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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Inspired air contains about 21% oxygen, 0.04% carbon dioxide and variable water vapour. Expired air contains about 16% oxygen, 4% carbon dioxide and is saturated with water vapour. Nitrogen stays approximately the same at 79%. The differences result from gas exchange in the alveoli.
This data table is a staple of the gas exchange topic. You need to know the approximate percentages, explain why each gas changes, and be able to describe how to test for the differences experimentally.
The composition data
Gas
Inspired air (%)
Expired air (%)
Change
Oxygen
~21
~16
Decreased
Carbon dioxide
~0.04
~4
Increased
Nitrogen
~79
~79
No change
Water vapour
Variable
Saturated (high)
Increased
Note that expired air still contains substantial oxygen (16%): not all oxygen is absorbed during each breath. This is why rescue breathing works: expired air contains enough oxygen to sustain a non-breathing person.
Explaining the differences
Oxygen decreases because it diffuses from the alveolar air into the blood to be used in aerobic respiration by body cells.
Carbon dioxide increases because it is produced by aerobic respiration in cells, carried by the blood to the lungs, and diffuses from the blood into the alveolar air.
Water vapour increases because the warm, moist surfaces lining the alveoli add water vapour to the exhaled air. Water is also a product of aerobic respiration.
Nitrogen is unchanged because nitrogen is not used or produced by the body: it passes through the lungs without reacting.
Testing the differences
To compare inspired and expired air: bubble each through limewater. Expired air turns limewater milky much faster than inspired air, showing it contains more carbon dioxide. To test for water vapour, breathe onto a cold surface or use cobalt chloride paper, which turns from blue to pink in the presence of moisture.
To test for oxygen differences, use a gas syringe or oxygen probe. Alternatively, a candle or glowing splint burns more brightly in inspired air than in expired air, though this is a rough test rather than a precise measurement.
Try a focused question (3 marks)
Explain why the percentage of carbon dioxide is higher in expired air than in inspired air.
Show the answer and marking guidance
Body cells carry out aerobic respiration, which produces carbon dioxide as a waste product.
Carbon dioxide is transported in the blood to the lungs.
It diffuses from the blood into the alveolar air down its concentration gradient and is exhaled.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying all oxygen is used up: expired air still contains about 16% oxygen.
Forgetting that nitrogen remains unchanged because the body does not use it.
Not linking carbon dioxide increase to cellular respiration.
Saying water vapour comes only from evaporation: it is also produced by respiration.