How do scientists measure how poisonous a chemical is?
Topic 8.12 Lethal Dose 50% (LD50): explain what LD50 measures and how it is used to compare the toxicity of substances.
A focused answer to APES Topic 8.12, covering what LD50 means, how it is measured and expressed (mass per body mass), how a lower LD50 means greater toxicity, the role of body mass, the limits of the measure, and its link to the dose-response curve, with a worked LD50 dose calculation.
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What this topic is asking
The College Board (Topic 8.12) wants you to explain what LD50 measures and how it is used to compare the toxicity of substances.
What LD50 means
Reading LD50: lower means more toxic
Limits of LD50
Why this matters
LD50 is the AP exam's standard toxicity calculation and the gateway to the dose-response curve of Topic 8.13. Its limits explain why chemicals such as the endocrine disruptors of Topic 8.3 are dangerous despite high LD50 values: they harm at low doses in ways lethality testing misses. On the exam you will typically be asked either to compare the toxicity of two substances from their LD50 values, remembering that lower means more toxic, or to calculate a lethal dose for an organism of a given body mass by multiplying the LD50 (in mg per kg) by the mass in kilograms. A frequent trap is being given LD50 values where the more dangerous chemical has the smaller number, so it pays to state the rule explicitly before answering. Pairing the calculation with a sentence on what LD50 does not capture, such as chronic or sublethal effects, shows the fuller understanding examiners reward.
Try this
Q1. State whether a chemical with a lower LD50 is more or less toxic. [1 point]
- Cue. More toxic (less of it is needed to kill half the population).
Q2. Explain why LD50 is expressed per kilogram of body mass. [2 points]
- Cue. Larger organisms need a larger total dose to reach the same effect, so giving the dose per kilogram of body mass allows toxicity to be compared fairly across organisms and species of different sizes.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)4 marksSection II (FRQ). (a) Define LD50. (b) Explain whether a chemical with a lower LD50 is more or less toxic. (c) Explain why LD50 is expressed per unit of body mass. (d) Calculate the lethal dose for a 70 kg person of a chemical with an LD50 of 5 mg per kg of body mass.Show worked answer →
A 4-point FRQ on LD50.
(a) Define (1 point): LD50 is the dose of a substance that kills 50% of a test population, usually expressed as mass of substance per unit of body mass (mg per kg).
(b) Explain (1 point): a lower LD50 means less of the substance is needed to kill half the population, so it is more toxic.
(c) Explain (1 point): larger organisms need a larger total dose for the same effect, so expressing the dose per kilogram of body mass allows comparison across body sizes and species.
(d) Calculate (1 point): 5 mg/kg times 70 kg equals 350 mg.
Markers reward the kills-half-the-population definition, the lower-LD50-means-more-toxic point, the body-mass scaling explanation, and the correct 350 mg calculation.
AP 2018 (style)1 marksSection I (multiple choice). Substance X has an LD50 of 2 mg/kg and substance Y has an LD50 of 500 mg/kg. Which statement is correct? (A) X is more toxic than Y (B) Y is more toxic than X (C) They are equally toxic (D) Toxicity cannot be compared this way. Justify your choice.Show worked answer →
A 1-point MCQ on LD50. The answer is (A).
A lower LD50 means a smaller dose is lethal, so the substance is more toxic. X (2 mg/kg) needs far less to kill half the population than Y (500 mg/kg), so X is more toxic. Y is less toxic (B is wrong), they are not equal (C), and LD50 is exactly how toxicity is compared (D). The trap is reading a higher number as more dangerous; for LD50, lower means more toxic.
Related dot points
- Topic 8.13 Dose Response Curve: interpret a dose-response curve and explain the difference between threshold and linear (non-threshold) responses.
A focused answer to APES Topic 8.13, covering how to read a dose-response curve, the difference between threshold and non-threshold (linear) responses, the role of the LD50 and ED50, why some chemicals have no safe dose, the limits of extrapolating from animal studies, with a worked dose-response reading example.
- Topic 8.14 Pollution and Human Health: describe how pollutants and pathogens affect human health and how infectious diseases spread through the environment.
A focused answer to APES Topic 8.14, covering the health effects of pollutants (heavy metals, particulates, toxins), waterborne and infectious diseases (cholera, typhoid, dysentery), pathogens and disease vectors, the difference between acute and chronic effects, dysentery and access to clean water, and prevention, with a worked disease-rate reasoning example.
- Topic 8.3 Endocrine Disruptors: explain what endocrine disruptors are and how they affect organisms by interfering with hormones.
A focused answer to APES Topic 8.3, covering what endocrine disruptors are, examples (atrazine, DDT, BPA, phthalates), how they mimic or block hormones, their effects on reproduction and development, why low doses can matter, and how to reduce exposure, with a worked frog-feminisation reasoning example.
- Topic 8.7 Persistent Organic Pollutants (POPs): describe the properties of persistent organic pollutants and explain why they are especially harmful.
A focused answer to APES Topic 8.7, covering the defining properties of persistent organic pollutants (persistence, fat solubility, long-range transport, toxicity), examples such as DDT, PCBs and dioxins, why they bioaccumulate and biomagnify, their effects, and international controls, with a worked persistence reasoning example.
- Topic 5.6 Pest Control Methods: compare chemical and biological pest control and explain the pesticide treadmill and the evolution of pesticide resistance.
A focused answer to APES Topic 5.6, covering chemical pesticides, their benefits and costs, biological control, the pesticide treadmill, pesticide resistance through natural selection, and broad-spectrum versus narrow-spectrum pesticides, with a worked resistance calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)