Radiation is the movement of energy across free space. Radioactivity is the property of certain unstable elements to change (decay) and emit radiation. Ionising radiation has sufficient energy to dislodge electrons from atoms thus changing the chemical properties of the substance it interacts with. Ionising radiation includes alpha particles, beta particles, gamma rays, x-rays and cosmic rays.

When a nuclide decays, what remains will not usually be the same chemical element as the original. The decay product may be stable and not decay any further. However, the decay product may be unstable and undergo another radioactive decay in due course. The time taken for half of a given amount of a nuclide to have undergone radioactive decay is called the half-life.

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This is the escape from containment and spread of radioactive material. Contact with radioactive material cannot normally make non- active substances radioactive.

Materials can apparently become 'radioactive' if they are contaminated by radioactive material. A patient who has been irradiated in the beam of an X-ray machine will not be radioactive. A worker who has been in a cloud of radioactive dust may have contamination of the clothing or skin and may inhale some of the material. S/he may need urgent decontamination to reduce his/her radiation dose and to protect others from radiation.

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Alpha particles are blocked by a piece of paper and do not penetrate intact skin. Beta particles can penetrate the body up to a centimetre or two. Gamma rays need feet of concrete or thick lead to shield. This means that alpha particles only pose a health risk if they contaminate open wounds, are breathed in or ingested. Ordinary clothing protects against alpha particles and reduces the effects of beta particles.

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Everyone is exposed to ionising radiation from naturally radioactive materials in the earth's crust and cosmic radiation from space. Man made radiation includes medical radiation and discharges from the nuclear and other industries that use radioactive materials. The total average radiation dose per year to people in the UK is 2.6 mSv of which about half is due to radon (see below for an explanation of the units). Londoners have lower radiation exposures than the national average because radon levels are generally low in London. None of the 350 homes which have been measured in Greater London were above the domestic action level, a level above which measures to reduce exposure are recommended.

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Radiation risks are estimated by extrapolation of information gained from the study of groups of people who have been exposed to known amounts of radiation (e.g. A-bomb survivors or radiotherapy patients), supplemented by the results of biological and cellular experiments. Radiation has two sorts of harmful effects on health because of radiation damage to tissues.

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These are increases in the risk of cancers and hereditary diseases. These occur many years or decades after the radiation dose. The risk is taken to depend linearly on the dose and there is not thought to be a dose threshold below which the risk is zero. In a few cases, radioiodine being the best example, radiation exposure results in a particular pattern of cancer (papillary thyroid carcinoma in children) which is very unlike the natural incidence pattern. However, in general, cancers caused by radiation cannot be distinguished from those with "natural causes".

The long term fatal cancer risk is estimated to be 5% per Sievert of whole body effective dose. This means that an additional radiation dose of 5 mSv would add an additional 0.025% to a person's 25% risk of dying from cancer. This would make their total cancer death risk 25.025%. Risk factors for non-fatal cancer and hereditary effects have also been estimated and are used for the purposes of radiological protection. Overall the risks of hereditary effects are judged to be substantially less than those of cancer.

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These always occur if a threshold dose is reached but they only occur after very high doses of radiation. They include radiation sickness and radiation burns. The symptoms start soon after exposure. The higher the radiation dose, the sooner and more severe the symptoms are.

In addition to direct effects on the body, Psychological effects may occur which are unrelated to the radiation dose. The way an incident is handled may affect the psychological consequences for those caught up in an incident.

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The number of radioactive decays per second is known as the activity and is measured in Bequerels (Bq). It depends on the amount of radioactive material present and the half-life of that material. This is not a useful unit to help define the health risk from a particular radioactive source. Bequerels are very small unit, for example 1kg of coffee would contain 1,000 Bq of natural radioactivity.

The units that are used to measure harm to people (radiation dose) and the risk of deterministic health effects (radiation sickness and radiation burns) define the energy that is deposited in body tissues. This deposition of energy is measured in Grays (Gy). 1 gray is 1 joule per kg. The different types of radiation alpha, beta, gamma, x-ray etc. differ in their ability to cause damage to tissues. Grays, multiplied by the relative biological effectiveness (RBE) of the radiation are used when considering the deterministic effects of radiation (radiation sickness etc). The RBE for x-rays and gamma rays is taken to be 1 and for alpha particles it is in the range 5-10. Deterministic effects always occur after a threshold dose (energy) has been received. An analogy here is a bruise caused by being punched. A bruise will appear if the dose (force) of the punch is big enough to break blood vessels in the skin. Below the threshold "dose" there is no bruise.

Ionising radiation can also increase cancer and hereditary disease risks in later life. Again, the different types of radiation differ in their ability to cause these late or stochastic effects. A unit is needed that takes account the type of radiation and the part of the body irradiated to allow a simple measure of long term risks. Sieverts(Sv) are those units (more commonly thousandths of Sieverts or millisieverts, mSv); they measure quantities called equivalent dose to individual tissues or effective dose to the whole body. There is no simple relationship between Bequerels and Sieverts. Therefore, you would need help to estimate effective dose and hence risk. Industries that use radiation, medical physicists and organisations such as NRPB can calculate effective dose using prewritten and extensively tested models.

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A millisievert (1 mSv) is about:

An abdominal CT scan gives a radiation dose of about 10 mSv, equivalent to about 4 years of background radiation. In the event of a radiation or nuclear accident that affects the public, current advice is to consider countermeasures such as sheltering if the radiation dose that would be received outdoors is more than about a year's worth of background radiation (3 mSv).

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Extensive environmental measurements are made throughout the UK by a number of organisations, including local authorities and Government regulatory bodies. These include air measurements to look for radioactive plumes and regular monitoring of ionising radiation in food crops. See the FSA report on radioactivity in food and the environment at Foodstandards website.

Industries that use radioactive materials are strictly regulated by the HSE, Nuclear Installations Inspectorate and, in some cases local authorities. Workers at risk undergo personal monitoring of radiation dose. The Central Index of Dose Information (CIDI) is the Health & Safety Executive's national database of occupational exposures to ionising radiation. Radioactive sources and discharges into the environment have to be authorised under the Radioactive Substances Act (1993) by the Environment Agency. Control of medical radiation is divided between the Health and Safety Executive, concerned with occupational exposure, and the Department of Health, concerned with patient exposure. Businesses that use radioactive sources are required to have a 'radiation protection adviser' usually a physicist, who oversees equipment and procedures.

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There is on average one major radiation event in the world per year. Lost source's accidents are becoming more common. Lost sources may turn up in the scrap metal industry. Scrap metal yards in the UK have radiation monitors to detect any unknown radiation in consignments they receive.

The UK has a nuclear industry and in addition many thousands of radioactive sources are used in a huge variety of industrial processes (e.g. industrial radiography, thickness gauges, smoke alarms, medical diagnosis and treatment). Therefore, there is scope for radiation accidents in the UK. The UK National Arrangements for Incidents involving Radioactivity (NAIR) scheme, which provides a first response to the police in the UK if they suspect they have found an uncontrolled radiation source, is invoked a handful of times per year.

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1. Criticality accidents. This is where a nuclear chain reaction occurs when not intended and radiation is released to the environment. An example is the accident in Tokai-Mura, Japan in 1999. A mixture of enriched uranium in nitric acid, which exceeded the critical mass was created by mistake and a nuclear chain reaction occurred in a research laboratory giving very large doses to two people. A detonated nuclear weapon spreads fission products over a wide area because of a nuclear chain reaction.
2. Reactor accidents. A working reactor allows a nuclear chain reaction to occur in controlled circumstances. If the structure that contains the reactor is breached, or the nuclear reaction goes out of control, then fission products can be released into the environment. The accident at Chernobyl in 1986 is the best recent example.
3. Fires dispersing a plume of radioactive material. Here there is no nuclear chain reaction, but a plume disperses the radioactivity from a fire. If a nuclear weapon were involved in an accidental fire or conventional explosion, then plutonium from the weapon would disperse in this way. This happened in Palomares, Spain in 1966 when a B52 plane, carrying nuclear missiles crashed. This is not the same as detonation of the weapon.
4. Lost sources. The most harmful known lost source accident to date occurred in Goiania, Brazil in 1987. A radiotherapy source was broken open and radioactivity scattered widely within a city by hand-to-hand contact and cross contamination of the environment. Thousands of people were contaminated.

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In the UK, the emergency plans for nuclear sites and the MOD are practised regularly. Different scenarios are used to ensure as many of the potential scenarios have been tried as possible. Lessons learnt from each exercise and real events influence emergency planning and response. Emergency plans are based on the worst credible accident. Plans must also show how they could be extended to cope with a larger scale emergency.

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In the UK, the emergency plans for nuclear sites and the MOD are practised regularly. Different scenarios are used to ensure as many of the potential scenarios have been tried as possible. Lessons learnt from each exercise and real events influence emergency planning and response. Emergency plans are based on the worst credible accident. Plans must also show how they could be extended to cope with a larger scale emergency.

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All countermeasures have risks are well as benefits. There are 5 main countermeasures:

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Legislation requires robust packaging and contsingency plans. The organisations that transport nuclear fuels and many other radioactive materials jointly operate the Radsafe scheme that offers first line help and advice (see the Radsafe website).

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