Page last reviewed: 13/07/2011

Radiation is a general term that refers to any sort of energy that can travel through space either as a wave or a particle. Examples of radiation include light, radio waves and microwaves.

There are two types of radiation, which are described in more detail below:

  • non-ionising radiation (low energy)
  • ionising radiation (high energy)

When people talk about the dangers of radiation, they are referring to ionising radiation (see Radiation - Risks for more information). However, ionising radiation is only a threat to health when a person is exposed to a significantly high dose, such as after the nuclear plant disaster at Chernobyl in the former Soviet Union in 1986.

Ionising radiation actually has important medical uses - for example, X-rays are used to diagnose certain conditions, and radiotherapy is used to treat cancer. For more information, see Radiation - When it is used.

Non-ionising radiation

Non-ionising radiation is a less powerful type of radiation. It does not usually pose a threat to human health (with the exception of very high doses that can generate heat and burn the skin).

Examples of non-ionising radiation include:

  • visible light
  • microwaves
  • radio and radar waves
  • mobile phone signals (see box, left)
  • 'wi-fi' - wireless internet connections

Ionising radiation

Ionising radiation is a more powerful form of radiation than non-ionising radiation. It is capable of damaging living cells and, in high enough doses, poses a threat to human health.

Examples of ionising radiation include X-rays and gamma rays, which are widely used in medicine for diagnostic and treatment purposes. Gamma rays are usually more powerful than X-rays and are sometimes used to sterilise certain materials.

To date, there have been more than 200 documented cases of accidents involving lost and mishandled sources of ionising radiation, particularly for industrial purposes.

These types of incidents appear to be becoming more common as the use of ionising radiation increases around the world, including places where controls are less stringent than in Europe.

Mobile phone safety

In recent years, there has been concern that exposure to the non-ionising radiation generated by mobile phones may pose a threat to human health.

For example, there have been (as yet unproven) claims that mobile phone use may increase a person’s risk of developing some types of cancer or may interfere with the normal workings of the brain.

As a result of these concerns, independent scientists were commissioned to investigate this and produce a number of reports. The most recent (2007) report concluded that there is no evidence that short-term use of mobile phones increases the risk of cancer or affects the normal functioning of the brain.

However, the report pointed out that as mobile phones have only been widely used for about 10 years, it may be too early to detect possible long-term risks or problems that may be associated with mobile phones.

See the Health A-Z topic about Mobile phone safety and Radiation - risks  for more information.

Page last reviewed: 13/07/2011

There are two main medical uses for radiation:

  • to help diagnose certain conditions using X-rays 
  • to treat cancer using radiotherapy


X-rays are similar to light waves in that they are made up of tiny packets of energy called photons. However, the photons in X-rays have a much higher energy level, which means the waves can pass through the human body (light is only just capable of penetrating the skin).

The photons are absorbed at different rates as they pass through the body. The pattern of absorption shows up on an X-ray image.

The parts of the body that are made up of dense material, such as the bones, absorb lots of photons and show up as clear white areas on X-ray images. The parts that are made of softer material, such as the heart and lungs, absorb smaller amounts of photons and show up as darker areas. The resulting contrast helps doctors to make diagnoses.

An X-ray machine consists of an X-ray tube, lead shielding and a photographic plate. The X-ray tube is like a giant light bulb that uses high-voltage electricity to generate the X-rays. The lead shielding is used to direct the X-rays towards a specific part of your body and prevents them escaping in all directions. The photographic plate captures the image of the X-rays as they pass through your body.

See the Health A-Z topic about X-rays for more information about when they are used, how an X-ray scan is performed, and the risks involved.

Computerised tomography (CT) scans

A computerised tomography (CT) scan is a more advanced type of diagnostic test involving X-rays.

A CT scanner is a large ring-shaped machine. The X-ray scanner inside the ring rotates in small movements around your body as you lie on a bed that moves backwards and forwards through the ring. As you move through the machine, the scanner uses a series of X-ray beams to scan parts of your body and build up detailed images.

The images produced by a CT scan are called tomograms. These are more detailed than standard X-rays. They can provide a cross-sectional (slice-like) view of the internal organs and structures of the body, such as blood vessels, bones and tumours.

See the Health A-Z topic about CT scans for more information about this type of scan.


High doses of ionising radiation can kill human cells by destroying the genetic material that the cells need in order to live and reproduce.

In the same way, radiotherapy uses the power of ionising radiation to kill cancerous cells, but manages to minimise the damage to healthy cells.

There are two main types of radiotherapy:

  • External beam radiotherapy, where a machine is used to direct high-energy rays at the part of the body affected by cancer.
  • Internal radiotherapy - also known as brachytherapy. A radioactive implant is used to treat tumours such as cervical or prostate cancer. A drink or injection can also be used to deliver radiation directly to an area inside the body to treat conditions such as an overactive thyroid or cancer.

Radiotherapy is usually given in a number of sessions over the space of several weeks, for safety reasons. If the required dose was delivered in one session, it would expose the person to a very high amount of radiotherapy that would be damaging and, in extreme cases, fatal.

See the Health A-Z topic about Radiotherapy for more information. 

Radiation from the sun

Your skin produces vitamin D when it is exposed to ultraviolet (UV) light from the sun. We need vitamin D to form strong and healthy bones.

However, it is important to moderate your exposure to sunlight, as too much exposure can trigger mutations (changes) in your skin cells, which over time can turn into skin cancer.

Overexposure to the sun can also cause wrinkling and premature skin ageing.

So take sensible precautions when out in the sun: use sun cream with a sun protection factor (SPF) that is suitable for your skin type and limit the amount of time you spend in the sun.

Page last reviewed: 13/07/2011

Non-ionising radiation

In the past, most experts felt that non-ionising radiation posed no threat to human health because it was not powerful enough to affect human cells in the same way as ionising radiation.

However, some people have argued that due to the increased use of telecommunications devices that use non-ionising radiation, such as mobile phones or wi-fi, the potential dangers may need to be investigated again.

A number of international studies have not identified any health risks associated with these devices. However, the widespread use of these devices only began at the end of the 1990s, so there may be long-term risks that have not yet come to light.

See Mobile phone safety - recommendations for more information about ways that you can minimise your exposure to non-ionising radiation when using a mobile phone or other wireless device.

Ionising radiation

Ionising radiation is more powerful than non-ionising radiation. It poses a threat to human health because it can damage human cells at the molecular level. Exposure to ionising radiation can cause three main health problems:

  • Acute radiation sickness. When a person is exposed to a massive amount of radiation, such as that released during a nuclear weapon explosion, it results in illness and symptoms such as vomiting, bleeding and diarrhoea. In recent years there has only been one reported case in England (the deliberate poisoning of the former Russian agent Alexander Litvinenko in 2006).
  • Acute local injury, such as burns to an area of skin. 
  • An increased risk of cancer. Moderate to large amounts of ionising radiation can alter the genetic material in cells, which can trigger the onset of cancer, usually many years after the initial exposure.

However, the above cases are rare, and most people only become exposed to the harmlessly low levels of radiation found naturally in the atmosphere (background radiation). Background radiation includes:

  • Radon - this naturally occurring radioactive gas is found in low levels in the atmosphere.
  • Cosmic rays - this radiation originates from space (from the sun and stars).
  • The earth - soil and rocks contain various radioactive materials that have been present since the earth was formed. These contribute to our exposure, as do building materials that are made from soil, rocks and stones.
  • Food and water - for example, nuts, bananas, red meat and potatoes all contain small traces of radiation.

Measuring the strength of radiation

The strength of radiation in relation to long-term risk is measured using units known as millisieverts, or mSv for short.

Some examples of different types of radiation exposure and their associated measurement in mSv are listed below.

  • A single chest X-ray (0.02 mSv). This is equivalent to the amount of radiation that you would be exposed to during a return flight from London to Spain (the higher up in the atmosphere you are, the less protection you have against cosmic rays).
  • A year's worth of medical tests (0.4 mSv). This is the average dose for each person each year as a result of medical tests, such as X-rays.
  • Natural radiation (2.2 mSv) is the average annual dose that a person receives from natural sources.
  • A mammogram (2 to 5 mSv) is the amount of radiation that a woman receives after having a mammogram (a type of X-ray used during breast cancer screening); a mammogram is a very safe procedure.
  • Working with radiation (20 mSv) is the legal limit, that a classified person who works with radiation may be exposed to in any given year. However, most workers receive considerably less than this.

Occupational risk

There is conflicting evidence about the risks faced by people who regularly work with radiation. These people include nuclear power workers and medical professionals who use radioactive technology, such as X-rays and CT scanners.

A study conducted in France found that people who worked in the nuclear power industry actually had lower rates of cancer than the population at large. This could be because French nuclear power workers tend to have an above-average income and high standards of living, which usually translates into a lower risk of getting cancer later in life. This is sometimes known as the healthy worker effect.

However, a pan-European study carried out in 15 different countries found that people who spent most of their life working with radiation, with a lifetime's worth of radiation exposure equivalent to 100mSv, had a slightly higher risk of getting cancer compared with the population at large.

But it should be stressed that improvements in safety standards mean it is now estimated that only 1 in 20 radiation workers are likely to be exposed to such levels during their career. 

Studies in the UK and Canada both found that the rates of heart disease and stroke were slightly higher among nuclear power workers. However, these results need to be interpreted with caution. In the UK and Canada, the nuclear power industry is a very male-dominated workforce, and men tend to smoke and drink alcohol more than women. Drinking and smoking are both independent risk factors for heart disease and stroke.

There is no evidence that the children of people who work with radiation have an increased risk of developing serious health conditions such as birth defects or leukaemia.

Smoking and radiation

Many smokers are unaware that their smoking habit is exposing them to significant levels of radiation.

All tobacco contains traces of an element called polonium-210, which is one of the most radioactive substances in the universe and was the same element that was used to poison Alexander Litvinenko. However, the amount of polonium-210 that is found in tobacco is very small.

It is estimated that people who smoke 30 cigarettes a day receive an annual radiation exposure that is equivalent to having 300 chest X-rays a year. American researchers have estimated that the polonium-210 found in cigarettes is responsible for 11,700 deaths each year across the globe.

See the Health A-Z topic about Quitting smoking for more information about the health risks that are associated with smoking, and advice about the best way to quit.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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