Why isn the value for total DDTs in the marine TEL/PEL, ERL/ERM, or AET benchmarks simply the sum of the individual three isomers?
A. This is a great example of why an understanding of the derivation of these various benchmarks is key to their proper application. The total DDT benchmark is not simply a mathematical summation of the other three benchmarks. It is derived by the same data evaluation process as any other compound. Therefore, it is responsive to the most potent of the isomers that were present in the samples used to generate the benchmark. The total DDT benchmark should only be used for screening when there is no more specific information about the constituent composition of total DDT. Because each isomer has its own potency, individual concentrations of each should be screened whenever possible.
