1) Why do traffic policeman wear masks at crowded crossroads??
2) How do plants and animals maintain the balance of oxygen and CO2 in the air ( or in theatmosphere)???
Plants take carbon dioxide to prepare food and release oxygen during daytime. This oxygen is taken in by animals and carbon dioxide is released. Thus, plants and animals help in maintaining balance of oxygen and carbon dioxide.
Traffic policemen wears mask to protect himself from the polluted air coming from the vehicles that may enter this nose and lead him to serious problems.