What is Radiation?

Sunshine is familiar - it delivers light, heat and suntans but we control our exposure to it. It consists of radiation in a range of wavelengths from long-wave infra-red to short-wavelength ultraviolet.



Beyond ultraviolet are higher energy kinds of radiation, which we all get in low doses from space, from the air, and from the Earth and which are used in medicine. Collectively we can refer to these higher-energy kinds of radiation as ionizing radiation. It can cause damage to matter, particularly living tissue. It is therefore dangerous at high levels, so it is necessary to control our exposure.








However, everyone is exposed to low levels of ionizing radiation. Naturally occurring background radiation – alpha, beta and gamma - is the main source of exposure for most people, though there is no evidence of harm at low levels. Background radiation, which is naturally present in our environment comes from naturally radioactive materials in the ground, or from cosmic rays. A lot of our natural exposure to background radiation is due to radon, a gas which seeps from the Earth's crust and is present in the air we breathe.

Many of us owe our lives and health to ionizing radiation produced artificially, such as medical and dental X-rays. Other kinds of ionising radiation are used to diagnose ailments and treat diseases.

Types of Ionising Radiation

Ionising radiation comes from the radioactive decay of unstable atoms. Most atoms are stable. But certain atoms change or disintegrate into totally new atoms. These kinds of atoms are said to be 'radioactive'.

Ionising radiation produces electrically-charged particles called ions in the materials it strikes. This process is called ionisation. In the large chemical molecules, of which all living things are made, the changes caused may be biologically important.

There are several types of ionising radiation:

X-rays and gamma rays have great penetrating power and can pass through the human body. Mass is required to shield us from them.

Alpha particles consist of two protons and two neutrons, in the form of atomic nuclei. They have little penetrating power and can be stopped by the first layer of skin or a sheet of paper. However, if alpha sources are taken into the body, for example by breathing or swallowing radioactive dust, alpha particles can affect the body's cells.

Beta particles are fast-moving electrons ejected from the nuclei of many kinds of atoms. These are small and can penetrate up to 1 to 2 centimetres of water or human flesh. They can be stopped by a sheet of aluminium a few millimetres thick.

Cosmic radiation consists of very energetic particles, mostly protons, which bombard the earth from outer space. It is more intense at higher altitudes than at sea level where the Earth's atmosphere is most dense and gives the greatest protection.

Neutrons are particles which are also very penetrating. On Earth they mostly come from the splitting, or fissioning, of certain atoms inside a nuclear reactor. Water and concrete are the most commonly used shields against neutron radiation from the core of the nuclear reactor.

Most materials in their natural state (including body tissue) contain measurable amounts of radioactivity. Naturally occurring background radiation is the main source of exposure for most people.

Health Effects

It has been known for many years that large doses of ionising radiation, very much larger than background levels, can cause a measurable increase in cancers after some years delay. Very large doses in the short term cause sickness and even death.

The body has defence mechanisms against damage induced by radiation as well as by chemical and other carcinogens. These can be stimulated by low levels of exposure, or overwhelmed by very high levels.

On the other hand, large doses of radiation directed specifically at a tumour are used in radiation therapy to kill cancerous cells, and thereby often save lives (usually in conjunction with chemotherapy or surgery).

Much larger doses are used to kill harmful bacteria in food, and to sterilise bandages and other medical equipment. Radiation has become a valuable tool in our modern world. See also The Peaceful Atom.

Standards and Regulation

Radiation protection standards are based on the conservative assumption that the risk is directly proportional to the dose, even at the lowest levels, though there is no evidence of risk at low levels.

In any country, radiation protection regulations are set by government authorities, generally in line with recommendations by the International Commission on Radiological Protection, and coupled with the requirement to keep exposure as low as reasonably achievable - taking into account social and economic factors.