Guide
LD50 and LC50 Explained: Reading Toxicity Numbers
LD50 is the dose of a substance that kills 50 percent of a group of test animals under stated conditions, usually given in milligrams per kilogram of body weight. LC50 is the equivalent for a concentration in air or water. A lower value means a more acutely toxic substance. They measure short-term lethality, not long-term or non-lethal effects.
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What the terms mean
"LD" stands for lethal dose and "LC" for lethal concentration. The "50" means that the figure is the dose or concentration expected to be lethal to half the animals in a test. Because it is a statistical midpoint, it is more reliable than the lowest dose that kills any animal. The value is only meaningful with its conditions: the species (rat, mouse, rabbit), the route of exposure (oral, dermal, inhalation), the form of the substance, and for LC50, the exposure duration, typically a few hours. See the definition in the glossary.
Units and routes
- Oral LD50: mg per kg of body weight, by swallowing.
- Dermal LD50: mg per kg applied to the skin.
- Inhalation LC50: concentration in air for a stated exposure time; GHS classification uses 4-hour values, in ppm by volume for gases and in mg/L for vapors, dusts and mists.
- Aquatic LC50 / EC50: concentration in water that kills (or affects) half of fish or other aquatic organisms in a given time, found in Section 12.
Values for different routes are not interchangeable. A substance may be moderately toxic if swallowed but dangerous when inhaled.
Why lower means more toxic
A smaller dose is needed to kill half the animals, so the substance is more potent. If substance A has an oral LD50 of 50 mg/kg and substance B has 500 mg/kg, A is ten times more acutely toxic by that route. Be careful with this comparison because the species, vehicle and test conditions must be similar.
How GHS uses the numbers
The GHS sorts acute toxicity into categories based on ranges. For the oral route, the category boundaries (in mg/kg body weight) are: up to 5 for Category 1, above 5 up to 50 for Category 2, above 50 up to 300 for Category 3, above 300 up to 2000 for Category 4, and, in systems that adopt it, above 2000 up to 5000 for Category 5. Dermal and inhalation ranges are different. The hazard statements, such as H301 for toxic if swallowed, link to these categories; Categories 1 to 3 carry the skull and crossbones pictogram, and Category 4 carries the exclamation mark. Browse the pictograms. Not all countries adopt Category 5, and OSHA's HazCom does not include it.
What LD50 cannot tell you
- Chronic effects: cancer, reproductive harm and organ damage after repeated low exposures.
- Irritation, corrosion and sensitization: a substance with a high LD50 can still burn skin or cause allergy.
- Human variability: animal results are estimates; people differ in size, health and sensitivity.
- Mixtures: the sheet may give calculated acute toxicity estimates (ATE) rather than measured values; see SDSs for mixtures.
Use LD50 and LC50 alongside the other data in Section 11 and the exposure limits in reading exposure limits.
Reading an example line
A line like "LD50 oral rat: 500 mg/kg" says that in rat studies, 500 mg of the substance per kilogram of body weight was lethal to half the group. It is not a safe-dose or human-dose figure, and animal results cannot simply be scaled to people. Treat the number as a ranking tool and a classification input, not as a basis for deciding how much of a chemical is acceptable.
A value such as "> 2000 mg/kg" means that the highest dose tested did not kill half of the animals, so the true LD50 is above that dose. Always read the qualifier.
Frequently asked questions
What is LD50?
The dose of a substance that is lethal to 50 percent of a test animal population under stated conditions, normally expressed in mg per kg of body weight.
What is the difference between LD50 and LC50?
LD50 is a dose (amount given), used for oral and skin exposure. LC50 is a concentration in air or water, used for inhalation and aquatic toxicity, with an exposure time.
Does a high LD50 mean a chemical is safe?
No. A high LD50 means low acute lethality. The chemical can still be corrosive, irritating, sensitizing or harmful after repeated exposure.
Where is LD50 on the SDS?
In Section 11, Toxicological information, and sometimes in Section 2 when it explains the classification. Aquatic values are in Section 12.