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Median lethal dose
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A median lethal dose (sometimes written as LD50) is a medical term for the dose of a chemical that has a 50% chance of killing you, typically a measurement of oral dose per kilogram of mass (for example, 50g/kg). It is used as a measurement to help determine safe doses of potentially harmful chemicals or drugs. It is related to the expression "The dose makes the poison", where the dose would kill half the population. For inhalation or injection routes, the LC50 is used to represent the median lethal concentration in a population. Sometimes, a comparable measure is known and reported called the LDLo or LCLo: the lowest known dose or concentration that caused a death (in humans, usually from accident or suicide). For ethical reasons, the median lethal dose is rarely known for humans.
Examples
For a simple example, MSG requires about 16.6g/kg given orally to have an LD50 in a rat. So if you had 100 rats at a mass of half a kg (0.5kg) living in the same conditions and fed all of them 8.3g of MSG, 50 would die from MSG poisoning.
On the flip side, stuff like Vitamin C has an even lower value, being 11.9g/kg. Some chemicals are hard to quantify, such as drinking massive amounts of water is very difficult as your body tries really hard to remove it from your body (it is assumed to be more than 90ml/kg).[1]
Sources
The National Institute for Occupational Safety and Health Registry of Toxic Effects of Chemical Substances (RTECS)[2] is regarded as the most authoritative source for minimum values of LD50, LC50, LDLo, and LCLo. Access to RTECS is by paid license, but other sources for these toxicity values can often be found, e.g., Material Safety Data Sheets (MSDS).