Pollution ultimately matters because of what it does to living bodies — including ours. Two very different threats dominate: pathogens (living agents like bacteria and viruses in contaminated water that cause diseases such as cholera) and toxins (chemicals whose harm depends on the dose). A pesticide sprayed at a harmless trace can become lethal after it concentrates step by step up a food chain, which is why bald eagles, not insects, nearly went extinct from DDT. This lesson gives you the tools to think quantitatively about harm: the dose-response curve, the LD50, and the twin processes of bioaccumulation and biomagnification — plus a guaranteed dose calculation.
Pathogens (disease-causing bacteria, viruses, protozoa, parasites) spread through contaminated water and poor sanitation, especially in developing regions. Key waterborne diseases: - Cholera (bacteria) — severe diarrhea/dehydration; from sewage-contaminated water. - Dysentery — bloody diarrhea from bacteria/protozoa. - Typhoid, hepatitis, and others.
Water quality is monitored via fecal coliform counts (indicating sewage contamination). Solutions: sanitation infrastructure, wastewater treatment (Lesson 26), safe drinking water, and disinfection. Other pollution-linked health issues include respiratory disease (air pollution, Lesson 23), cancers, and neurological damage (lead, mercury).
Toxicology studies how chemicals harm organisms. The central principle: "the dose makes the poison" — nearly any substance is harmful at a high enough dose, and many toxins are harmless at low doses.
A dose-response curve plots the response (e.g., % of test organisms harmed or killed) against the dose. From it: - LD50 (lethal dose 50%) = the dose that kills 50% of a test population. A lower LD50 means a more toxic substance (less is needed to kill half). - ED50 (effective dose 50%) = dose producing a given effect in 50%. - Threshold dose = the dose below which no effect is observed (for threshold toxins).
[DIAGRAM: Dose-response curve — x-axis = dose, y-axis = % of population responding (S-shaped curve rising from 0 to 100%). LD50 marked where the curve crosses 50% mortality. Note: a curve shifted LEFT (lower LD50) = more toxic; shifted RIGHT = less toxic.]
Toxins are classified by effect: carcinogens (cause cancer), teratogens (birth defects), neurotoxins (nervous system, e.g., lead, mercury), mutagens (DNA mutations), endocrine disruptors (hormone interference). Toxicity also depends on exposure duration: acute (short, high) vs. chronic (long-term, low-dose).
Two related but distinct processes for persistent (long-lasting), fat-soluble toxins: - Bioaccumulation — a toxin builds up within a single organism over time, because it's absorbed faster than it's excreted (stored in fatty tissue). - Biomagnification — the toxin increases in concentration at each higher trophic level up a food chain, because each predator eats many contaminated prey and retains the toxin. Top predators accumulate the highest concentrations.
[DIAGRAM: Biomagnification pyramid. Water/producers: 0.001 ppm DDT → small fish (1° consumers): 0.05 ppm → large fish: 2 ppm → fish-eating birds (top predator): 25 ppm. Each level's concentration rises sharply; the top predator carries the highest load.]
Classic example — DDT: the pesticide DDT bioaccumulated and biomagnified up aquatic food chains; in fish-eating birds (bald eagles, ospreys, pelicans) it caused eggshell thinning, crashing their populations. DDT was banned in the US (1972), and raptor populations recovered. Rachel Carson's Silent Spring (1962) publicized these effects.
Persistent organic pollutants (POPs) — synthetic, fat-soluble, long-lived chemicals (DDT, PCBs, dioxins) that biomagnify and travel globally. Mercury (from coal burning, gold mining) biomagnifies in fish, poisoning people who eat predatory fish (Minamata disease, Japan). Because they biomagnify, health advisories often warn against eating large predatory fish.
Environmental risk = probability of harm × severity of consequences. Risk assessment weighs hazard, exposure, and vulnerability; risk management adds economic, political, and social factors. People often misjudge risk (fearing rare dramatic events over common ones). The precautionary principle advises caution (limiting a substance/activity) when potential harm is serious even if the science is not fully certain.
Dose-response/LD50 interpretation and biomagnification are among the most tested Unit 8 concepts, and dose calculations (mg toxin per kg body weight) are common FRQ math. The DDT/mercury cases and the LD50-toxicity relationship appear repeatedly.
Chemical A has an LD50 of 5 mg/kg; Chemical B has an LD50 of 500 mg/kg. Which is more toxic?
Solution: Chemical A is more toxic. A lower LD50 means a smaller dose kills half the population, so less of it is needed to be lethal.
Interpretation: Lower LD50 = more toxic (it takes less to kill).
Distinguish bioaccumulation from biomagnification.
Solution: Bioaccumulation is the buildup of a toxin within one organism over its lifetime (absorbed faster than excreted). Biomagnification is the increase in concentration at each higher trophic level as toxins pass up a food chain, so top predators carry the highest concentrations.
Interpretation: Bioaccumulation = within an individual; biomagnification = up the food chain.
Explain why bald eagles were more affected by DDT than the insects it was sprayed to kill.
Solution: DDT is persistent and fat-soluble, so it bioaccumulates in organisms and biomagnifies up the food chain. Eagles are top predators, eating many contaminated fish that each ate contaminated prey, so DDT concentrated to high levels in the eagles, causing eggshell thinning and reproductive failure — far higher exposure than in the low-trophic-level insects.
Interpretation: Top predators integrate toxins from everything below them — highest biomagnified dose.
A toxin's LD50 is 10 mg per kg of body weight. Estimate the lethal-threshold (LD50) dose for a 70 kg adult.
Strategy: multiply the per-kg dose by body mass (dimensional analysis).
Solution:
10 mg/kg × 70 kg = 700 mg
Answer: ~700 mg would be the LD50 dose for a 70 kg person.
Interpretation: (mg/kg) × kg = mg — the kilograms cancel, leaving total milligrams. Toxin doses scale with body weight.
(A) Probability of harm × severity.
Persistent pesticides are fat-soluble and long-lasting, so they bioaccumulate in individual organisms (absorbed faster than excreted) and biomagnify up the food chain — each predator concentrates the toxins from all the contaminated prey it eats. Top predators therefore carry the highest concentrations and suffer the most harm. Example: DDT thinning bald-eagle eggshells (or mercury in predatory fish).
(a) 4 mg/kg × 25 kg = 100 mg. (b) 4 × 80 = 320 mg. (c) Children have lower body weight, so the same total dose delivers a higher dose per kg, and developing bodies are more sensitive.
FRQ rubric (10 pts):
- (a) 1 pt chemical is persistent/fat-soluble; 1 pt bioaccumulates within organisms and biomagnifies up trophic levels; 1 pt birds are top predators, so they carry the highest concentration. (3)
- (b) 1 pt setup 50 mg/kg × 2 kg; 1 pt = 100 mg. (2)
- (c) 1 pt names a human risk (eating contaminated fish → toxin/mercury-type poisoning, neurological/developmental harm); 1 pt explains via biomagnification/consumption. (2)
- (d) For each of two actions: 1 pt name + 1 pt justification (stop/ban the chemical at source; fish-consumption advisories; remediate the lake; treat industrial discharge); 1 pt both plausibly reduce exposure. (3)
A lake is contaminated with a persistent, fat-soluble industrial chemical. Small fish show 0.2 ppm, large predatory fish show 5 ppm, and fish-eating birds show 40 ppm. The chemical's LD50 in birds is 50 mg/kg.
(a) Explain, using bioaccumulation and biomagnification, why the birds have the highest concentration. (3 pts) (b) A 2 kg bird ingests the chemical. Calculate the total dose (in mg) that would represent its LD50. Show work. (2 pts) (c) Explain one risk to humans from this contamination. (2 pts) (d) Propose two actions to reduce the health risk, and justify each. (3 pts)
MC: 1. (B) Kills 50% of a test population. 2. (B) More toxic. 3. (B) Bioaccumulation. 4. (B) Biomagnification. 5. (C) Top predators like bald eagles. 6. (B) Persistent and fat-soluble. 7. (B) Pathogens in contaminated water. 8. (B) Harm depends on the amount received. 9. (B) Biomagnification. 10. (A) Probability of harm × severity.
Persistent pesticides are fat-soluble and long-lasting, so they bioaccumulate in individual organisms (absorbed faster than excreted) and biomagnify up the food chain — each predator concentrates the toxins from all the contaminated prey it eats. Top predators therefore carry the highest concentrations and suffer the most harm. Example: DDT thinning bald-eagle eggshells (or mercury in predatory fish).
(a) 4 mg/kg × 25 kg = 100 mg. (b) 4 × 80 = 320 mg. (c) Children have lower body weight, so the same total dose delivers a higher dose per kg, and developing bodies are more sensitive.
FRQ rubric (10 pts):
- (a) 1 pt chemical is persistent/fat-soluble; 1 pt bioaccumulates within organisms and biomagnifies up trophic levels; 1 pt birds are top predators, so they carry the highest concentration. (3)
- (b) 1 pt setup 50 mg/kg × 2 kg; 1 pt = 100 mg. (2)
- (c) 1 pt names a human risk (eating contaminated fish → toxin/mercury-type poisoning, neurological/developmental harm); 1 pt explains via biomagnification/consumption. (2)
- (d) For each of two actions: 1 pt name + 1 pt justification (stop/ban the chemical at source; fish-consumption advisories; remediate the lake; treat industrial discharge); 1 pt both plausibly reduce exposure. (3)
⭐ Exam strategy: Two anchors — lower LD50 = more toxic (counterintuitive, so drill it), and biomagnification concentrates persistent fat-soluble toxins in top predators. For dose math, mg/kg × kg = mg, and smaller bodies get a higher dose per kilogram from the same amount.
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