Unfortunately, most of the air we breathe is not pure. It contains harmful substances called pollutants which cause air pollution. Air pollution is one of the major problems of the world. Air pollution can be reduced by planting more trees. Trees cleanse the air of carbon dioxide by absorbing it for the process of photosynthesis. Trees also bring rain which helps in cleansing the atmosphere. Factories and power stations are provided with tall chimneys which carry smoke and waste gases high up into the air. In some industries, to settle down pollutants, wastes are sprayed with jets of water in scrubbers, before they reach the chimneys. In a few modern industries, electric current is used to check smoke and dust particles. Nowadays, cars are fitted with a special device called a catalytic converter. The catalytic converter converts hydrocarbons and carbon monoxide into harmless water vapour and carbon dioxide. New car engines are designed to use lead-free or unleaded petrol which is less harmful than the leaded one.