Answer:

Because,

Carbon dioxide is considered a pollutant only when its level of concentration in the air is too high. Whereas sulphur dioxide, carbon monoxide and nitrogen dioxide are hazardous gases and are considered pollutants even at low levels of concentration.

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Answer:

Pollutant is a relative term. A pollutant is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. At certain levels, 405 ppm of the atmosphere, it is not a pollutant. When it starts increasing, it causes global warming that adversely affects the global temperature, causing seasonal changes that are not common and other undesirable effects to the ecosystem we live in i.e. the earth. Hence, beyond a particular level, it will be termed as a pollutant.

For the record , oxygen ,beyond a particular concentration is a pollutant as well.