The LD50 is a standardized measure for expressing and comparing the toxicity of chemicals. The LD50 is the dose that kills half (50%) of the animals tested (LD = "lethal dose"). The animals are usually rats or mice, although rabbits, guinea pigs, hamsters, and so on are sometimes used. In all these tests, the dose must be calculated relative to the size of the animal. The most common units are milligrams of chemical per kilogram of test animal (mg/kg or ppm).
|Chemical||Category||Oral LD50 in Rats (mg/kg)|
|Diflubenzuron ("Dimilin")||Chitin inhibitor||10,000|
Table 12.12.1 gives the LD50 values for some insecticides. In each case, the chemical was fed to laboratory rats. Note that the lower the LD50, the more toxic the chemical. Even adjusting for the test animal's weight, the LD50 for one species is often quite different from that for another. Thus any LD50 value gives only a rough estimate of the risk to humans. The way in which the chemical is administered also has a marked effect on LD50 values. The chemical may be fed, injected, applied to the animal's skin, etc., and each method usually generates a different LD50.
Because a single test may kill as many as 100 animals, the United States and other members of the Organization for Economic Cooperation and Development agreed in December 2000 to phase out the LD50 test in favor of alternatives that greatly reduce (or even eliminate) deaths of the test animals.