CO2 is the symbol carbon dioxide Photosynthesis requires CO2 and sunlight and water and releases oxygen as a by product. It uses the pollution (CO2) meaning that less of the CO2 would still be in the air. As you may know CO2 allows the earths atmosphere to allow more infrared radiation, causing damage to wildlife and the world in general (ice caps melting, sea level rising, floods and tropical cyclones becoming more common etc) however photosynthesis takes in that and releases oxygen which is needed.
