PAH model (Swartz et al. 1995) which attempts to predict the

toxicity of mixtures of PAH compounds using the concept of toxic units (Appendix

B). This model attempts to predict the probability of significant acute toxicity to

benthic infauna from exposure to sediment concentrations of a mixture of PAHs.

The obvious current limitation is that it does not address chronic effects: the critical

body residue or narcosis models (e.g. McCarty et al. 1992; McCarty and Mackay

1993) which attempt to assess the acute (and in some cases chronic) toxicity of

mixtures of hydrophobic neutral narcotic chemicals. This model is appropriate for

use when the exposure is expressed as a body burden.

Risk Characterization

This section describes the general methods used to make qualitative and

quantitative characterizations of risk. These include the use of the toxicity quotient

Chapter 3 Ecological Exposure Assessment