Human influences on the environment — Edexcel International GCSE Biology
Pollution by sulfur dioxide, carbon monoxide and sewage, greenhouse gases and global warming, eutrophication, and the effects of deforestation.
Revision notes
6 short notes, in the order of the specification. Each one 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.
Greenhouse gases are the gases that cause the greenhouse effect: carbon dioxide, methane, nitrous oxide, CFCs and water vapour. Human activities add to them. Burning fossil fuels and deforestation increase carbon dioxide, cattle, rice paddy fields and landfill release methane, nitrogen fertilisers release nitrous oxide, and old refrigerators and aerosols released CFCs.
The enhanced greenhouse effect is the extra warming caused when human activities increase the amount of greenhouse gases. These gases absorb infrared radiation given out by the Earth and radiate it back, so more heat is trapped and the average surface temperature rises, called global warming. Consequences include rising sea levels, extreme weather and changes in species distribution.
Untreated sewage pollutes rivers and lakes because it contains organic matter, pathogens and mineral ions. Decomposers feed on the organic matter and use up the dissolved oxygen in aerobic respiration, so fish and other aquatic animals die and biodiversity falls. Pathogens in sewage spread diseases such as cholera and typhoid, and its nitrates and phosphates can cause eutrophication.
Eutrophication is when too many mineral ions, such as nitrates and phosphates from fertilisers, are leached into lakes and rivers. Algae grow quickly, forming an algal bloom that blocks light, so plants below cannot photosynthesise and die. Decomposing bacteria feed on the dead material and use up the oxygen in aerobic respiration, so fish die.
Deforestation
Biology onlyDeforestation is the removal of large areas of forest, usually by cutting or burning trees. Without roots, minerals are leached and soil is eroded, which can cause flooding. Fewer leaves reduce evapotranspiration, so rainfall falls. Less photosynthesis, plus burning and decay of felled trees, raises atmospheric carbon dioxide, adding to the enhanced greenhouse effect.
Exam questions
7 exam-style questions (25 marks), each with its mark scheme.
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19 cards: flip them, mark what you knew, and practise the rest.
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The whole of ecology and the environment on one page, so you can see where this subtopic fits.
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Quick check questions
What gas causes acid rain?
Show answer
Sulfur dioxide.
Why is carbon monoxide poisonous?
Show answer
It combines with haemoglobin, so the blood carries less oxygen.
Name the five greenhouse gases in the spec.
Show answer
Water vapour, carbon dioxide, nitrous oxide, methane and CFCs.
Why does sewage pollution kill fish?
Show answer
Decomposers feeding on it use up the oxygen in the water.
Biology only How does deforestation increase soil erosion?
Show answer
Tree roots no longer hold the soil together.
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.
How does carbon monoxide affect the body?
Carbon monoxide combines with haemoglobin in red blood cells much more strongly than oxygen does. Haemoglobin combined with carbon monoxide cannot carry oxygen, so less oxygen reaches the tissues and cells cannot respire aerobically enough. This causes headaches, dizziness and fainting, and a high dose can cause death.
How does sulfur dioxide cause acid rain?
Sulfur dioxide is released when fossil fuels containing sulfur, such as coal and oil, are burned. It dissolves in rain water to form acidic solutions, making acid rain. Acid rain lowers the pH of lakes and rivers so fish die, damages tree leaves and washes minerals out of the soil.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026