LD50: What It Measures and What It Cannot Tell You

LD50 is a toxicology measurement, not a guide to a safe amount of a substance.

What LD50 measures

The median lethal dose, or LD50, is an estimate of the amount expected to cause death in half of a test population under specified conditions. It usually comes from animal experiments and is reported relative to body weight, commonly in milligrams per kilogram (mg/kg). It describes acute lethal toxicity. OECD: median lethal dose definition, Annex 1

An estimate belongs to its study: the species, route of exposure, substance tested, and observation period matter. OECD’s acute oral toxicity method reports an LD50 estimate and, when possible, a confidence interval reflecting statistical uncertainty. OECD Test Guideline 425

What the number cannot tell you

  • An animal value is not a personal human threshold. Species and individual differences complicate extrapolation. Multiplying an animal LD50 by a person’s weight does not establish a safe or lethal human dose.
  • Exposure routes are not interchangeable. A result for swallowing a substance cannot be treated as a result for another route. Inhalation studies commonly report a median lethal concentration, LC50, with an exposure duration.
  • Survival is not the same as absence of harm. LD50 does not identify the amounts that cause serious, nonfatal effects or predict what will happen to an individual. CCOHS: interpreting LD50 and LC50

Acute toxicity also does not establish long-term safety. Repeated exposure can produce different effects, and immediate and chronic toxicity cannot generally be predicted from one another. CCOHS: What Makes Chemicals Poisonous

Reading toxicity claims

Look for the original source, species, exposure route, method, and uncertainty behind a number. A bare number or a ranking across different experiments leaves out essential context. LD50 is one endpoint in a broader assessment, not an overall ranking of how safe substances are. CCOHS: using an LD50 value

Continue with compound risks and interactions or set, setting, purity, and dose. Suggest a correction.