Air pollution: sulfur dioxide and carbon monoxide

Ecology and the environment · Human influences on the environment · note 1 of 6

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Air pollution: sulfur dioxide and carbon monoxideSpec 4.12

In short

Sulfur dioxide and carbon monoxide are air pollutants from burning fuels. Sulfur dioxide dissolves in rain to form acid rain, which lowers the pH of lakes so fish die and damages tree leaves. Carbon monoxide, from incomplete combustion, combines with haemoglobin more strongly than oxygen, so less oxygen is carried to the tissues for aerobic respiration.

Burning fuels releases pollutant gases into the air. Two that you must know about are sulfur dioxide and carbon monoxide.

Sulfur dioxide (SO₂)

Sulfur dioxide is released when fossil fuels that contain sulfur, such as coal and oil, are burned (for example in power stations). It dissolves in rain water to form acidic solutions, so the rain becomes acid rain.

Acid rain lowers the pH of lakes and rivers. Many fish and other aquatic organisms cannot survive in acidic water, so their numbers fall.
Acid rain damages the leaves of trees, so they photosynthesise less. It also washes minerals out of the soil, so the trees grow poorly or die.
Some lichens are very sensitive to sulfur dioxide, so they die out where the air is polluted. Their absence shows that the air is polluted.
Sulfur dioxide gas also irritates the lungs and can cause breathing problems.

Carbon monoxide (CO)

Carbon monoxide is a poisonous gas formed by the incomplete combustion of carbon-containing fuels, when there is not enough oxygen. Sources include car engines and faulty gas heaters. It is colourless and has no smell.

Carbon monoxide combines with haemoglobin in red blood cells, much more strongly than oxygen does.
Haemoglobin that is combined with carbon monoxide cannot carry oxygen, so less oxygen is transported to the tissues.
Cells then cannot respire aerobically enough. The effects are headaches, dizziness and fainting, and a high dose can cause death.

Exam tip:

For carbon monoxide, link three ideas: it binds to haemoglobin, so less oxygen is carried, so less aerobic respiration. For sulfur dioxide, link it to acid rain and then to the effect on lakes or trees.

A power station burning coal or oil releases sulfur dioxide, which dissolves in water droplets in clouds to form acid rain; the rain damages tree leaves, washes minerals out of the soil and lowers the pH of a lake so that fish die; an inset shows lichen on a tree trunk, which is sensitive to sulfur dioxide. — Sulfur dioxide forms acid rain, which harms trees, soil and the organisms in lakes.

Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026

Frequently asked questions

What is eutrophication and what causes it?

Eutrophication happens when too many mineral ions, such as nitrates and phosphates from fertilisers, are leached into lakes and rivers. Algae grow rapidly and block light, so plants below die. Decomposing bacteria feed on the dead material and use up the oxygen in aerobic respiration, so fish and other animals die.

How does deforestation affect the carbon cycle?

Biology only Deforestation raises the carbon dioxide concentration of the air. With fewer trees there is less photosynthesis, so less carbon dioxide is removed and less oxygen is released. Burning the felled trees releases carbon dioxide by combustion, and decay releases it by respiration of decomposers. This adds to the enhanced greenhouse effect.

How does the enhanced greenhouse effect cause global warming?

The Earth gives out infrared radiation, and greenhouse gases absorb some of it and radiate it back towards the Earth. Human activities, such as burning fossil fuels, increase these gases, so more infrared radiation is trapped. The average surface temperature rises, which is global warming, causing rising sea levels and more extreme weather.

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