Publications

Information Sheet No. 5

Reviewed September 2009

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Sources, Effects and Risks of Ionising Radiation

Introduction

Ionising radiations are electromagnetic radiations and sub-atomic particles with sufficient energy to cause ionisation (the removal of electrons from individual atoms) when they pass through or are absorbed in materials. Ionising electromagnetic radiations include x-rays and gamma-rays and subatomic particles include electrons (beta particles) and neutrons.

Ionising radiations can be divided into directly ionising and indirectly ionising categories. Charged particles such as beta and alpha particles (two neutrons and two protons) emitted by radioactive materials are directly ionising. X-rays, gamma-rays and neutrons, which are uncharged, 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 (which is the quantity fundamentally related to the risk of biological effects) is the gray (Gy), 1 Gy being equal to 1 joule of energy deposited per kilogram of material. Studies of the biological effects of radiation have shown that heavily ionising particles such as alpha particles and neutrons are more damaging for the same absorbed dose than lightly ionising radiation such as beta particles or gamma-rays. In general terms the ability of radiation to harm biological matter is dependent on the way in which energy is transferred. Heavily ionising particles transfer their energy in a shorter distance than lightly ionising 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 w_R is used to take into account the different biological effectiveness of different types of radiation. Absorbed dose in tissue (D_T) weighted by this radiation weighting factor is called the equivalent dose (H_T), ie, H_T = D_Tw_R. The unit of equivalent dose is the sievert (Sv), with commonly used submultiples of millisievert (mSv) and microsievert (µSv).

Different tissues in the body have different sensitivities to ionising radiation, and the quantity effective dose, E, has been defined to account for this. The effective dose is the equivalent dose which if given uniformly to the whole body, 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 = ∑_TW_TH_T, 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
Medical: 500
Fallout: 5
Air Travel: 6
Occupational: 0.5
Luminous Dials: <2

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 radiopharmaceutical 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

Electrons are involved in the bonding of atoms into molecules and their removal by ionisation will result in chemical changes. 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 people comes mainly 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). Data from these committees and other sources are reviewed by the International Commission on Radiological Protection (ICRP) enabling it to derive and publish recommendations and guidance on radiological protection. In its Publication 103 the ICRP as an indicative risk for fatal cancers states

“… the approximated overall fatal risk coefficient of 5% per Sv … continues to be appropriate for the purposes of radiological protection”.

At younger ages, the probability of induction of cancer following exposure to ionising radiation is higher than the nominal value of 5% per Sv, and as age increases this probability decreases. Therefore, ICRP prior to Publication 103 introduced age-dependent renormalisation factors to correct risk factors for the age effect. These are as below.

Age (years)	1	5	10	15	50	70
Renormalisation factor	3.0	2.5	2.0	1.5	0.5	0.3

The correction factor for adults between the ages of 15 and 50 is taken to be unity.

Dose limits

The ICRP have recommended both occupational and public dose limits in its Publication 103. The dose 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 limits in planned situations^a

Type of limit	Occupational	Public
Effective dose	20 mSv per year, averaged over defined periods of 5 years^d	1 mSv in a year^e
Annual equivalent dose in
Lens of the eye^b	150 mSv	15 mSv
Skin^c	500 mSv	50 mSv
Hands and feet	500 mSv	-

a: Limits on effective dose are for the sum of the relevant effective doses from external exposure in the specified time period and the committed effective dose from intakes of radionuclides in the same period. For adults, the committed effective dose is computed for a 50-year period after intake, whereas for children it is computed for the period up to age 70 years.
b: This limit is currently being reviewed by an ICRP Task Group.
c: Averaged over 1 cm² area of skin regardless of the area exposed.
d: 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.
e: 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.

The public dose limit of 1 mSv per year is about half the dose arising from natural radiation exposure.

For further information, contact:

National Radiation Laboratory
P O Box 25099
Christchurch
New Zealand

Phone:: + 64 3 366 5059
Fax:: + 64 3 366 1156
Internet:: http://www.nrl.moh.govt.nz

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