Where there are no existing fish-toxicity test data, it is recommended that a limit test is conducted using the LC50 or EC50 of the most sensitive test species of the other taxonomic groups tested. If no significant mortality occurs in this limit test (when compared with the control), it is unnecessary to undertake a full toxicity test and the endpoint of the fish test should be reported as greater than the concentration tested (> limit concentration).
Most substances exhibit a fairly similar degree of toxicity to both algae and crustacea, but there are occasions where a substance appears to be very much more toxic to one class of organism than the other. Should the apparent toxicity be found to be due to a physical effect, such as chelation, and not toxicity, OSPAR recommends that the limit test should be conducted at the LC50 or EC50 of the other species that was tested.
Limit-test results will only affect the ranking of products where the substance on which the HQ is based is more toxic to fish than to algae or crustaceans.