3.3.4.4 Exposure area decision units based on preferential exposure

As defined in the glossary, an exposure area is the specified area throughout which a potential receptor is exposed. Contact with all parts of the exposure area is equal through random movement within the exposure area over time. But as introduced in Section 3.3.4.1, in some situations standard-sized exposure areas do not reflect the known or suspected movement of the receptor; that is, the human or ecological receptor prefers some areas over others. Therefore, the CSM and the resulting sampling plan should consider the suspected or actual movement of the receptor by the use of smaller exposure area DUs within which the receptor can be expected to move randomly. Swing sets and sandboxes in residential yards are the classic example for human exposure; in such scenarios a child is expected to spend more time on and around such play areas than in the remainder of the yard. Movement within such smaller areas is expected to result in equal exposure to all parts of the area, and therefore it meets the definition of exposure area. In this example, the sampling plan should reflect two exposure area DUs, where two ISM samples are collected, one confined to the swing set area and the second across the remainder of the yard.