Toxicity Range by WHO


Toxicity is the capacity of a substance to poison or toxicity is the degree to which a substance can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell (cytotoxicity) or an organ (organotoxicity), such as the liver (hepatotoxicity). By extension, the word may be metaphorically used to describe toxic effects on larger and more complex groups, such as the family unit or society at large.
A central concept of toxicology is that effects are dose-dependent; even water can lead to water intoxication when taken in large enough doses, whereas for even a very toxic substance such as snake venom there is a dose below which there is no detectable toxic effect.


Measuring toxicity
Toxicity can be measured by its effects on the target (organism, organ, tissue or cell). Because individuals typically have different levels of response to the same dose of a toxin, a population-level measure of toxicity is often used which relates the probabilities of an outcome for a given individual in a population. One such measure is the LD50. When such data does not exist, estimates are made by comparison to known similar toxic things, or to similar exposures in similar organisms.


What is LD50
The LD50 is the dose of a pesticide that will kill half of a group of test animals from a single exposure by either the dermal, oral or inhalation routes. The larger an animal, the greater the lethal dose required to kill it. The LD50 is expressed in milligrams per kilogram of body weight of the test animal. For example, if a pesticide has an oral LD50 value of 10 mg/kg, and the test animals each weigh 1 kg, 50% of the animals would die of poisoning if each ate 10 mg of the pesticide. If the test animals weigh 25 kg each, the lethal dose to kill 50% of these animals would be 10 mg/kg X 25 kg = 250 mg each. In the same way we can determine LC50, LD90, LC90.


Note: A pesticide with a lower LD50 is more toxic than a pesticide with a higher number because it takes less of the pesticide to kill half of the test animals. The smaller the LD50 value, the more toxic the pesticide.