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

Explanation

Besides being essential for photosynthesis, `CO_2` is a greenhouse gas. Upto 350 ppm, it is both a good raw material as well as essential for keeping the earth warm. However, when its concentration rises, it becomes pollutant because it results in global warming. The letter is quite dangerous because it is melt polar ice, raise the water level of oceans and submerge several coastal areas and islands.