What is pollution? Properties of pollutants
Pollution means adding a harmful substance or energy (heat, noise, light) to the environment faster than nature can break it down. The harmful thing is a pollutant.
- Primary pollutants come straight from a source (CO from a car). Secondary pollutants form in the air from others (ozone in smog).
- Point sources are one clear place (a factory pipe). Non-point sources are spread out (farm run-off, traffic).
- How bad a pollutant is depends on its toxicity, persistence (how long it lasts), whether it dissolves in fat or water, and how much there is.
- Biodegradable pollutants (sewage) break down; persistent ones (plastics, some pesticides, heavy metals) stay for years.
Air pollutants and their sources
- Carbon monoxide (CO): from incomplete burning of fuels. It is toxic because it binds to haemoglobin and blocks oxygen transport.
- Particulates (PM₂.₅, PM₁₀): soot and dust from diesel, burning and building sites. They get deep into the lungs; they also dim sunlight (global dimming).
- Sulfur dioxide (SO₂): from burning fuels that contain sulfur (coal). Causes acid rain and breathing problems.
- Nitrogen oxides (NOx): formed when nitrogen and oxygen in air react at the high temperature inside engines. Cause acid rain and smog.
- Carbon dioxide (CO₂): from all burning of fossil fuels. Not toxic in normal amounts, but a greenhouse gas.
- Indoor pollutants: smoke from wood or dung cooking fires, radon gas, mould, and fumes from paints and cleaners. Good ventilation and clean cooking gas help.
Acid rain, smog and temperature inversion
Normal rain is slightly acidic (pH about 5.6) because CO₂ dissolves in it. Acid rain has pH below 5.6 because SO₂ and NOx form sulfuric and nitric acid. It harms lakes, forests, soils and stone buildings.
Photochemical smog forms when sunlight acts on NOx and unburnt fuel vapours, making ground-level ozone and other irritants. It is worst on sunny, still days in traffic-heavy cities.
Usually air gets cooler as you go up, so warm dirty air rises away. In a temperature inversion, a warm layer lies above cooler air. The cool air cannot rise, so pollution is trapped near the ground.
Water, land, noise and thermal pollution
Eutrophication: extra nitrates and phosphates (fertilisers, detergents, sewage) → algal bloom → algae die → bacteria decompose them and use oxygen → fish die.
Sewage also carries pathogens (germs) that cause diarrhoea, cholera and typhoid.
Bioaccumulation is the build-up of a persistent pollutant inside one organism over its life. Biomagnification is the rise in its concentration at each higher level of a food chain (e.g. mercury or DDT: water → plankton → small fish → big fish → birds or people). Persistent organic pollutants and some chemicals called endocrine disruptors act like hormones and upset growth and reproduction.
Thermal pollution: hot water from power stations lowers the oxygen water can hold. Noise pollution (above about 85 dB for long periods) damages hearing and causes stress. Marine pollution includes plastics, oil spills and run-off.
Pollution and health: dose and response
"The dose makes the poison." A dose-response curve plots how many organisms are affected as the dose goes up. LD50 is the dose that kills 50% of a test group. A lower LD50 means a more toxic substance.
Reducing pollution
- At the source: cleaner fuels, electric and public transport, catalytic converters (turn CO and NOx into CO₂ and N₂), low-sulfur fuel.
- Before release: scrubbers remove SO₂ with lime; electrostatic precipitators and filters catch particles.
- Sewage treatment: screening → settling (primary) → bacteria break down waste (secondary) → removing nutrients and disinfecting (tertiary).
- Solid waste: Reduce, Reuse, Recycle, composting; then landfill or incineration with energy recovery.
Place matters too: valleys, still air, warm weather and slow rivers make the same pollution more severe; wind, rain and fast water spread and dilute it.
Try it
Leave a white cloth near a busy road for a week and one indoors. Compare the dust. Then in the 3D, find the lowest emissions and highest filters that bring back all 10 fish.
Key formulas and definitions
- Primary pollutant: released directly (CO, SO₂, NOx, PM)
- Secondary pollutant: formed in air (ground-level O₃, acids)
- Acid rain: pH < 5.6 (SO₂ + water → sulfurous/sulfuric acid; NOx → nitric acid)
- CO: incomplete combustion; blocks oxygen in blood
- Eutrophication: nutrients → algae → decay → low oxygen → fish die
- Biomagnification: concentration rises up the food chain
- LD50: dose that kills 50% of a test group; lower = more toxic
Worked examples
1. Why does a car engine produce nitrogen oxides even though petrol has no nitrogen?
The air drawn into the engine is about 78% nitrogen. At the very high temperature of burning, nitrogen and oxygen from the air react to form NO and NO₂.
2. A lake has DDT at 0.00005 ppm. Plankton have 0.04 ppm, small fish 0.5 ppm, big fish 2 ppm and fish-eating birds 25 ppm. How many times more is the level in birds than in water?
25 ÷ 0.00005 = 500 000 times. This rise up the food chain is biomagnification.
3. Substance A has LD50 = 5 mg/kg, B has LD50 = 500 mg/kg. Which is more toxic?
A. It needs a much smaller dose (5 mg per kg of body mass) to kill half the test animals.
4. Put in order what happens after fertiliser washes into a pond.
Nutrients rise → algae grow fast (bloom) → light is blocked, plants below die → algae and plants die → bacteria decompose them and use up oxygen → fish and other animals die.
Common mistakes
- Thinking all rain with any acid is "acid rain": normal rain is already about pH 5.6.
- Mixing up ozone: high up it protects us; at ground level in smog it is a harmful pollutant.
- Saying CO₂ is toxic like CO. CO is the poisonous one; CO₂ mainly warms the planet.
- Confusing bioaccumulation (one organism over time) with biomagnification (up the food chain).