Carbon dioxide in air is essential as plants use it to make food. When does it become a pollutant? What damage does it cause?
Carbon dioxide is required by plants to make food. Hence, a certain level of it is essentially required in the atmosphere. But it becomes a pollutant due to its excessive concentration in the atmosphere; when its level is much more than what is required.
Carbon dioxide can trap radiation coming from sunlight which increases the average temperature of the earth. This causes global warming. This results in melting of ice in the polar region, which leads to expansion of oceans (raising the water level), which ultimately results in flooding in coastal regions. Also, this can cause climate change, affect rain patterns, agriculture, plants and animals.