Describing The Extent Of Exposure

So far, we've talked about two of the steps involved in characterizing exposure: describing the source and describing the distribution of the stressor or disturbed environment. Now we'll examine the third key step: describing the extent of the exposure.

A receptor does not have to actually contact a stressor for adverse effects to occur. As illustrated in the reveal activity below on co-occurrence, stressors can simply be nearby to take effect. For some assessments, assessors may need to consider past, current, and future scenarios.

Reminder! Exposure can be described in three ways:

• Contact between a stressor and receptor.
• Co-occurrence between a stressor and receptor.
• Uptake of a stressor by a receptor.

Some stressors must contact receptors to cause an effect. The extent of contact is determined by:

1. The amount or extent of the stressor in an environmental medium.
2. The activity or behavior of the receptors.

An example of the second case is the fact that predators often hunt in specific areas (rather than at random points) at a specific site such as a meadow or forest edge. It is unlikely that such predators would contact contaminants in a wetland, for instance, because it does not feed or rest there.

Some ways to quantify chemical exposure have been developed and are elaborated on in the In-Depth unit below. Contact between biological stressors and ecological receptors are more difficult to measure. Risk assessors usually rely on professional judgment to evaluate this type of contact. Contact is often assumed to occur in areas and during times where the stressor and receptor are both present.

Co-Occurrence    text-only version

Absorption and Uptake    text-only version

Evaluating Contact with Chemical Stressors

Relevant guidelines section(s): 4.2.1.3