Question

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

Answer 1

because excess amount of carbon dioxide in out environment is harmful for our man

Answer 2

carbon dioxide is a necessary for plants but excessive amount of carbon dioxide in atmosphere will be harmful for living organism as it cause global warming by from a blanket like structure around the earth which will prevents heat from earth's surface to escape into the atmosphere. So, as a necessary element for plants but also considered as a pollutant.

hope you understand.