Risk Characterization (Cont.)

where

hazard quotient

EPC

exposure point concentration, dose, body burden, or dietary

concentration reflecting exposure for relevant exposure areas;

these may be point estimates or summary statistics; this is

expressed in the same units as the TF

TF =

the selected toxicity factor appropriate for the chemical and

receptor.

HQs in excess of "1" are indicative of potential risk. Because these are often

based on threshold TF values, it is difficult to judge the magnitude of risk.

Nevertheless, the degree to which TF exceeds "1" provides a qualitative indication

of magnitude. Quotient methods can be utilized in weight-of-evidence and

probabilistic approaches. For the latter, distributions of TF and EPC values can be

derived (Suter et al. 1993).

Weight-of-Evidence or Lines-of-Evidence Approaches

The risk assessment can apply weight-of-evidence approaches when relating

multiple measurement end points to an assessment end point. Typically, these

approaches consider:

a. The weight or level of confidence given to the individual measurement end

points used to evaluate the assessment end point based on strength of

association between assessment and measurement end points, data quality,

and study design and execution as described earlier in connection with

selecting the measurement end points.

b. The magnitude of response of each measurement end point based on

absolute magnitude, spatial extent, and duration.

c. Concurrence among the measurement end points (i.e., if all the

measurement end points agree, this increases the weight of the overall

assessment).

These three elements permit the investigator to assess the overall weight of

evidence or to resolve information that may be disparate. The USEPA espoused

weight of evidence but provides no guidance for executing an approach. Menzie et

al. (1996) provide a quantitative and qualitative method based on the efforts of a

workgroup comprised of industry and government representatives. Sample,

Opresko, and Suter (1996) developed a qualitative approach. Both the weight-of-

evidence or lines-of-evidence approaches underscore the importance of being open,

consistent, and less subject to hidden biases.

Chapter 3 Ecological Exposure Assessment