Question

Carbon dioxide is necessary for plant why do we then consider it as pollutant

Answer 1

Yes carbon dioxide is necessary for plants because it helps in their breathing and making food. But it is harmful for human beings.It's quantity is fixed in air composition so when it will rise the quantity of oxygen will decrease and without oxygen no body can survive. So it is useful but in it'fixed amount.

Answer 2

it is it is true that carbon dioxide is necessary for plants as they need it to synthesise carbohydrates during photosynthesis but excessive amount of Co2 in atmosphere becomes harmful for living organism and hence it is considered as pollutant.

Excess of excess of Co2 in atmosphere is causing global warming by forming an annual up around the earth which prevent heat from the Earth surface to escape into the atmosphere.