A toxicant is a chemical compound that has an effect on organisms. Toxicants are typically introduced into the environment by human activity.The effects depend on the concentration of the compound. Generally three concentration ranges can be delineated of all chemical compounds

• too little: the metabolic behaviour deviates more from the natural behaviour for decreasing concentrations
• enough: the metabolic behaviour is rather insensitive to changes in the concentration
• too much: the metabolic behaviour deviates more from the natural behaviour for increasing concentrations

The effect of toxicants can be quantified on the basis of the concentration inside the organism. The Dynamic Energy Budget theory can be used to specify the mode of action of the compound for the individual organism under study: one or more parameters of the DEB model change as function of the internal concentration.

Some toxicants are specifically developed to fight pests,the so-called pesticides such as

• insecticides to kill insects (or protect plants as some would prefer to stress)
• herbicides to kill "unwanted" weeds,
• food conservation products to kill micro-organisms that feast on our food

Many other toxicants are unwanted by-products of some production process, or accidental spoils.