Atmospheric pollution cannot be caused by which of the following?

An atmospheric pollutant is a substance released into the air that, either directly or through the compounds it forms, degrades air quality, harms living organisms, or disrupts the climate; a gas is considered clean/non-polluting when its release or combustion does not introduce such harmful by-products into the atmosphere.

Among the four gases listed, sulphur dioxide, carbon monoxide, and carbon dioxide are each established atmospheric pollutants, whereas hydrogen's combustion product is water vapour with no toxic or acidic residue -- so hydrogen is the substance that cannot cause atmospheric pollution, making Hydrogen the correct answer.

• Carbon monoxide (CO): forms from incomplete combustion of carbon-based fuels and binds far more strongly to haemoglobin than oxygen does, which is why it is classified as a recognised air pollutant.

• Sulphur dioxide (SO2): released when sulphur-containing fuels are burned; it dissolves in atmospheric moisture to form acidic compounds, a major cause of acid rain, and so is a recognised air pollutant.

• Carbon dioxide (CO2): released by combustion and respiration; although plants use it in photosynthesis, its build-up in the atmosphere traps infrared radiation (the greenhouse effect), so excess CO2 is treated as a pollutant/contributor to global warming.

• Hydrogen (H2): its combustion reaction, 2H2 + O2 → 2H2O, produces only water vapour with no toxic or acidic by-product, so it does not contribute to atmospheric pollution.