Step 3: Selecting and developing toxicity factors

Step 3: Selecting and developing toxicity factors

The selected toxicity factors must meet two general criteria:

a. They must relate to the assessment end point chosen during problem

formulation.

b. They must be specific to the receptor species identified during problem

formulation.

To meet the first of these criteria, the risk assessor must compare the toxic end

point and described effect to the assessment end point. For example, if the

assessment end point was protection of a commercial fishery, the toxicity factor

must have an end point and described effect which relates to the maintenance or

reproductive success of the species of commercial interest. A toxicity factor

associated with reduced reproduction in fish applies, but a factor which may indicate

eggshell thinning in shore birds is not applicable in this example. The two toxicity

factors may be very different in magnitude, but only the value appropriate to the

assessment end point applies. This is why it is so important to have an updated

summary of the toxicity factors and their associated effects. It is only through this

description that the risk assessor can judge whether a particular toxicity factor is

applicable to the assessment end point.

This subsection provides several methods to calculate toxicity factors. The

effects assessment should attempt to identify or develop toxicity factors for the

selected receptors. If a receptor-specific toxicity factor is available, the risk

assessment should use it. However, in many instances, such receptor-specific

information will not be available from the literature or the sediment evaluation

procedures, and the risk assessor will have to develop a toxicity factor based on

information from other species. In such instances, the risk assessor may extrapolate

from related information.

This subsection provides several methods for making these extrapolations.

The product of this section is a summary of available toxicological data and a

selection of a toxicity factor for each COC. The selection should include the reason

for selecting the particular toxicity factor and an explanation of how it relates to the

receptor of concern and the assessment end point.

Chapter 3 Ecological Exposure Assessment