The 19th century chemical industry developed methods of assessing dangers of chemicals in the workplace. A factory is enclosed, relatively small. Workers come in contact with a small number of chemicals, easily tracked. Factory dangers are easy to check. When U.S. EPA and Environment Canada were created in the early 1970s, these agencies thought of the environment as one big workplace.
Epidemiology tries to link a chemical exposure to some observed ill.
Such a study might ask, for instance, if the level of organochlorines in your food causes declining sperm count. You may or may not find the criminal, but failure doesn't mean no criminal exists. Likewise, concluding no link exists between organochlorines and declining sperm count or any other human ill based only on an epidemiologic study would be bad use of science. Toxicologists inject or feed test-chemicals to lab animals, then extrapolate results to people. Rats and mice live short lives and the test chemical that shows no harm in the rodent may, in fact, injure long-lived humans.