Home / Courses / Environmental Science / Unit 6

Environmental Science Unit 6: Pollution

Pollution harms air, water and land. Some pollutants build up in food chains, and some come from sources that are hard to pin down.

Covers: Point and nonpoint sources, air pollutants, ozone depletion, eutrophication, biomagnification, solid waste, noise and light pollution

Study guide

Pollution is classified by its source. Point source pollution comes from a single identifiable place, like a factory pipe or smokestack, and is easier to regulate. Nonpoint source pollution comes from many diffuse sources, like farm and urban runoff, and is much harder to control.

Major air pollutants include carbon monoxide, nitrogen oxides, sulfur dioxide, particulate matter, lead and ground-level ozone. Primary pollutants are emitted directly; secondary pollutants form in the air, like photochemical smog from nitrogen oxides and volatile organic compounds in sunlight. Sulfur and nitrogen oxides form acid rain, which harms lakes, forests and buildings. Indoor air pollutants include radon, mold and smoke. The Clean Air Act regulates emissions, and scrubbers and catalytic converters reduce them.

Stratospheric ozone absorbs harmful UV radiation. Chlorofluorocarbons (CFCs) released from old refrigerants and aerosols break down ozone molecules, creating the ozone hole over Antarctica. The Montreal Protocol phased out CFCs, and the ozone layer is slowly recovering.

Eutrophication happens when excess nutrients (nitrogen and phosphorus from fertilizer and sewage) enter water. Algae bloom, die and decompose, and the decomposition uses up dissolved oxygen, creating dead zones where fish die.

Some toxins, like mercury, DDT and PCBs, are persistent and fat-soluble. Bioaccumulation builds them up in an organism over its life, and biomagnification increases their concentration at each higher trophic level, harming top predators.

Solid waste is managed by landfills, incineration, composting and recycling, but reducing waste is most effective. Noise and light pollution disrupt animal behavior and human health.

Cold water holds more dissolved oxygen than warm water. Biological oxygen demand (BOD) is the oxygen that decomposers use while breaking down organic matter. High BOD, from sewage or manure for example, lowers dissolved oxygen, so the two are inversely related. Eroded sediment makes water cloudy (turbid). This blocks sunlight from aquatic plants and algae, reducing photosynthesis and oxygen, clogs fish gills and makes it harder for predators that hunt by sight to find food.

Volatile organic compounds (VOCs) evaporate from gasoline, paints and solvents, and they react with nitrogen oxides in sunlight to form ground-level ozone and smog. Vapor recovery nozzles at gas pumps capture gasoline fumes during refueling so they don't escape into the air. Some aquatic macroinvertebrates, like mayfly and stonefly nymphs, need clean, oxygen-rich water. Others, like some worms and midge larvae, tolerate pollution. Scientists survey which ones are present to judge a stream's water quality.

Key ideas

• Point sources come from one place (a pipe); nonpoint sources are spread out (farm runoff).
• Eutrophication: excess nutrients → algae bloom → decomposition uses up oxygen → dead zones.
• Biomagnification: persistent toxins like mercury and DDT concentrate up food chains.
• CFCs destroy stratospheric ozone; ground-level ozone is a harmful smog component.
• Reduce, reuse, recycle: reducing waste matters most.

Formulas and rules

Common mistakes

• Mixing up good ozone (stratosphere) with bad ozone (ground level).
• Confusing bioaccumulation (within one organism) with biomagnification (up the food chain).

Words to know

Point source pollution

Pollution from a single identifiable source, like a pipe.

Eutrophication

Nutrient overload that causes algae blooms and low oxygen.

Biomagnification

Toxins becoming more concentrated at higher trophic levels.

CFCs (chlorofluorocarbons)

Chemicals that destroy stratospheric ozone.

Reduce, reuse, recycle

The 3 Rs; reducing is most effective.

Nonpoint source pollution

Diffuse pollution like farm and urban runoff.

Primary pollutant

Emitted directly, like CO or SO₂.

Secondary pollutant

Forms in the air, like ground-level ozone.

Photochemical smog

Forms from NOx and VOCs in sunlight.

Acid rain

Precipitation acidified by sulfur and nitrogen oxides.

Montreal Protocol

The treaty that phased out CFCs.

Dead zone

Low-oxygen water caused by eutrophication.

Bioaccumulation

Toxin buildup in one organism over its life.

Persistent pollutant

Breaks down slowly, like DDT, mercury and PCBs.

Sanitary landfill

A lined landfill designed to contain waste and leachate.

Clean Air Act

U.S. law regulating air pollutant emissions.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.