Substances that cause illness or death in living things.

A poison is a chemical that causes some dysfunction, or toxic reaction, in a person or other living thing. By definition, a toxin is a poisonous chemical of biological origin, produced by a microorganism, plant, or animal. In common usage, however, the words poison and toxin are often used interchangeably. The study of poisons is called toxicology.

Individuals differ in their tolerance of potentially toxic chemicals, and children are much more sensitive to poisons than adults. Individuals who are extremely sensitive to poisoning by a particular chemical are said to be hypersensitive to that chemical. People are exposed daily to potentially toxic chemicals in the environment through food, medicine, water, and the atmosphere.

Toxicity

All chemicals have the potential to be toxic when the dose (or exposure) is large enough to affect the functioning of...