It is difficult to be completely sure that a certain chemical leads to certain health problems. One reason for this uncertainty is that some effects may appear only very slowly. Some types of cancer, for example, may need twenty years to develop. Another reason for the uncertainty is the methods which are used to test the chemicals. It is impossible, of course, to test the chemicals on human beings, so experiments are conducted with animals. However, some scientists, especially scientists who are working for industrial companies, criticize the animal experiments. They claim, for example, that the animals are exposed to very large amounts of the chemicals. These amounts are much greater than the amounts which a human would absorb during one lifetime.