Which of these is NOT a pollutant?

Analyze the question: We need the one substance that is not a pollutant, meaning it is a normal, harmless part of clean air.

Concept: A pollutant is a substance that, when added to the environment in harmful amounts, damages living things or the environment.

Going through the options:

• A. Carbon dioxide (CO₂): It occurs naturally, but in excess from burning fuels and deforestation it is a pollutant. It drives global warming through the greenhouse effect. It is commonly classed as a pollutant in exam questions.
• B. Lead oxide: It comes from vehicle exhaust (leaded petrol) and industrial fumes. It is toxic and damages the nervous system, so it is a pollutant.
• C. Nitrogen gas (N₂): It makes up about 78% of the air. It is chemically inert and harmless, and it is used in the nitrogen cycle. It is not a pollutant.
• D. Nitrogen oxide (NO, NO₂): It comes from vehicle engines and industry. It causes acid rain and respiratory problems, so it is a pollutant.
• E. Sulphur dioxide (SO₂): It comes from burning fossil fuels and causes acid rain, so it is a pollutant.

Answer: The only substance that is a natural, harmless major component of air is nitrogen gas, so the answer is C.

Tip: Do not confuse nitrogen gas (N₂) with the nitrogen oxides (NO, NO₂). When a question lists oxides of nitrogen, sulphur, carbon or lead, think pollutant. The plain elemental gas N₂ is harmless.