<Li> Carbon dioxide ( CO ) - Because of its role as a greenhouse gas it has been described as `` the leading pollutant '' and `` the worst climate pollution '' . Carbon dioxide is a natural component of the atmosphere , essential for plant life and given off by the human respiratory system . This question of terminology has practical effects , for example as determining whether the U.S. Clean Air Act is deemed to regulate CO emissions . CO currently forms about 405 parts per million ( ppm ) of earth 's atmosphere , compared to about 280 ppm in pre-industrial times , and billions of metric tons of CO are emitted annually by burning of fossil fuels . CO increase in earth 's atmosphere has been accelerating . </Li>
where do some of the natural air pollutants come from
Carbon dioxide