Carbon dioxide is necessary for plants.

Besides being essential for photosynthesis, carbon dioxide is a green house gas. Upto $350 \mathrm{ppm}$, it is both a good raw material as well as essential for keeping the earth warm. However, when its concentration rises (as presently it is $400 \mathrm{ppm}$ ), it becomes pollutant because it results in global warming. The latter is quite dangerous because it will melt polar ice, raise the water level of oceans and submerge several coastal areas and islands.