Animal tests can easily miss effects that occur in less than 1 percent of the population, mostly due to limits on the number of animals in the test. To find effects in a small percentage of the population requires an unmanageable number of animals. Animal studies can produce a false sense of safety. A chemical that causes cancer in 8 percent of the exposed human population, might fly through the standard cancer study and produce no effects at all. If the entire U.S. population were exposed to this chemical, as a food additive, or example, this would translate into 22.4 million people with a cancer that did not even show up in an animal study.
Animal tests are notoriously bad at picking up learning and behavioral problems.
This is largely because the human brain is much more complex and vulnerable than the rat brain, and learning tests for rats are just not as easy to devise as learning tests for children. The toxic dose for these compounds was only determined after decades of human injury. Pound for pound, humans can get higher doses of many chemicals than laboratory animals - simply because of basic differences in biology and metabolism. Sorting fact from fiction. Drug Metabolism Reviews.