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K324 / 2027
Maintaining Air Quality

Lesson 1 of 5

What is in air, and where do pollutants come from?

Separate the normal mixture from substances causing harm.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can describe dry-air composition.Syllabus

    Syllabus K324, 12(a). Describe dry-air composition

  • I can name atmospheric pollutants.Syllabus

    Syllabus K324, 12(b). Name atmospheric pollutants

  • I can identify pollutant sources.Syllabus

    Syllabus K324, 12(c). Identify pollutant sources

What is in air, and where do pollutants come from?

Separate the normal mixture from substances causing harm.

Dry air contains about 78% nitrogen and 21% oxygen by volume. Most of the remaining 1% is noble gases, mainly argon, with carbon dioxide and other trace gases. Dry air excludes water vapour; the water-vapour content of actual air varies.

Pollutants and sources
SubstanceSource or formationMain concern
Carbon monoxide, COIncomplete combustion of carbon-containing fuels when oxygen supply is insufficientToxic; reduces oxygen transport in blood
Nitrogen oxides, NO and NO2High-temperature reactions involving air during lightning and in internal combustion enginesRespiratory irritation and acid rain
Sulfur dioxide, SO2Volcanoes and burning fossil fuels containing sulfur compoundsRespiratory irritation and acid rain
Methane, CH4Livestock, decomposition without oxygen, and natural-gas leakageGreenhouse gas
Ozone, O3Can form near ground level through sunlight-driven reactions involving other pollutantsHarmful to respiration at ground level
Unburned hydrocarbonsFuel that escapes combustion, including engine exhaust and evaporationContribute to photochemical air pollution

Distinguish CO from CO2. Both contain carbon and oxygen, but CO is a toxic incomplete-combustion product. Complete combustion produces CO2, which is important in the carbon cycle and greenhouse effect. A fuel containing no sulfur cannot directly supply sulfur dioxide simply by burning; identify which elements the source provides.

Check your understanding

Air is often quoted as 78% nitrogen and 21% oxygen. Do these figures apply directly to humid air?

  1. Not quite. They describe dry air; water vapour lowers the other percentages.
  2. No. Humid air has more oxygen, because water molecules contain oxygen.
  3. Yes. Water vapour is a liquid, so it is not counted as part of the air.
Show the answer

Not quite. They describe dry air; water vapour lowers the other percentages.

The figures are for dry air. In humid air, water vapour takes up part of the sample, so nitrogen and oxygen make up slightly smaller shares.