Toxicology is the study of the effect of chemical and biological substances on human health. It is the means by which scientists can evaluate the risks posed by the introduction of man-made or man-imposed species into the environment. These species can emerge through the disposal of industrial waste, chemical runoff from agricultural or other treatment processes, human excretion or disposal of pharmaceuticals, or the collateral deposition of substances such as heavy metals through mining or smelting. Obviously, toxicology is a pivotal field in the advancement of environmental chemistry because understanding what chemicals might lead to negative health consequences, and at what level of exposure, is at the very heart of strategizing the replacement and remediation of such substances. Toxicological studies can involve direct testing on animals, epidemiological studies on humans, or in vitro studies, in which chemicals and potential target species represented by tissue cultures are mixed directly in the laboratory.