Question

Carbon dioxide is necessary for plants. Why do we consider it as a pollutant?

Answer 1

It is true that $C{O}_2$ is necessary for. plants as they need it to synthesise carbohydrate during photosynthesis, but excessive amount of $C{O}_2$ (more than normal concentration) in atmosphere becomes harmful for living organisms and hence is considered as pollutant. The $C{O}_2$ is formed inside body of living organisms as a result of cellular oxidative metabolism. It diffuses out of body through diffusion. Its excess amount inside body is harmful. Moreover excessive amount of $C{O}_2$ is causing global warming (greenhouse effect) by forming an envelope around earth which prevents the heat from earth's surface to escape into atmosphere.