A thick fog- like layer in the atmosphere which is made up of smoke and is called smog. The smoke released is released into the atmosphere due to various activities like automobile exhausts, power plants, factories and burning of firewood and dung cakes etc., The smoke may contain the oxides of nitrogen which combines with other air pollutants and fog to form smog. The smog is very harmful in the following ways.

● The smog is very harmful and causes breathing difficulties such as asthma, cough and wheezing in children.

● Smog reduces visibility which may result in road accidents.