Toxicity is the capacity of a substance to poison. Swiss physician Paracelsus (1493–1541) defined poison as follows: "What is there that is not a poison? All things are poison and nothing without poison. Solely the dose determines that thing is not a poison." The toxicity of a substance is therefore not an inherent property but the detrimental manifestation of its biochemical effect in a living system. The severity of a substance's toxicity is the function of its interaction with the physiology of a particular organism. For example, chocolate is moderately toxic to canines but minimally to other animals. Ingested in very large quantities, even vitamins can exhibit toxicity in humans.

The comparison of two substances in terms of their relative toxicity is difficult because every substance has its own mode of action and target organ(s). Hence, the short-term poisoning potential (acute toxicity) of a substance is measured by the amount needed to kill half the population of a test species, called the LD50 (lethal dose for 50 percent). The measurement is expressed as milligrams per kilogram of body weight (mg/kg). For example, administering to mice a substance with an LD50 for mice of 10 mg/kg would kill 50 percent of a population of mice. In environmental studies, the term used to measure toxicity in air or water is LC50 (lethal concentration for 50 percent), defined similarly to LD50 and expressed as parts per million, parts per billion, or milligrams per liter (mg/l).

The route of entry of a toxicant affects its LD50 value. Since the lethality of a substance is related to its ability to block vital cellular functions by interacting with specific biomolecules, the site of exposure, its dissemination speed, and the physiological importance of the target tissue all factor into its toxicity. For example, the common household pesticide dichlorovos has an LD50 for rats of 56 mg/kg if taken orally (through the mouth) and an LD50 of 15 mg/kg if injected intraperitoneally (into the abdominal cavity).

The toxicity of a chemical also varies from one animal to the next. The LD50 for the common insecticide Diazinon is 300 to 400 mg/kg in rats, while in birds it is 2.75 mg/kg. Theobromine, a chemical found in chocolate, is toxic to dogs but not to rodents. Although largely extrapolated from animal tests, a rating system for acute chemical toxicity for humans has been devised and is presented in Table 1.

Chronic or cumulative toxicity is manifested as a result of continuous exposure to a chemical. A common example is the "genotoxicity" of benzene, a chemical present in car exhausts and cigarette smoke. The metabolism of benzene in the liver results in the formation of highly reactive free-radicals. These in turn may cause damage to the genetic material of a cell, in some cases leading to cancer.


LD50 is the amount of a hazardous substance that results in the death of 50 percent of the individuals exposed. LD50 is commonly measured by exposing rats or mice to increasing amounts of the toxic substance until a dosage is reached that kills half the exposed animals within a certain time period (usually fourteen days).

—Dan M. Sullivan

Table 1. Toxicity rating for humans (70 kg [154 lb.] body weight).
Table 1. Toxicity rating for humans (70 kg [154 lb.] body weight).

Rating/Class Dose
SOURCE: Gosselin, et al. (1984). Clinical Toxicology of Commercial Products. Baltimore: Williams & Wilkens.
6. Super toxic <5 mg/kg
5. Extremely toxic 5–50 mg/kg
4. Very toxic 50–500 mg/kg
3. Moderately toxic 0.5–5 g/kg
2. Slightly toxic 5–15 g/kg
1. Practically nontoxic >15 g/kg

Although traditional investigations into the toxicity of chemicals in the natural environment have focused on animals, the toxicity of agrichemical and environmental pollutants to plants (phytotoxicity) has gained interest. Despite being well documented in literature, phytotoxicity is measured in various ways by agronomists and plant scientists; a standard quantification unit is therefore difficult to devise.

SEE ALSO Venom .

Hiranya S. Roychowdhury


Gosselin, Robert E.; Smith, Roger P.; Hodge, Harold C.; and Braddock, Jeanette E., eds. (1984). Clinical Toxicology of Commercial Products. Baltimore: Williams & Wilkins.

Klaassen, Curtis D., ed. (2001). Casarett and Doull's Toxicology: The Basic Science of Poisons. New York: McGraw-Hill Medical Publishing Division.

Internet Resources

Moorman, Gary W. "Plant Disease Facts: Phytotoxicity." The Pennsylvania State University. Available from .

"OSH Answers." Canada's National Occupational Health and Safety Resource. Available from .

Also read article about Toxicity from Wikipedia

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