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Unit 8
7–10% of examThis unit follows pollution into water and soil, then into living things, including you. You'll learn where pollution comes from, why some chemicals pile up in food chains, how trash and sewage are handled, and how scientists measure how toxic something is.
Longer videos that cover the whole unit. Good for a first pass or a final review.
A point source of pollution comes from one spot you can identify, like a factory pipe or a sewage outlet. A nonpoint source is spread out, like fertilizer, oil and trash washing off farms and streets in runoff, which makes it much harder to control.
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Every species can only survive within a certain range of conditions, and pollution can push its habitat outside that range. Coral reefs are hurt by warmer water, sediment runoff and destructive fishing; oil spills coat and poison marine life; and plastic trash gets swallowed by or tangles up ocean animals.
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Endocrine disruptors are chemicals that interfere with hormones, the chemical signals that control growth, development and reproduction. They have been linked to birth defects, developmental problems and male fish developing female traits. DDT is a classic example; the weedkiller atrazine is often discussed too, though scientists still debate how much harm it does to wildlife at the levels found in nature.
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Wetlands and mangrove forests filter water, soak up floodwater, protect coasts from storms, store carbon and shelter young fish and birds. People drain and fill them for farms and buildings, pollute them and divert their water, which destroys these free ecosystem services.
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When fertilizer, manure or sewage adds extra nitrogen and phosphorus to water, algae grow out of control in an algal bloom. When the algae die, bacteria break them down and use up the dissolved oxygen, causing fish kills and dead zones. Water low in nutrients is called oligotrophic; water rich in nutrients is eutrophic.
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Thermal pollution happens when water is heated, usually because power plants and factories use it for cooling and send it back warmer. Warm water holds less dissolved oxygen, which can stress or kill fish and other aquatic life.
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Persistent organic pollutants (POPs) are human-made, carbon-based chemicals like DDT and PCBs that break down very slowly. They dissolve in fat instead of water, so they build up in living things, and they can travel long distances on wind and water before settling.
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Bioaccumulation is when a chemical builds up in one organism's body over its life because it takes the chemical in faster than it gets rid of it. Biomagnification is when the chemical becomes more concentrated at each higher level of a food chain, so top predators carry the most; DDT thinning bald eagle eggshells and mercury in large fish are classic examples.
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Much of the trash in the US goes to sanitary landfills, which are lined with clay and plastic, collect leachate (polluted liquid) and methane, and are covered with soil. Incineration shrinks trash but makes air pollution and toxic ash, and e-waste needs special handling because it contains toxic metals.
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The best way to deal with waste is to make less of it: reduce first, then reuse, then recycle. Composting turns food and yard scraps into soil, methane from landfills can be captured to make energy, and recycling saves resources but still takes energy and money.
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Wastewater treatment happens in stages: primary treatment lets solids settle out, secondary treatment uses bacteria to break down organic matter, and tertiary treatment removes leftover nutrients and chemicals. The water is disinfected with chlorine, UV light or ozone before release, and the leftover sludge has to be disposed of or reused.
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LD50 is the dose of a substance that kills half of a test population. It's used to compare toxicity: the lower the LD50, the more toxic the substance.
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A dose-response curve plots how a group of organisms responds as the dose of a substance goes up. You can read the LD50 off it, along with the threshold dose (the highest dose with no detectable effect) and the ED50 (the dose that causes a set effect in half the group); because testing on people would be unethical, these data usually come from animal studies.
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It's hard to prove exactly how one pollutant affects people, because everyone is exposed to many things at once and effects can take years to appear. Some links are clear, though: dysentery from sewage-contaminated water, mesothelioma from breathing asbestos, and lung damage from ground-level ozone.
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Pathogens are germs, like viruses, bacteria and parasites, that cause disease, and many are spread by vectors such as mosquitoes. Climate change, crowding and poor sanitation help them spread; diseases to know include malaria, tuberculosis, cholera, plague, West Nile virus, Zika, SARS and MERS.
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