Editing Median lethal dose (section)

== Conventions ==
The LD<sub>50</sub> is usually expressed as the mass of substance administered per unit mass of test subject, typically as [[milligram]]s of substance per [[kilogram]] of body mass, sometimes also stated as [[nanogram]]s (suitable for [[botulinum toxin]]), [[microgram]]s, or [[gram]]s (suitable for [[paracetamol]]) per kilogram. 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). For substances in the environment, such as poisonous vapors or substances in water that are toxic to fish, the concentration in the environment (per cubic metre or per litre) is used, giving a value of LC<sub>50</sub>. But in this case, the exposure time is important (see below).

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 LD<sub>50</sub> is not the lethal dose for all subjects; some may be killed by much less, while others survive doses far higher than the LD<sub>50</sub>. Measures such as "LD<sub>1</sub>" and "LD<sub>99</sub>" (dosage required to kill 1% or 99%, respectively, of the test population) are occasionally used for specific purposes.<ref>{{cite web|editor=Doris V. Sweet|date=July 1997|url=https://www.cdc.gov/niosh/pdfs/97-119-a.pdf|title=Registry of Toxic Effects of Chemical Substances (RTECS) / Comprehensive Guide to the RTECS|publisher=U.S. Department of Health and Human Services|url-status=dead|archive-url=https://web.archive.org/web/20130516165953/http://www.cdc.gov/niosh/pdfs/97-119-a.pdf|archive-date=2013-05-16 |id=DHHS (NIOSH) Publication No. 97-119}}</ref>

Lethal dosage often varies depending on the method of [[Drug administration|administration]]; for instance, many substances are less toxic when administered orally than when [[Intravenous therapy|intravenously]] administered. For this reason, LD<sub>50</sub> figures are often qualified with the mode of administration, e.g., "LD<sub>50</sub> i.v."

The related quantities LD<sub>50</sub>/30 or LD<sub>50</sub>/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]], for [[ionizing radiation]], as survival beyond 60 days usually results in recovery.

A comparable measurement is LCt<sub>50</sub>, which relates to lethal dosage from exposure, where C is concentration and t is time. It is often expressed in terms of mg-min/m<sup>3</sup>. ICt<span style="font-size:100%;"><sub>50</sub></span> is the dose that 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&nbsp;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&nbsp;mg/m<sup>3</sup> is equivalent to 10 minutes of 10&nbsp;mg/m<sup>3</sup> (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. In these cases, the lethal concentration may be given simply as LC<sub>50</sub> and qualified by a duration of exposure (e.g., 10 minutes). The [[Material safety data sheet|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 (ID<sub>50</sub>) 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 LD<sub>50</sub>s to some test animal. In [[biological warfare]] infective dosage is the number of infective doses per cubic metre of air times the number of minutes of exposure (e.g., ICt<sub>50</sub> is 100 medium doses - min/m<sup>3</sup>).