Air pollution

When a volcano erupts huge clouds of smoke, steam and ash are shot high into the air. If you were close to the volcano you would also smell invisible fumes escaping from cracks in the volcano's rocky sides. An erupting volcano is a fine example of air pollution and it has been occurring naturally on the Earth since the planet first formed.

Today, humans are causing much worse air pollution than any volcano but it does not occur in such a spectacular way. Every time you switch on a light there is a chance that you are causing a tiny amount of air pollution. It does not occur by the light switch or the room you are in but at a power station many kilometres away. Most power stations use a fuel such as coal, oil or gas. When the fuel is burnt, energy is released which is used to make electricity. This flows along cables to your town and home then through wires in the wall to the switch. The burnt fuel is reduced to carbon dioxide and sulphur dioxide gases, steam and smoke. While the steam and smoke can clearly be seen escaping from the power station's chimneys, the gases, being colourless, enter the atmosphere undetected.

Carbon dioxide occurs naturally in the air in small amounts. It is produced by living things when they respire and is used by green plants to make food. It also helps to hold the Sun's heat in the atmosphere. As there are a large number of power stations throughout the world producing carbon dioxide the amount of this gas in the air is increasing. Many people fear that it will hold more of the Sun's heat and cause the weather to change in almost every place. The process in which the Earth warms up due to the extra carbon dioxide in the air is called global warming.

While the carbon dioxide may cause global warming, the sulphur dioxide makes an acid in the clouds. The sulphur dioxide dissolves in water droplets and combines with oxygen in the air to make sulphuric acid. This can then fall as rain or even snow. After a shower of acid rain the water drains into the ground. It dissolves some of the minerals in the soil which plants need and carries them away to streams and rivers. In time, after many showers of acid rain, the soil contains so few minerals that plants can no longer grow in it.

Acid rain not only causes plants to die on land it also causes fish and other water life to die in the streams, lakes and rivers nearby. Each water creature is sensitive to acids in water. If the amount of acid in the water rises too high the creatures simply die. Fish are also faced with another problem. Acid rain breaks up minerals containing aluminium. They flow into watercourses and stop the gills of the fish working properly. This leads to the fish dying of suffocation.

As the dangers of air pollution are being discovered new ways are being found to reduce it. For example, coal can be turned into a fuel called coke, which produces less smoke and fewer harmful gases when it burns. Even soot can be removed from smoke. Metal plates are charged with static electricity. This is the kind of electricity you can make on a plastic pen by rubbing it on your sleeve. The smoke is then passed between the plates and the soot is attracted to them just as you can attract pieces of dust with the static electricity on your pen.

Even more ways will have to be found to reduce air pollution. Perhaps you can think of one?

Why do cars cause air pollution?
They cause air pollution by the burning of petrol in their engines. Inside the engine are cylinders. In each cylinder is a piston. When petrol is burnt in a cylinder hot gases are produced quickly that push down on the piston. The pistons are connected by levers to the car's wheels and as they push down they make the wheels turn. The hot gases are quickly removed from the cylinders so more petrol can be burnt. They form the exhaust fumes, which escape from the tailpipe into the air. The exhaust fumes contain carbon dioxide, carbon monoxide and nitrogen dioxide.

Is carbon monoxide like carbon dioxide?
No, it is very poisonous. If it is breathed in, it enters the blood and stops the haemoglobin taking up oxygen. This can prove fatal. Carbon monoxide is not just produced by car engines. It can also be produced by a gas fire that has been badly fitted. There are simple detectors that can be bought to test for carbon monoxide in a room. If there are dangerous levels of gas in the room the detector changes colour.

What makes the brown haze over a city?
It is nitrogen dioxide. Nitrogen usually does not take part in changes with other substances. However, in a car engine it is so hot that nitrogen joins with oxygen to make nitrogen dioxide. In hot weather the air over a city may move very little. All the nitrogen dioxide produced by cars in the city collects above the streets and forms a brown cloud.

How is ozone made from exhaust gases?
It is made in the brown cloud of nitrogen dioxide when the sun shines on it. The energy in sunlight breaks up the nitrogen dioxide and ozone forms. If it descends to the streets it irritates the eyes and makes the throat sting.

Can anything be done to remove the harmful gases in exhaust fumes?
Yes. New cars are fitted with catalytic converters. This is a device which is attached to the exhaust pipe. It contains metals that change the exhaust gases as they pass over them. The deadly carbon monoxide is converted to the less harmful carbon dioxide and nitrogen dioxide is changed to nitrogen, which can safely join the atmosphere.

Why do some kinds of petrol have lead in them and some do not?
Lead is added to petrol to make the petrol burn well and give the engine plenty of power. For most of the twentieth century petrol for cars contained lead. It was then discovered that the lead formed tiny particles and entered the air with the exhaust fumes. The lead particles settled on food and skin and were inhaled by people. Children were found to absorb lead into their bodies more readily than adults. It was discovered that lead caused damage to the brain and nerves so a way of making car engines run on unleaded petrol was found. Today most cars run on unleaded petrol but some older cars can only run on leaded petrol.