The air is a natural mixture of gases. But we are altering this natural mixture. We do this when we burn fossil fuels such as coal, oil and natural gas.
Most of the fuel we use is burned in power stations and in motor cars. This releases carbon dioxide gas (the cause of global warming) and also gases that mix with water vapour to make acids (or acid rain), and a brown gas that causes smog and irritates the eyes and throat.
