LD50 and LC50 Explained: How Toxicity Values Appear in an SDS

LD50 stands for lethal dose, 50 percent. It is the amount of a substance, given in a single dose, that is expected to kill half of a group of test animals. LC50 is the equivalent for inhalation: the concentration in air that is lethal to half the test group over a stated exposure time. These numbers are the basis of the GHS acute toxicity hazard class, and they appear in Section 11 of nearly every Safety Data Sheet.

How to read the numbers

LD50 is expressed in milligrams of substance per kilogram of body weight (mg/kg), so a result of 200 mg/kg means 200 milligrams for each kilogram of the animal. The key rule is that a lower value means a more toxic substance, since a smaller dose is enough. A substance with an oral LD50 of 5 mg/kg is far more dangerous than one with 5,000 mg/kg. The route is always stated: oral, dermal (skin) or inhalation, and the species, commonly rat or rabbit. Inhalation LC50 values are stated in mg/L, ppm or mg/m3 for a stated time, usually four hours for gases and vapours, and the units matter.

From numbers to categories

The GHS converts these values into acute toxicity categories, with Category 1 the most toxic. The oral thresholds, in mg/kg, are broadly: category 1 at 5 or less, category 2 above 5 up to 50, category 3 above 50 up to 300, category 4 above 300 up to 2,000, and category 5 above 2,000 up to 5,000, which some jurisdictions do not adopt. Dermal values are higher for the same categories, and inhalation values differ for gases, vapours and dusts and mists. Each category maps to hazard statements. For example, category 3 oral is H301, toxic if swallowed, and category 4 oral is H302, harmful if swallowed. Categories 1 to 3 use the signal word Danger and the skull and crossbones pictogram, while category 4 uses Warning with the exclamation mark.

Acute toxicity estimates for mixtures

For a mixture, you do not usually have an LD50 for the exact product. Instead, GHS uses an Acute Toxicity Estimate (ATE) for each component, and combines them with an additivity formula weighted by concentration. A small amount of a very toxic ingredient can dominate the result, and unknown-toxicity components are handled with specific rules. Our guide to GHS mixture classification walks through the calculation, and our mixture ATE calculator lets you test a recipe.

What Section 11 should include

Section 11 should report the available data honestly, route by route: acute toxicity values with species and test conditions, skin corrosion or irritation, serious eye damage, sensitisation, germ cell mutagenicity, carcinogenicity, reproductive toxicity, and target organ effects after single or repeated exposure, plus aspiration hazard. It should also describe symptoms and likely routes of exposure. When there are no data, say that no data are available rather than leaving it blank or inventing a number, and, where a classification is based on a calculation, say so. Our guide to the 16 sections explains how this section fits with the rest.

Limits of LD50

LD50 measures only short-term lethality in one species, so it says nothing about cancer, reproductive harm, allergy or the effects of repeated exposure. Substances can have a high LD50 (low acute toxicity) and still be serious long-term hazards, as formaldehyde shows: it has acute toxicity, but its most important classification is carcinogenicity. See our formaldehyde guide. Also, modern regulations encourage using existing data, read-across and in vitro methods instead of new animal tests.

Where to find reliable values

Use harmonised classifications where they exist, peer-reviewed data, regulatory dossiers such as those under REACH, and supplier SDSs that cite their sources. Record the source and retrieval date so your classification can be audited. The substance pages in our chemical database show the harmonised hazard statements with their sources.