Which of the following is not an air pollutant?
A: ${H_2}$
B: $S{O_2}$
C: ${O_3}$
D: $N{O_2}$

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Hint: Pollution is the presence of contaminants into the natural environment that cause adverse changes in the environment. It can take the form of chemical substances or energy, like, heat, light, or noise. The components that cause pollution are called pollutants. These substances can be either foreign substances or energies or naturally occurring contaminants.

Complete answer:
We know that pollutants are substances that cause pollution. In this question, we have to find the substance which is not an air pollutant among the given options. Hydrogen is used as a fuel. It is zero-emission fuel burned with oxygen. Water is the byproduct when hydrogen is used as a fuel. $S{O_2}$ Is sulfur dioxide. It is a toxic gas that is responsible for the smell of burnt matches. It is the main cause of acid rain. Naturally, it is formed by volcanic activity and as a byproduct of burning fossil fuels containing sulfur compounds. This means it is a pollutant. ${O_3}$ Is ozone. It is produced when two primary pollutants (nitrogen oxides and volcanic compounds) react in sunlight and stagnant air. It is considered a secondary pollutant (as it is produced from primary pollutants). Primary pollutants are substances which are emitted directly from a process like volcanic eruption or ash. $N{O_2}$ Is nitrogen dioxide. Nitrogen oxides are the primary pollutants. Nitrogen oxides are also the main cause of acid rain. So, this is also an air pollutant. From this above discussion, we can conclude that hydrogen is the substance that is not an air pollutant.

So the answer to this question is option A that is ${H_2}$.

Sulfur dioxide is not acid rain. It is the reactant that combines with water in the atmosphere to form sulphuric acid that causes acid rain. Sulfur dioxide is a toxic gas that is responsible for the smell of burnt matches.