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National Radiation Laboratory |
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Information Sheet No. 5
February 1998
Sources, Effects and Risks of Ionising Radiation
Introduction
Ionising radiations are electromagnetic radiations and sub-atomic particles with sufficient energy to cause ionisation when they pass through or are absorbed in materials. Ionising electromagnetic radiations include x‑rays and gamma rays.
Ionising radiations can be divided into directly ionising and indirectly ionising categories. Charged particles such as beta and alpha rays emitted by radioactive materials are directly ionising. X-rays and gamma rays, which are electromagnetic radiations, and neutrons, which are uncharged particles, cause ionisation indirectly, by giving rise to secondary charged particles (electrons and protons) when they pass through materials.
Radiation units
The SI unit of absorbed dose is the gray (Gy), equal to 1 joule of energy deposited per kilogram of material. Studies of the biological effects of radiation have shown that heavily ionising particles are more damaging per gray than beta or gamma radiation. In general terms the ability of radiation to harm biological matter is dependent on the way in which energy is transferred. Heavy particles such as alpha particles and neutrons transfer their energy in a shorter distance than gamma rays and beta particles of the same energy. The average energy loss per unit track length is called the linear energy transfer or LET. For the same dose or energy absorbed per unit mass the higher the LET the greater the biological damage.
For the purpose of radiological protection a radiation weighting factor wR is used to take into account the different biological effectiveness of different types of radiation. Absorbed dose in tissue (DT) weighted by this radiation weighting factor is called the equivalent dose (HT), ie, HT = DTwR. The unit of equivalent dose is the sievert (Sv), with submultiples of millisievert (mSv) and microsievert (µSv).
In the case of non-uniform irradiation of the body the effective dose, E, is determined. This is the equivalent dose which if given uniformly would give rise to the same risk of effect as the non-uniform irradiation. The effective dose is determined by summing weighted equivalent doses for different body organs. E = ∑TWTHT, where the tissue weighting factors take account of the differing sensitivities of different organs to radiation effects.
Sources of radiation exposure
In New Zealand approximate contributions to mean population radiation exposure may be summarised as in the Table.
Average per caput radiation dose in New Zealand
|
Source |
E (µSv/y) |
|
Natural |
1800 |
Natural background radiation is by far the largest contributor. It is made up of a number of components. Cosmic radiation from space amounts to about 300 µSv per year at sea level with approximately a doubling in dose level for every 1500 metres of altitude. Moving a further 35 m up a hill would therefore add around another 10 µSv. Gamma radiation from radioactive materials naturally present in soils and building materials, principally the uranium and thorium series and potassium-40, contributes an average of about 150 µSv per year in New Zealand. This is low compared with levels in most countries. Variations from one place to another, however, are quite large, and even moving from a wooden to a brick house could add some 100 µSv per year.
In some parts of the world the terrestrial component of natural background radiation reaches more than 100 times greater than the world average value. Radioactive material present naturally in our bodies (principally potassium-40 present in a constant proportion of 0.0118% of all potassium) contributes about 200 µSv per year. Another important contribution is received from radioactive decay products of radon. Radon is a gas and the daughter of radium. The radioactive decay of radium in soils leads to the emanation of radon gas into the atmosphere, and inhalation of radon daughters leads to irradiation of the bronchial epithelium.
The medical use of radiation for diagnostic x-rays, radiotherapy and radio-pharmaceutical procedures contributes about 500 µSv per year averaged over the population. Diagnostic x-rays are the most important component.
Radioactive fallout from atmospheric nuclear weapons tests gave rise to a maximum dose (effective dose equivalent) in New Zealand of about 30 µSv per year around 1965-66, and this fell to about 10 µSv per year by 1980 and is continuing to decline. The contribution from air travel arises from the increased dose rate from cosmic radiation at aircraft cruising altitudes.
The effects of radiation
Radiation affects materials by causing ionisation, which is the removal of electrons from individual atoms. Since these electrons are involved in the bonding of atoms into molecules it is not surprising that chemical changes result. In living matter the DNA macromolecules carrying the genetic information required for the development and division of cells are considered the most critical targets. Radiation may alter a small part of the DNA molecule such as a single gene or it may break one or both strands of the DNA in one of many places, destroying or altering some of the information carried. The damage is often reparable but in some cases it can result in cell death or cell transformation. Dead cells are normally absorbed or rejected by the organism. However, if a sufficient number of cells are killed, functions will be affected and the organism may die. Cell transformations or mutations, however, do not necessarily lead to any deleterious effects in the organism. Indeed very large numbers of such cellular changes occur normally. Very rarely, however, they may result in a cancer or, in the case of cells responsible for reproduction, in hereditary damage in later generations.
The evidence for effects in man comes from studies of the Hiroshima and Nagasaki bomb survivors, from patients who have received doses of radiation for medical purposes, and small numbers of workers such as radium dial painters, and some miners who have been exposed to relatively high levels of radiation. Radiation effects have been observed in many animal studies and much detailed information on cell damage and on various types of mutations has been obtained from studies of mammalian cell cultures.
Delayed effects from radiation exposure are divided into two types, somatic (malignant disease in the exposed individual) and hereditary (inherited abnormalities in the descendants of exposed individuals). Comprehensive reviews of these effects are carried out by the Committee on the Biological Effects of Ionizing Radiation (BEIR) of the National Academy of Sciences, USA, and the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). The more significant somatic effects are leukaemia and cancers of a variety of organs, and the data are sufficient to estimate risks in terms of cancer deaths per 10 000 of exposed population per sievert for many of these. The total of all fatal cancers as an average for both sexes and all ages which may ultimately result from a given uniform whole-body exposure is about six to ten times that for leukaemia alone. For doses of around 1 Sv the total fatal malignancy risk is estimated to be of the order of 10 x 10-2 per Sv. At lower doses the risk coefficient for low LET radiation, averaged over all ages, is about 5 x 10-2 per Sv. Children and a foetus exposed in utero are somewhat more sensitive with a risk rate of about twice this.
Dose limits
Dose limits have been recommended by the International Commission on Radiological Protection (ICRP) for both occupational and public exposure. The limits are set at levels such that continuous exposure at the limit does not represent an unacceptable risk of detrimental effects. The limits are set out in the Table.
Recommended dose limits1
| Application |
Dose limit |
|
| Occupational | Public | |
| Effective dose | 20 mSv per year, averaged over defined periods of 5 years2 | 1 mSv in a year3 |
| Annual equivalent
dose in the lens of the eye the skin4 the hands and feet |
150 mSv 500 mSV 500 mSV |
15 mSv 50 mSV - |
| 1 | The limits apply to the sum of the relevant doses from external exposure in the specified period and the 5‑year committed dose (to age 70 years for children) from intakes in the same period. |
| 2 | With the further provision that the effective dose should not exceed 50 mSv in any single year. Additional restrictions apply to the occupational exposure of pregnant women. |
| 3 | In special circumstances, a higher value of effective dose could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv per year. |
| 4 | The limitation on the effective dose provides sufficient protection for the skin against stochastic effects. An additional limit is needed for localised exposures in order to prevent deterministic effects. |
Using the ICRP risk coefficient for a working population (18-64 years) of 4 x 10-2 per Sv, continuous exposure over working life at the dose limit of 20 mSv per year represents a lifetime risk of fatal cancer of about 4 x 10-2, and a maximum risk rate of the order of 1 x 10-3 per year attained at an age of about 80 years. This risk rate is comparable with that of prompt fatality in the most hazardous types of employment. The lifetime risk can be compared with the 'natural' lifetime risk of fatal cancer in New Zealand which is about 0.25.
The lifetime risk from exposure at the public limit of 1 mSv per year is about 3 x 10-3, or about half that arising from natural radiation exposure.
For further information, contact
National Radiation Laboratory
P O Box 25099
Christchurch
Phone: (03) 366 5059
Fax: (03) 366 1156
Internet: http://www.nrl.moh.govt.nz