Chronic Toxicity
Conducting Chronic Tests
In chronic and acute tests, we expose the same organisms to chemicals. The difference is that in chronic tests, the organism is exposed for a substantial portion of its lifetime. Then we look for effects on growth, reproduction and survival.
Testing through the Entire Life
Typically, young organisms are more sensitive to chemicals than older organisms. So we usually start tests with very young organisms and expose them to a chemical for as long a period of time as is practical. Since reproduction is generally a sensitive endpoint, tests are often continued until reproduction begins. Many species produces several offspring every few days, and during a toxicity test we get dozens to hundreds of young produced.
In all of these tests, the health of the treated organisms is compared with that of control organisms. Except for the presence of the chemical, both the treated and control organisms are treated the same. As long as the exposed organisms can survive, grow and reproduce as well as or better than the control group, we know our chemical is not harming them. This concentration without effect on the test chemical is called the "No Observed Effect Concentration" (or NOEC).
In all of these tests, the health of the treated organisms is compared with that of control organisms. Except for the presence of the chemical, both the treated and control organisms are treated the same. As long as the exposed organisms can survive, grow and reproduce as well as or better than the control group, we know our chemical is not harming them. This concentration without effect on the test chemical is called the "No Observed Effect Concentration" (or NOEC).
Conditions
We will test anywhere, from one to fifteen species in chronic toxicity tests with our chemicals. These species are tested one at a time in the laboratory under carefully controlled conditions. These conditions help to keep the organisms healthy and increase their growth rate and reproduction. In the environment, species do not live alone with all of their needs (food and water) provided. Instead, organisms depend on each other to provide food and shelter in a food-web or a community of organisms. The complex interactions that can occur in a community cannot be studied by isolating organisms and testing each organism separately in the laboratory. To make sure we are not upsetting any of the interactions among species, we sometimes conduct tests with many organisms at the same time. This research is called model ecosystem research.