Basic concepts of radiation
Updated 25 May 2022
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Introduction
This page answers key questions about radiation: what is radiation, how does radiation affect people and how are we exposed to radiation? Radiation refers to the transfer of energy from one place to another.
There are 2 broad categories of radiation:
- non-ionising radiation, such as visible light, signals from mobile phones and radio waves
- ionising radiation, such as radiation emissions from uranium ore and high frequency waves in the electromagnetic spectrum (such as X-rays)
Each type of ionising radiation is capable of disrupting stable atoms and causing them to have an imbalance of charge (ionisation). This can cause chemical changes in living matter which may cause harm to people’s health, depending on the radiation dose received.
Ionising radiation
Unstable atoms in a material are said to ‘decay’ giving out ionising radiation in the process. The property of a material to decay and emit ionising radiation is called radioactivity and the material is said to be radioactive. As the unstable atoms in a radioactive material decay the atoms change to another form.
The time taken for half the unstable atoms in a material to decay and change is known as the half-life. Each radioactive material has its own half-life. These vary from less than a few seconds to more than thousands of years.
There are 3 main types of ionising radiation: alpha (α), beta (β) and gamma (γ). Alpha and beta radiations are particles, while gamma radiation is a wave similar to X-rays. These forms of radiation differ in their ability to penetrate into the body or other materials and also in their ability to cause harm to people.
Alpha particles
As they are relatively big, heavy and slow, alpha particles are not able to penetrate very far through materials. They are stopped by a few centimetres of air or a sheet of paper and even by the dead layer of skin on the outside of our bodies.
As they usually cannot penetrate into the body, alpha particles do not pose a significant hazard from outside the body. However, radioactive materials emitting alpha particles can get into the body by inhalation, ingestion or through open wounds. They can then damage tissue and have a greater potential to cause cancer than beta particles and gamma rays.
Beta particles
These are relatively light, small and fast, so they may travel several metres in air and can penetrate through exposed skin. Consequently, beta particles can present a hazard from inside or outside the body.
They can be stopped by thin sheets of aluminium or perspex.
Gamma rays
These rays have no weight and can penetrate through the body, depositing some of their energy on the way and so causing harm. Gamma rays are therefore a hazard both inside and outside the body.
They can be stopped or exposure can be reduced by the use of thick, heavy shielding.
How radiation affects people
The term ‘radiation dose’ is used to describe the amount of energy absorbed by a material from ionising radiation passing through it. The most common measure of radiation dose to people is called effective dose, measured in units called sieverts. Effective dose takes account of the different sensitivities of organs in the body and the effects of different types of radiation.
A sievert is a large dose of radiation and in most cases radiation dose will be given in microsieverts (µSv, one millionth of a sievert) or millisieverts (mSv, one thousandth of a sievert).
At low levels radiation causes no immediate perceptible damage to people. However, any exposure to radiation is considered to be capable of increasing the lifetime risk of cancer and of passing on hereditary illnesses to children.
Individuals exposed to very high doses of radiation may receive burns to the skin, damage to the gastrointestinal, cardiovascular or nervous systems, and exceedingly high doses can cause death.
Radiation exposures
People have always been exposed to low levels of radiation from natural sources. On average, people in the UK receive an annual dose of 2.7 mSv. Natural sources make up approximately 84% of this dose, with the remainder coming from a variety of artificial sources.
Average annual radiation exposure of a person in the UK (2.7 mSv overall)
In the UK, 84% of the 2.7 mSv average annual radiation exposure of a person comes from natural sources, made up of 48% radon gas from the ground, 13% gamma rays from the ground and buildings, 12% cosmic rays, and 11% intakes of radionuclides exluding radon. The remaining 16% comes from artificial sources, including medical radiation, with <1 % from occupational exposures, fallout, nuclear discharges and products.
Natural radiation sources include gamma rays from the natural radioactivity in the earth and in building materials, the small amounts of natural radioactivity in food and drink, and cosmic rays which bombard the earth from space. However, by far the greatest contribution comes from breathing radon gas which is given off by natural radioactive materials in the earth.
Inhalation of radon leads to alpha-particle irradiation of the lungs and has been shown to cause lung cancer.
Artificial (man-made) sources are dominated by medical exposures. All other artificial sources contribute in total less than 0.5% of the average annual exposure.
For further information please visit Radiation: detailed information.