APES Unit: Unit 8 — Aquatic & Terrestrial Pollution (Topics 8.13–8.15)
Big Idea: "The dose makes the poison." — Paracelsus, 1538
You will:
Estimated time: One block (90 min) or two 45-min periods. Data collection at home is 5–7 days.
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In 1538, the Swiss physician Philippus Aureolus Theophrastus Bombastus von Hohenheim — better known by his self-appointed Latin name Paracelsus — published a now-famous aphorism: Alle Ding' sind Gift, und nichts ohn' Gift; allein die Dosis macht, dass ein Ding kein Gift ist. "All things are poison, and nothing is without poison; the dosage alone makes a thing not a poison." This single sentence is the foundational principle of modern toxicology, and it remains the lens through which environmental scientists evaluate chemical risk five centuries later. Pure water can kill a marathon runner who drinks gallons of it (hyponatremia); the botulinum toxin used in cosmetic injections is — by mass — the most lethal substance known to science, yet a few nanograms relax a frown line. Toxicity is not a property of a chemical; it is a property of the relationship between a chemical, an organism, and a dose.
To quantify this relationship, toxicologists run controlled bioassays. A standard population of test organisms — typically lab rats, daphnia, fathead minnows, or radish seedlings — is divided into groups, and each group is exposed to a different concentration of the substance under investigation. After a fixed exposure window, the response (death, paralysis, failed germination, reduced root length) is recorded for each group. When response (% affected) is plotted against the logarithm of dose, the resulting curve is almost always sigmoidal — an S-shape with a flat lower tail, a steep middle, and a flat upper plateau. The dose at which 50 % of the population shows the response is called the LD50 (Lethal Dose 50) when the endpoint is death, or the EC50 (Effective Concentration 50) when it is a sublethal effect like impaired growth. The LD50 is a single number that lets regulators compare apples to apples: a substance with an LD50 of 2 mg/kg is roughly 1,000× more acutely toxic than one with an LD50 of 2,000 mg/kg.
The shape of the dose-response curve carries important information beyond LD50. The flat lower tail represents a threshold — a concentration below which the body's enzymatic detoxification, DNA repair, and excretion systems handle the exposure with no measurable effect. Threshold-type curves apply to most non-carcinogenic toxicants and are the basis for regulatory Reference Doses (RfDs) and tolerable daily intakes. Carcinogens and certain endocrine disruptors, in contrast, are often modeled with non-threshold (linear-no-threshold, or LNT) curves, where any exposure carries some incremental risk. The 1986 Safe Drinking Water Act, for example, sets the maximum contaminant level goal (MCLG) for known carcinogens at zero — a regulatory acknowledgment that no safe threshold can be confidently identified.
Two more concepts complicate environmental toxicology in important ways. The first is the difference between acute and chronic exposure. Acute exposure is a single high dose over a short window — a chemical-tanker spill, an accidental pesticide ingestion. Chronic exposure is repeated low doses over months to decades — atrazine in drinking water, particulate matter in urban air, microplastics in seafood. The two produce very different dose-response curves and very different health outcomes. A dose that is harmless acutely may, over a lifetime, produce cancer, neurodevelopmental damage, or reproductive failure. The second is the way certain substances move through ecosystems. Bioaccumulation is the buildup of a substance within a single organism, because intake exceeds the organism's ability to metabolize or excrete it; fat-soluble compounds like DDT, PCBs, and methylmercury accumulate in adipose tissue. Biomagnification is the increase in tissue concentration of a bioaccumulated substance as it moves up a food chain — a phytoplankton may carry 0.04 ppm DDT, a herring 2 ppm, a herring gull 75 ppm, and the eggshells of the gull's chicks fail to calcify properly. The chemistry that determines whether a pollutant bioaccumulates — lipid solubility, slow metabolic clearance, environmental persistence — is the same chemistry that drove Rachel Carson's 1962 alarm in Silent Spring and the eventual U.S. ban on DDT in 1972.
In this lab you will measure a true dose-response curve using Raphanus sativus (the common garden radish). Radish seeds are an ideal bioassay organism: they germinate in 3–5 days, root elongation is easy to measure with a ruler, and they respond to a wide spectrum of environmental toxicants — heavy metals, road salt (NaCl), excess nitrogen fertilizer, household detergents, herbicides — in a reproducible, dose-dependent way. You will prepare a serial dilution of one toxicant, expose 10 seeds per concentration, measure germination after 5 days, plot your data, and graphically estimate the EC50. The data almost always resolves into the same clean S-shape Paracelsus described in words half a millennium ago.
Background. In a real wet lab, you would prepare six petri dishes, each lined with filter paper, and pipette 5 mL of a different concentration of road salt (sodium chloride, NaCl) onto the paper. You would then place 10 radish seeds in each dish, cover, and incubate at 22 °C for 5 days. After incubation, you would count germinated seeds and measure root length. This simulation reproduces that protocol with realistic biological variation, so you can collect a full data set in minutes and analyze the dose-response relationship.
Atrazine is a chlorotriazine herbicide registered for use in 1958 and is, by mass applied, the second-most-used pesticide in the United States — roughly 70 million pounds per year, almost entirely on corn, sorghum, and sugarcane. It is also the most commonly detected pesticide in U.S. surface and groundwater. The U.S. EPA Maximum Contaminant Level (MCL) for atrazine in drinking water is 3 parts per billion (3 µg/L), set based on a one-year-old rat reproductive-toxicity LD50 study. The European Union banned atrazine in 2004, citing chronic exposure concerns; the U.S. completed a re-registration review in 2006 and again in 2020, retaining the 3 ppb MCL.
In 2002, U.C. Berkeley endocrinologist Tyrone Hayes published a study in Proceedings of the National Academy of Sciences reporting that male African clawed frogs (Xenopus laevis) exposed to atrazine at concentrations as low as 0.1 ppb — thirty times below the U.S. drinking water limit — developed reduced testosterone, retarded gonadal development, and, at higher exposures (25 ppb), fully functional ovaries and viable eggs in genetic males. Critically, Hayes's data did not produce a classical monotonic sigmoidal curve. The strongest feminizing effect occurred at 0.1 ppb, with a partial recovery at 25 ppb. This non-monotonic dose-response (NMDR) pattern is characteristic of endocrine-disrupting chemicals (EDCs) and is one of the most contested ideas in modern toxicology.
Hayes's findings have been replicated by multiple independent labs and challenged by industry-funded studies. The regulatory significance is profound: classical risk assessment assumes that "the dose makes the poison" — that response is a monotonic function of dose, that an LD50 exists, and that a safe threshold can be identified by testing high doses and extrapolating downward. If endocrine disruptors operate by hijacking hormone-receptor signaling at parts-per-billion or even parts-per-trillion concentrations — concentrations at which a normal toxicant would produce no effect — then high-dose testing is exactly the wrong way to characterize their risk, and the entire architecture of U.S. chemical regulation, codified in the 1976 Toxic Substances Control Act (TSCA), may systematically miss these compounds.
In 2016, Syngenta — the principal manufacturer of atrazine — settled a $105 million class-action lawsuit brought by 1,085 U.S. municipal water systems whose drinking water had been contaminated. In 2022, the EPA's Office of Inspector General released a report finding that the 2020 atrazine re-registration had been improperly influenced by political appointees who overrode the agency's own science staff. As of 2025, the 3 ppb MCL remains in force in the United States; atrazine remains banned in Europe.
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