LD50, which is frequently encountered in safety data sheets, toxicology papers and venom research, is a measure that summarizes the acute toxicity of a substance in a single number. The abbreviation stands for lethal dose, 50%: the dose expected to cause death in half of the test animals when given once under defined conditions.
The smaller the value, the more toxic the substance, because a smaller amount produces the same result. This article explains the units of LD50, how it differs from LC50, how it is determined and how it translates into acute toxicity categories in chemical classification.
LD50 and LC50: units and difference
LD50 is defined in terms of dose and is usually given in milligrams per kilogram of body weight (mg/kg). This unit is used for oral and dermal administration. Venom research also reports subcutaneous, intravenous and intraperitoneal injection routes; some studies give the value in micrograms per animal.
LC50, by contrast, is not a dose but a concentration: the concentration in inhaled air, or in the water an organism lives in, that is expected to cause death in half of the test organisms over a set period. For inhalation toxicity, milligrams per litre (mg/L) are used for dusts, mists and vapours, and parts per million by volume (ppmV) for gases; the standard exposure time is 4 hours. For aquatic organisms, LC50 is also given in mg/L.
How is LD50 determined?
In the classical method, large numbers of animals were assigned to several dose groups and the 50% point was calculated statistically from the mortality rates. This approach was abandoned under the principle of reducing animal use; the OECD's classical acute oral toxicity guideline, TG 401, was deleted in the early 2000s. Today, three methods that reach a result with fewer animals are used for acute oral toxicity:
- TG 420, fixed dose procedure: Uses clear signs of toxicity rather than death as the endpoint.
- TG 423, acute toxic class method: Places the result directly in a classification range.
- TG 425, up-and-down procedure: Doses animals one at a time and estimates the LD50 with a confidence interval.
Guidelines TG 402 for dermal toxicity and TG 403 and TG 436 for inhalation toxicity are also available. The result varies with the animal species, sex, route of administration and the vehicle in which the substance is dissolved. An LD50 value should therefore not be compared with another value without stating the conditions under which it was obtained.
For venoms, even the way the sample is obtained can affect the result. A study on Androctonus crassicauda reported that venom collected by electrical stimulation was markedly more toxic than an extract prepared from telson tissue (Ozkan et al., 2007). This is why the way a sample was obtained is also recorded in venom characterization; the general framework of the analytical methods is on the analysis page.
From LD50 to category: acute toxicity limits
In the classification of chemicals, LD50 and LC50 values correspond directly to category limits. The Globally Harmonized System (GHS), the CLP Regulation in the European Union and the SEA Regulation in Türkiye use the same limits:
| Category | Oral | Dermal | Inhalation, dusts and mists (4 hours) |
|---|---|---|---|
| Category 1 | ≤ 5 mg/kg | ≤ 50 mg/kg | ≤ 0.05 mg/L |
| Category 2 | 5 – 50 mg/kg | 50 – 200 mg/kg | 0.05 – 0.5 mg/L |
| Category 3 | 50 – 300 mg/kg | 200 – 1000 mg/kg | 0.5 – 1.0 mg/L |
| Category 4 | 300 – 2000 mg/kg | 1000 – 2000 mg/kg | 1.0 – 5 mg/L |
In the ranges in the table, the lower limit is excluded and the upper limit included. Mixtures are not tested component by component; an acute toxicity estimate (ATE) is calculated from the known values of the components, and the same table is applied to this estimate. The label equivalent of a category is the pictogram, signal word and hazard statement: for categories 1 and 2, H300 is used for the oral route, H310 for the dermal route and H330 for inhalation. How the codes are read is explained in the article on the GHS06 pictogram and H statements.
Reading the product classification
In the safety data sheet published for our facility's product, the classification is given as Acute Tox. 2 (H300) by the oral route, Acute Tox. 2 (H310) by the dermal route and Acute Tox. 1 (H330) by inhalation. According to the table, these categories correspond to the ranges of 5 to 50 mg/kg, 50 to 200 mg/kg and 0.05 mg/L or less, respectively. This does not mean that a single LD50 value has been disclosed for the product; it shows the hazard range in which the classification was made. The role of the expert who performs the classification is the subject of the article on the chemical assessment expert, and the product forms are the subject of the Products page.
The limits of LD50
LD50 only summarizes the lethal outcome of a single, short-term exposure. It says nothing about the effects of long-term exposure, irritation, sensitization or the mechanism of action. Nor can the value be transferred directly from the animal species in which it was obtained to humans; LD50 is therefore not interpreted as a safety threshold or a dose for use.
In toxicology, the principles of reducing, refining and replacing animal use are becoming increasingly decisive. Cell-culture-based tests and computational approaches can, in some cases, provide data for classification without animal testing. In laboratories that work with venom, what matters most is knowing what the value means and setting up a system that prevents exposure; one part of that system is described in the article on personal protective equipment.
