| The following gives examples of radiation doses and their effects: | |
| Dose | Effect |
| 2 mSv/year | Typical background radiation experienced by everyone (av 1.5 mSv in Australia, 3 mSv in North America). |
| 1.5 to 2.0 mSv/year | Average dose to Australian uranium miners, above background and medical. |
| 2.4 mSv/year | Average dose to US nuclear industry employees. |
| up to 5 mSv/year | Typical incremental dose for aircrew in middle latitudes. |
| 8 mSv | CT chest X-ray. |
| 9 mSv/year | Exposure by airline crew flying the New York - Tokyo polar route. |
| 10 mSv/year | Maximum actual dose to Australian uranium miners. |
| 20 mSv/year | Current limit (averaged) for nuclear industry employees and uranium miners. |
| 50 mSv/year | Former routine limit for nuclear industry employees. It is also the dose rate which arises from natural background levels in several places in Iran, India and Europe. |
| 100 mSv/year | Lowest level at which any increase in cancer is clearly evident. Above this, the probability of cancer occurrence (rather than the severity) increases with dose. |
| 350 mSv/lifetime | Criterion for relocating people after Chernobyl accident. |
| 1,000 mSv/cumulative | Would probably cause a fatal cancer many years later in 5 of every 100 persons exposed to it (ie. if the normal incidence of fatal cancer was 25%, this dose would increase it to 30%). |
| 1,000 mSv/single dose | Causes (temporary) radiation sickness such as nausea and decreased white blood cell count, but not death. Above this, severity of illness increases with dose. |
| 5,000 mSv/single dose | Would kill about half those receiving it within a month. |
| 10,000 mSv/single dose | Fatal within a few weeks. |
Radiation is the process of emitting energy in the form of waves or particles.
The human race has evolved over millions of years in a radioactive environment. The earth is naturally radioactive, so is the air we breathe, the food we eat and the ground we stand on. It is only when we are subjected to relatively large doses of radiation that it becomes a danger to life or health.
Everything in the world is made up of atoms. Some atoms are stable, and others are unstable. An unstable atom – for example the nuclei may be too big to hold it together – will shake off particles and surplus energy from their nucleus until they become stable.
The process of shaking off these particles and energy is called ‘radioactivity', and the particles and energy which are released are known as ‘nuclear radiation'.
Measuring radiation
1000 microsieverts (µSv) = 1 millisievert (mSv)
1000 millisieverts = 1 sievert (Sv)
Radiation can arise from natural sources or from artificial sources from human activities. Natural and artificial radiations are not different in kind or effect.
Most radiation exposure is from natural sources such as radioactivity in rocks and soil of the earth's crust; radon, a radioactive gas from the earth and present in the air; and cosmic radiation. About one quarter of natural radiation comes from the human body itself.
Natural sources of radioactivity account for up to 85% of the annual radiation dose for humans.
In Australia, the average background radiation dose is about 1.5 millisievert per year, however in some parts of the world doses can be more than 50 mSv/yr.
The highest known level of natural background radiation affecting a substantial population is in Kerala and Madras states in India, where approximately 140,000 people receive doses which average over 15 millisievert per year from gamma radiation in rocks and soil, in addition to a similar dose from radon. Comparable levels occur in Brazil and Sudan, with average exposures of up to about 40 mSv/yr.
Several places are known in Iran, India and Europe where natural background radiation gives an annual dose of more than 50 mSv and up to 260 mSv (at Ramsar in Iran). Lifetime doses from natural radiation range up to several thousand millisievert. However, there is no evidence of increased cancers or other health problems arising from these high natural levels.
Radiation arising from human activities typically accounts for about 15% of the public's exposure every year. X-rays and other medical procedures account for most exposure from this area. The rest comes from coal burning, appliances, and other industrial and research procedures.
About 1% of exposure is due to the fallout from past testing of nuclear weapons or the generation of electricity in nuclear, as well as coal and geothermal power plants.
Natural sources of Radiation- 50% radon: The air is naturally radioactive, because it contains radon. This gas is a product of radioactive decay in uranium and thorium, and it continuously seeps out of the earth's crust in small quantities.
Uranium is distributed throughout the earth's crust in an average ratio of 2 parts per million, and 3 parts per million in sea water. Its concentration is higher in certain rocks such as granite. Normally, radon concentration is low, but it can build up inside buildings (entering through cracks in the floor or directly from the building materials used). This is the biggest single source of background radiation. - 14% gamma rays from rocks, soils and building materials: The rocks in the earth's crust are also naturally radioactive, which means that building materials like bricks and concrete are radioactive because they are made of materials taken from the earth, like sand and clay. The average annual dose from gamma rays is 0.35 mSv. Any individual's own dose depends on where they live and of what materials their house or office is built. If you live or work in a building made of granite or keep your doors or windows shut then this will increase the level of radioactivity you are exposed to.
- 11.5% food and drink: Plants and animals take in naturally radioactive materials from the earth, making everything we eat or drink radioactive. Some foods, like Brazil nuts, tea, coffee and bread, contain more radioactive materials than others. The average dose from this source is 0.3 mSv. This also means that human beings become sources of radiation in our own right.
- 10% cosmic radiation: A lot of radiation comes from the sun and from outer space, but the atmosphere shields us from most of it. People who live in the mountains, for example, get more radiation because there is less air to shield them. Someone flying in a plane could get up to 100 times the amount of radiation as someone at sea level. The average annual dose at sea level is 0.25 mSv, and for every 30m above sea level someone lives, 0.0001 mSv should be added. Every transatlantic flight gives a dose of 0.05 mSv.
Artificial sources of radiation- 14% medicine: In most countries, medicine is the largest source of artificial exposure to radiation. The most common sources are X-ray examinations; the most powerful sources are radiation doses to treat cancer. Radioactive materials are used in many other areas of medicine to prevent and treat diseases. Typical doses from X-ray examinations are: dental 0.02 mSv; chest 0.05 mSv; pelvis 1.2 mSv; spine 2 mSv.
- 0.5% miscellaneous, including: Power stations: Both nuclear and coal-fired power stations contribute radioactive materials to the environment – the ash from coal-fired power stations contains thorium and uranium which is radioactive, while nuclear stations release traces of radioactive gases. The typical annual dose if you live at the site boundary of a nuclear power station is 0.05 mSv; if you live about one mile away, 0.005 mSv; if you live one mile from a coal-fired power station, 0.0004 mSv; and if you live more than five miles away from any power station, the dose is undetectable.
- In the home and office: Most people are exposed to small radiation doses from a variety of everyday objects such as smoke detectors. The average annual dose is 0.0004 mSv.
- Chernobyl: The 1986 accident at the Chernobyl nuclear reactor in Ukraine released large amounts of radioactive material into the air. The average dose from this to people in the United Kingdom that year was about 0.03 mSv, just over 1% of total exposure, though it was undetectable in subsequent years.
- Fallout from atomic weapons tests of the 1950s and 1960s: Material thrown into the upper atmosphere by these tests is still gradually falling to earth, giving an annual dose of about 0.05 mSv.
