8.11 Lethal Dose 50% (LD50)

What is LD50?

LD50 (Lethal Dose 50%) is the dose of a toxin required to kill 50% of a test population, usually determined through controlled lab studies on animals like rats or mice. It's measured in mg of toxin per kg of body weight (mg/kg). LD50 lets scientists compare the relative toxicity of different chemicals.

Interpreting LD50 values

A lower LD50 means a substance is more toxic — a smaller dose kills half the population. A higher LD50 means a substance is less toxic, since a much larger dose is needed to cause the same lethality. LD50 helps rank pesticides, chemicals, and pollutants by danger level for regulatory decisions.

Dose-response curves & limitations

LD50 is found using a dose-response curve, plotting % mortality against dose. Factors like age, sex, body size, and species affect toxicity, so LD50 in animals may not perfectly predict effects in humans. LD50 doesn't measure sublethal effects (like reproductive or developmental harm), which can still be serious at lower doses.

Key terms

LD50

The dose of a substance required to kill 50% of a test population, measured in mg/kg body weight.

Low LD50 value

Indicates a highly toxic substance, since only a small dose is needed to cause death in half the population.

High LD50 value

Indicates a less toxic substance, since a large dose is needed to kill half the population.

Dose-response curve

A graph plotting percent mortality against dose, used to determine the LD50 of a substance.

Limitation of LD50

Doesn't capture sublethal effects (e.g., reproductive/developmental harm) and results from animal testing may not fully apply to humans.

Try an AP-style question

What does LD50 represent?

1. The dose that kills 50% of test subjects
2. The dose needed for full recovery
3. The dose that kills all subjects
4. The dose that causes no effect