"and assumed
that the LD50's of all detected pesticides were additive (a not
unreasonable assumption). "

How reasonable this assumption is depends a lot on mode of action.  If you summed all the pesticides that kill by cholinesterase inhibition summing may be reasonable.  Or it may also be unreasonable if those agents have different biochemical degradation pathways and part of the LD50 results from degradation pathway saturation.  But, summing things that kill by other biochemical routes than cholinesterase inhibition with cholinesterase inhibitors is not at all reasonable.

Dick

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