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Dose–response and LD50: a virtual toxicity test

GeographySoils, Biosphere & EnvironmentAges 17–18Virtual lab

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Use with my class

A virtual bioassay in three views. Toxicity test: expose lettuce seeds, brine shrimp larvae or small fish to a control and five concentrations of copper(II) sulfate, sodium chloride, ethanol or a detergent, count how many are affected after the standard time, and plot the dose–response curve on a log scale to read the LC50 (EC50 for seeds) and the threshold (NOEC and LOEC). Repeat to pool replicates, change the range and spacing of the doses, and add a second substance to see additive, synergistic or antagonistic effects. LD50 values: compare oral and skin LD50 values of common substances with the GHS hazard categories. Hormone mimic: a fish population exposed to a few nanograms per litre of the synthetic estrogen EE2 kills no fish directly but stops breeding and collapses.

Lesson: Toxicology: dose–response curves, LD50 and LC50, thresholds, routes of exposure, synergism and endocrine disruptors

What it shows

Toxicologists describe how harmful a substance is with a dose–response curve: the fraction of organisms affected rises from none to all as the concentration increases, usually as an S-shaped curve on a log scale. The concentration that affects half of them is the LC50 (EC50); the highest concentration with no clear effect is the NOEC. Each organism here has its own tolerance, so repeated tests scatter, and the curve is fitted by maximum likelihood. Values are illustrative but in realistic ranges. The LD50 table uses typical published values for rats; the fish population model is simplified.

How to use

On Toxicity test, choose Organism and Toxicant, set Lowest concentration and Dilution factor, then press Start test. Each test adds six rows to the table and points to the graph; press Repeat to add replicates. Tick Show fitted curve to see the LC50. Try a Second substance. On LD50 values, switch Route and tap a bar or choose a Substance. On Hormone mimic, set EE2 concentration and Exposure years and press Start test.

Parameters you can change

  • View Toxicity test, LD50 values, Hormone mimic
  • Organism Lettuce seeds, Brine shrimp, Small fish
  • Toxicant Copper(II) sulfate, Sodium chloride, Ethanol, Detergent (SDS)
  • Lowest concentration 0.01–10000 mg/L
  • Dilution factor ×2, ×3.16, ×10
  • Organisms per dish 10–30
  • Second substance None, B, additive, B, synergistic, B, antagonistic
  • Show fitted curve
  • Route of exposure Swallowed (oral), On the skin (dermal)
  • EE2 concentration 0–10 ng/L
  • Exposure years 1–10 years

Questions to explore

  1. Why is the dose–response curve plotted against the logarithm of the concentration?
  2. How do the LC50 and the NOEC tell you different things about the same toxicant?
  3. How can a substance that kills no fish still make a fish population collapse?

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