The Coastal Resource Coordinator's Bioassessment Manual

HAZMAT 93-1–Toxicity Tests
response (the endpoint) in the species used for the test. Positive control samples help
establish a dose-response relationship for the test species. These tests demonstrate the
responsiveness of the test organisms for the endpoint of the test.
EC 50 and LC 50
Toxicologists often report the toxicity of a substance as either an EC 50 or an LC 50 value. An
EC 50 is the concentration of a particular chemical associated with a sublethal response in 50
percent of the test organisms; EC stands for effective concentration. An LC 50 is the
concentration associated with death of 50 percent of test organisms; LC stands for lethal
concentration. The results of toxicity tests on environmental samples are rarely reported as
either EC 50 or LC 50 values because the samples consist of a mixture of chemicals, often
unknown, and the toxicity cannot be associated with one specific chemical. However, if
tests are run on a series of dilutions of the test sample, then the results can be reported as
either an EC 50 or an LC 50 with regard to the sample dilution, not the chemical
concentrations in the sample. For example, if toxicity tests were performed on a dilution
series of a contaminated water sample and 50 percent of the test organisms were killed by
the solution containing 40 percent test sample, the LC 50 would be 40 percent.
Other Factors Influencing Toxicity Tests
In all experimental measurements, and especially those involving living organisms, it is
important to identify outside factors that may interfere with a correct interpretation of the
test results. The effects of unknown co-contaminants, impurities, and degradation products
(such as ammonia or sulfides) in the test material can further complicate interpretation of
the toxicity of the test material. Another factor that can confuse interpretation of toxicity
test results is seasonal variation in the test organism's sensitivity to the substances being
tested. The physical characteristics of the sample matrix can influence toxicity test results
both by controlling the bioavailability of the contaminants and by directly affecting the test
organisms. The latter is of special concern when the test organisms do not occur naturally
in the area from which the test samples were taken. In the case of water these physical
characteristics include pH, salinity, and temperature, while in sediments grain size, total
organic carbon (TOC), and water content must also be considered. Very high concentrations
of fines in apparently uncontaminated sediments have been found to be toxic to the the
amphipod, Rhepoxynius abronius, who prefers fine sandy sediments (DeWitt et al., 1988).
The toxicity was believed to be due to either the fine grain size, the high sediment water
3-13 August 1997