Ionising radiation

Ultraviolet radiation, X-rays and gamma rays are high-energy radiations that can ionise atoms they hit. If these atoms are ones inside the body the radiation can damage or kill cells. If the DNA in a cell is damaged it may mutate. This can cause cells to grow out of control – they become cancer cells.

Ultraviolet radiation

Ultraviolet radiation from the Sun travels towards the Earth. The highest energy (highest-frequency) ultraviolet radiation is stopped by the ozone layer, a layer of the gas ozone in the upper atmosphere.

Unfortunately, using CFC gases in aerosol cans (as a propellant to force the contents of the can out of the nozzle) and as a refrigerant in refrigerators and freezers, caused CFC gas to increase in the atmosphere. This pollution reacted with the ozone and reduced the amount in the ozone layer.

Above the Antarctic an ‘ozone hole’ developed and began to grow.

International agreements have stopped the use of CFCs but the ozone layer will take time to recover. Where the ozone layer has been depleted, living organisms, especially animals, suffer more harmful effects from ultraviolet radiation.

The lower-energy ultraviolet radiation that passes through the ozone layer can cause skin cancer and damage the lens of the eye. The number of cases of skin cancer rose when people started to spend more time in the sun. Light skins are more easily damaged than dark skins. Dark skins absorb more of the ultraviolet radiation close to the surface; light skins allow the radiation to penetrate further into the body tissues.

Campaigns warn people to stay out of the sun during the hottest part of the day, and to cover up with a hat and clothes. They should also use a highprotection factor sunscreen, which reduces the ultraviolet radiation reaching the skin.

Sunscreens have a sun protection factor (SPF) number, which relates to how long a user can stay out in the sun. To find out the time you can stay in the sun wearing the sunscreen, multiply the time you could safely stay in the

Sun without the sunscreen by the SPF. For example, if a person could safely stay in the sun for 10 minutes, using a sunscreen with SPF 15 means they can can safely stay in the sun for 150 minutes. It is important:

• not to miss any area of skin
• to put the cream on in a thick enough layer
• to reapply it according to the instructions.

Ulltraviolet blocking sunglasses are recommended to protect the lens of the eye from damage.

Ionising radiation can cause cells to turn cancerous. Ultraviolet radiation from the Sun can cause skin cancer.

Using ultraviolet radiation

Ultraviolet radiation is used to detect forged bank notes. Bank notes have some features, for example the number of pounds, that fluoresce in ultraviolet radiation. This means they absorb the ultraviolet and re-emit visible light. You may have seen a detector in shops. The banknotes are placed under the light and the number shows up brightly, because it flouresces if the bank note is genuine.

X-rays

X-rays have high energy and can pass through the body tissues. They are stopped by denser materials such as the bones and by pieces of metal. They can be used to scan the body and show breaks in the bones. The diagram shows how a photographic plate can be placed behind the patient and X-rays directed towards the patient. The plate will darken where the X-rays strike it and leave white shadows of the bones, where the X-rays are absorbed.

The operator stands behind a lead screen so that he or she is not exposed to X-rays each time a patient is X-rayed.

The patient should not receive too many X-rays. They are used when the benefit (for example, finding a broken bone) is greater than the risk.

In the 1950s, all pregnant women were X-rayed to check on the development and position of the baby. This caused a few cancers among children. The benefits were not greater than the risks and routine X-rays were stopped.

X-rays are used for security scans of passengers’ luggage. They pass through the suitcase and clothes, but metal items and batteries stop the X-rays and Fig. 12.9 Taking an X-ray picture show up as shadows on the screen.