8.5.4.3 Decision unit versus exposure unit

Some DUs selected for a project may not match the definition of an exposure unit, for example, when DUs are designed solely for comparisons with cleanup values to reach "remediate/don't remediate" decisions for specific plots. Another situation is when there are different exposure units over the same area for different receptors but only one set of DUs for a site. Depending on how DUs are defined, it is possible to have a DU that is larger or smaller than an exposure unit. There are no statistical procedures in place to estimate an EPC when the DU is larger than the exposure unit, although some possibilities for estimating high end concentrations are discussed in Section 3.5. Similarly, when an exposure unit is composed of more than one DU, unless replicate ISM are available for each, there is no established method to combine results from the DUs to produce a robust, demonstrably conservative EPC. Limitations of methods in situations where the DUs do not match the exposure units may be a significant obstacle to the use of ISM data in risk assessment. Methods for overcoming these limitations are discussed in Section 4.2.