LD50 = dose that kills 50% of a test population; lower LD50 = more toxic. A dose-response curve plots effect vs. dose and reveals the threshold where harm begins.
Pollution & Human Health
Acute (single high-dose) vs. chronic (long-term, low-dose) exposure. Dose, exposure time, and toxicity together determine health risk.
Wetlands & Mangroves
Filter pollutants, buffer floods and storm surge, and shelter juvenile fish. Draining, filling, and development destroy these high-value ecosystems.
Pathogens & Disease
Polluted water + poor sanitation → spread of waterborne pathogens and infectious disease, especially in regions lacking treatment infrastructure.
Endocrine Disruptors
Interfere with hormone systems even at very low concentrations. Sources: some pesticides, plastics (BPA), personal care products.
The key terms you must know
Point source / nonpoint source — identifiable single-origin pollution vs. diffuse, widespread pollution.
Eutrophication — nutrient pollution triggering algal blooms and oxygen depletion.
Bioaccumulation — toxin buildup within a single organism over its lifetime.
Biomagnification — increasing toxin concentration at each trophic level.
Persistent organic pollutants (POPs) — long-lasting toxic chemicals like DDT and PCBs.
LD50 — the dose lethal to 50% of a test population, used to compare toxicity.
Dose-response curve — a graph of effect vs. dose that reveals toxicity and the threshold dose.
Acute vs. chronic exposure — a single high-dose event vs. repeated long-term low-dose exposure.
Leachate — contaminated liquid percolating through landfill waste.
Primary/secondary/tertiary treatment — the three stages of sewage treatment.
Pathogen — a disease-causing organism spread via contaminated water.
Endocrine disruptor — a chemical interfering with hormone systems.
Key themes to remember
Toxins concentrate as they move up food chains. Top predators bear the highest biomagnification burden.
Not all toxicity is acute. Endocrine disruptors and POPs can cause harm through chronic, low-dose exposure rather than a single lethal dose.
Waste management follows a clear hierarchy. Prevention (reduce) is always preferred over disposal.
Treatment happens in stages of increasing sophistication. Each sewage treatment stage removes a different category of contaminant.
Water pollution and nutrient cycles are connected. Eutrophication is essentially human-caused nutrient cycle disruption (ties back to Unit 1).
Common exam traps
Don't confuse bioaccumulation with biomagnification. Bioaccumulation = within one organism. Biomagnification = across trophic levels.
A high LD50 doesn't mean a chemical is harmless. Chronic, low-dose, or endocrine-disrupting effects aren't captured by LD50 alone.
Eutrophication kills fish through oxygen depletion, not direct toxicity — the mechanism is decomposer respiration, same as in Unit 1.
Landfills and incineration both have tradeoffs — don't describe either as a "clean" solution; landfills risk leachate, incineration risks air pollution and toxic ash.
Point source pollution is easier to regulate than nonpoint source — because nonpoint sources lack a single identifiable origin.
Endocrine disruptors can act at very low doses — don't assume "more dilute means safe" the way you might for acute toxins.