Median lethal dose
In toxicology, the median lethal dose, LD50 (abbreviation for “Lethal Dose, 50%”), or LCt50 (Lethal Concentration & Time) of a toxic substance or radiation is the dose required to kill half the members of a tested population. LD50 figures are frequently used as a general indicator of a substance's acute toxicity. The test was created by J.W. Trevan in 1927 but is now being phased out in favor of the Fixed Dose Procedure.
The term semilethal dose is occasionally used with the same meaning, particularly in translations from non-English-language texts, but can also refer to a sublethal dose; because of this ambiguity, it should generally be avoided.
Medical Subject Headings, Medical Subject Headings (MeSH) (ID D007928) defines LD50 as:
The dose amount of poisonous or toxic substance or dose of ionizing radiation required to kill 50% of the tested population. Year introduced: 1976(1971).
The LD50 is usually expressed as the mass of substance administered per unit mass of test subject, such as grams of substance per kilogram of body mass. Stating it this way allows the relative toxicity of different substances to be compared, and normalizes for the variation in the size of the animals exposed (although toxicity does not always scale simply with body mass). Typically, the LD50 of a substance is given in milligrams per kilogram of body weight. In the case of some neurotoxins such as batrachotoxin, one of the most deadly venoms known, the LD50 may be more conveniently expressed as micrograms per kilogram (µg/kg)of body mass.
The choice of 50% lethality as a benchmark avoids the potential for ambiguity of making measurements in the extremes, and reduces the amount of testing required. However, this also means that LD50 is not the lethal dose for all subjects; some may be killed by much less, while others survive doses far higher than the LD50. Measures such as 'LD1' and 'LD99' (dosage required to kill 1% or 99% respectively of the test population) are occasionally used for specific purposes.
Lethal dosage often varies depending on the method of administration; for instance, many substances are less toxic when taken by mouth than when intravenously administered. For this reason, LD50 figures are often qualified with the mode of administration, e.g. "LD50 i.v."
The related quantities LD50/30 or an LD50/60 are used to refer to a dose that without treatment will be lethal to 50% of the population within (respectively) 30 or 60 days. These measures are used more commonly within Radiation Health Physics, as survival beyond 60 days usually results in recovery.
A comparable measurement is LCt50 which relates to lethal dosage from exposure, where C is concentration and t is time. It is often expressed in terms of mg-min/m³. ICt50 is the dose which will cause incapacitation rather than death. These measures are commonly used to indicate the comparative efficacy of chemical warfare agents, and dosages are typically qualified by rates of breathing (e.g., resting = 10 l/min) for inhalation, or degree of clothing for skin penetration. The concept of Ct was first proposed by Fritz Haber, and is sometimes referred to as Haber's Law, which assumes that exposure to 1 minute of 100 mg/m3 is equivalent to 10 minutes of 10 mg/m3 (1 × 100 = 100, as does 10 × 10 = 100).
Some chemicals, such as hydrogen cyanide are rapidly detoxified by the human body, and do not follow Haber's Law. So in these cases the lethal concentration may be given simply as LC50 and qualified by a duration of exposure (e.g. 10 minutes). The Material Safety Data Sheets for toxic substances frequently use this form of the term even if the substance does follow Haber's Law.
For disease-causing organisms, there is also a measure known as the median infective dose and dosage. The median infective dose (ID50) is the number of organisms received by a person or test animal qualified by the route of administration (e.g., 1,200 org/man per oral). Because of the difficulties in counting actual organisms in a dose, infective doses may be expressed in terms of biological assay, such as the number of LD50's to some test animal. In biological warfare infective dosage is the number of infective doses per minute for a cubic meter (e.g., ICt50 is 100 medium doses - min/m3).
Animal rights concerns
Animal-rights and animal-welfare groups, such as Animal Rights International, have campaigned against LD50 testing on animals in particular as, in the case of some substances, causing the animals to die slow, painful deaths. Several countries, including the UK, have taken steps to ban the oral LD50, and the Organization for Economic Co-operation and Development (OECD) abolished the requirement for the oral test in 2001 (see Test Guideline 401, Trends in Pharmacological Sciences Vol 22, February 22, 2001).
- Oral LD50 of Vitamin C (ascorbic acid) in rats is 11.9 g/kg . 
- Oral LD50 of Grain alcohol: 10.6 g/kg in young rats, 7.06 g/kg in aged rats.
- Oral LD50 of Table Salt: 3 g/kg in rats
- Oral LD50 of Tetrahydrocannabinol (active ingredient found in Cannabis): 1270 mg/kg in male rats, 730 mg/kg in female rats.
- Oral LD50 of Nicotine: 50 mg/kg in rats.
- Subcutaneous LD50 of Batrachotoxin: estimated at 2 to 7 µg/kg in humans.
- LD50 of Polonium 210: estimated at 10 ng/kg (inhaled) to 50 ng/kg (ingested) in humans makes this one of the most toxic substances known. One gram in theory could poison 100 million people of which 50 million would die.
Other measures of toxicity
- Certain safety factor
- Fixed Dose Procedure
- Lowest published toxic concentration (TCLo)
- Lowest published lethal dose (LDLo)
- IC50 (50% inhibitory concentration)
- Draize test
- TCID50 Tissue Culture Infective Dosage
- EID50 Egg Infective Dosage
- ELD50 Egg Lethal Dosage
- Plaque forming units (pfu)
- What is an LD50 and LC50
- LD50 test ban welcomed
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