The study of the harmful effects of chemicals on humans and the environment is called "toxicology." The term "toxicity" is used to describe the pattern of harmful effects that a chemical can produce. Over four hundred years ago, a European doctor named Paracelsus established the basic premise of toxicology:

"All substances are poisons; there is none which is not; the dose differentiates a poison from a remedy."

- Paracelsus, 1493-1541

This statement says that everything can be toxic if the dose is high enough. For some highly toxic chemicals only a very small amount is needed to produce harm. For others a large amount may be necessary to produce harm. This difference in the amount of chemical needed to produce the same level of effect is called "potency."