Question

carbon dioxide is necessary for plants why do we consider it as a pollutant

Answer 1

Plants require C02 in an optimum amount for the process of photosynthesis. But, high concentration of  C02 is harmful and considered as a pollutant. Higher concentration of Co2 is one of the causes of greenhouse effect and global warming as it absorbs the infrared radiations. It increasing the temperature of Earth. This leads to many  atmosphere problems.

Answer 2

because it is green house gas which causes green house effect and global warming