Air Pollutants


Between 1969 and 1972 twelve American astronauts walked on the Moon. These are the only people who have ever seen the Earth from the surface of the Moon. From that distance it is very small, but the view on the return journey was like the picture above.

They could see the swirls of cloud in the atmosphere (the thin layer of air covering the planet). They saw what a beautiful planet Earth is and it became more obvious that what happens to the atmosphere in one part of the world could have an effect on it in another part.

Air is a mixture of gases that supports plant and animal life and keeps the surface of the Earth warm enough for the things that live on it. But ever since humans started burning wood, we have been adding gases and small particles to the air we breathe. Where these gases and particles affect our health and the health of plants they are called air pollutants. This only became a problem when we started burning fossil fuels, which are mostly coal, oil and gas. Now the rapid increase in the world population has made some of these problems even greater.

Air pollution can irritate the eyes, throat and lungs. Different people can react very differently. Some may notice chest tightness or cough, while others may not notice any effects at all.

People with heart disease, such as angina (chest pain), or with lung disease, such as asthma or emphysema, may be very sensitive to air pollution. They may notice symptoms when the majority of people do not.

img:Sheffield pollutionAir can be polluted by gases or particles. When we talk about air pollution we are usually thinking of it as being man-made, or ‘anthropogenic‘ air pollution, but the air contains naturally occurring pollutants such as volcanic dust, smoke from forest fires and desert sand. But as we have little control over these, the following pages will be referring to air pollution as being man-made.

Pollutants can be classed as either primary or secondary. Primary pollutants are substances that come directly from a source, for example carbon monoxide gas from a motor vehicle exhaust. Secondary pollutants do not get emitted from a source but form in the air when primary pollutants react with other gases and particles. A good example of a secondary pollutant is ground level ozone, as there are no man-made sources of ozone. It is formed by a chemical reaction between other pollutants in the atmosphere.

Sometimes it is possible to see air pollution, such as when smog is formed in the summer or during the winter when a weather event, called an inversion, keeps a lid on the air over a city and traps pollution.

Invisible or visible air pollution

img:Sheffield pollutionUp to the middle of the 20th century, air pollution was so bad that it could be seen, smelt and tasted in some parts of the UK. Before effective clean air laws were passed and anti-pollution technology was available, chimneys from every house and every factory belched out smoke.

These days most of the air pollution we experience is invisible and odourless and is found at much lower concentrations.

Is it smoke?

A common mistake that people can make is to think that the “smoke” rising from power station cooling towers is air pollution. However this is simply water vapour being emitted as steam. The picture below shows a power station with its cooling towers producing large quantities of steam. This is not air pollution. If you look closely at the picture, there is a thin tall chimney. It is the emissions coming from this outlet that, although almost invisible, are causing air pollution.

img:Sheffield pollution

If you imagine the air around the cooling towers as a big sponge, when it is dry it can soak up the water vapour coming from the towers. However, in damp or rainy conditions the air is like a wet sponge, which means the water vapour coming out of the cooling towers has no place to go and when it cools down it turns into the steam we see (it condenses). Cold weather also causes this effect in the same way as on cold winter days we can see our breath.

Sometimes the steam can appear to be different colours; this is because it just looks different in different light and weather conditions. This happens in the same way the sea appears to change colour; some days the sea looks grey, and on others it looks blue.

Smoke is caused when fuel is burnt inefficiently so that not all the fuel is combusted. Extremely small pieces of un-burnt material rise together in the hot gases and we see them as smoke, the more pieces there are the blacker the smoke. This smoke is composed of small particles that can be very bad for your health.

Air pollutants that are measured by the Government and local councils include:

Fine particles >
Nitrogen dioxide >
Sulphur dioxide >
Ozone >

Find out more about measuring air pollution here

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