Fact Sheet 4

α particles do not even penetrate the dead outer layers of the skin. Alpha emitting materials are therefore of concern only if they are taken into the body – for example by inhalation or ingestion. Once inside the body, however, α particles are more damaging than either β particles or γ radiation.

β particles penetrate up to a few millimetres into the body. β emitting substances can therefore be of concern both outside and inside the body.

γ rays penetrate deep into the body and can indeed pass right through it. γ emitters are mainly of concern as sources of radiation external to the body.

MEASUREMENTα and β particles and γ radiation are all easily detected and measured with instruments such as what used to be called a geiger counter.

Several units are used to quantify radiation as it passes through the body. The unit of radiation damage (dose) is the sievert (Sv) or, for lower radiation exposures, the millisievert (mSv). For very low exposures, the microsievert (µSv) is used. Thus, throughout the world, our average annual radiation exposure due to naturally occurring radioactive materials in the environment and in our bodies is conveniently expressed as 2.4 millisieverts (mSv) per year - but could also be written 0.0024 Sieverts (Sv) or 2400 microsieverts (µSv).

RADIATION AND HEALTHRadioactive materials and how they come to be present on Earth, naturally and man-made, are discussed in NIASA Fact Sheet 3. All radioactive materials emit ionising radiation, i.e. radiation with suffi ciently high energy to interfere with the structure of the atoms and molecules in the matter through which it passes.

High levels of radiation exposure can be very damaging. Low level exposure is much less so.

� ere are three main types of ionising radiation of concern to radiation protection specialists. � ey are known as alpha (α), beta (β) and gamma (γ) radiation.

Alpha and beta radiation consists of fast moving particles. More precisely, alpha ‘rays’ are composite particles consisting of two protons and two neutrons. Typically, alpha particles are emitted by heavy radioactive nuclei such as uranium and thorium.

Beta ‘rays’ are fast moving electrons. In living tissue, the most commonly encountered beta-emitter is potassium-40, the radioactive isotope of potassium that, like uranium and thorium, has forever been present in rocks in the Earth’s crust.

� e emission of alpha and beta particles is usually accompanied by gamma radiation. � is can be compared with very penetrating X-rays.

For radiation protection purposes, α, β and γ radiation can be distinguished by their very diff erent penetrating power.

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Radiation monitor measuring radiation levels

Radiation workers handling new uranium fuel assembly

Fact Sheet 4Uranium and its daughter products (see Fact Sheet 3) contained in granite also leads to elevated radiation dose levels, the best known example in South Africa being in the Paarl area where radon gas in poorly ventilated buildings can reach levels at which remedial action should be taken.

Altitude also plays a part. Relative to Cape Town, there is less atmosphere above Johannesburg, for example, to shield Gauties from cosmic radiation. � e cosmic component of typical radiation dose levels in Johannesburg is therefore around 0.3 mSv/y higher than in Cape Town. A single twelve-hour fl ight to Europe adds another 0.05 mSv (or 50 µSv) to your annual radiation exposure.

Medical exposure also varies widely. Doses associated with particular procedures range from around 0.1 mSv for a chest X-ray to 20 mSv for an extensive CT-scan.

OCCUPATIONAL EXPOSUREWorkers at nuclear power stations are permitted to receive 20 mSv/y. Typically they average 1 mSv/y in addition to their natural background exposure of around 2.4 mSv/y. In emergency situations, however, they are allowed to receive more. At Fukushima, about twenty workers received between 100 and 200 mSv, the authorised emergency limit being 250 mSv. � is may have increased their risk of developing cancer in later life by up to one percentage point. See page 4

BACKGROUND RADIATION EXPOSUREWe are constantly being irradiated by radioactive materials naturally present in the environment. The greatest source is radioactive radon gas in the air we breathe (see NIASA Fact Sheet 3). By irradiating the fine structure deep in our lungs, largely with damaging α particles, radon and its radioactive daughter products contribute about half of our total ‘background’ radiation exposure. Potassium-40 and carbon-14 in our food and radon gas dissolved in drinking water add a further 10%.

Cosmic radiation from outer space contributes around 13% as do gamma rays from the rocks and soil beneath our feet and from building materials that surround us.

� e other major contributor is medical exposure. In Britain medical procedures contribute a further 15% to the national average radiation exposure. People whose work brings them into contact with ionising radiation add just 0.2% to the national average exposure. Finally, weapons testing fall-out largely from the 1960s still accounts for a similar 0.2%.

Actual levels vary widely from place to place. � is chart refers to Britain. � ere the segments add up to an annual average exposure of 2.7 millisieverts per year (2.7 mSv/y).

� e place with the highest known natural radiation exposure in the world is the coastal town of Ramsar in Iran where the population receives radiation doses up to 300 mSv or more per year. Hot springs bring radium and radon gas up from underground uranium deposits. Studies of the health of populations in places such as Ramsar have failed to reveal any ill-effects attributable to radiation.

Other examples are coastal areas in Brazil and in Kerala in India where naturally radioactive thorium in the sand gives rise to exceptionally high radiation levels. In South Africa the highest known levels are due to thorium in rock in the vicinity of the thorium mine at Steenkampskraal on the West Coast.

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Afrikaans Language Monument, Paarl

Fact Sheet 4

SOURCES OF KNOWLEDGELiving organisms are made up of billions of cells performing essential functions to keep the organism alive and well. ‘Ionising’ radiation has enough energy to disrupt the bonds that bind atoms together to form the molecules that constitute the cell.

If the radiation damage is intense, the body’s regular repair mechanisms may be overwhelmed and the cell may die. If the damage is slight, the repair mechanism will deal with the problem. If the damage is to the DNA molecules in the cell’s nucleus, the cell may be able to reproduce itself although defective. � e daughters of such cells may later begin to replicate uncontrollably as a cancer. What triggers this later development ten, twenty or even more years after irradiation is not understood.

Types of cells that divide frequently such as blood and hair precursor cells and germ cells are more susceptible to radiation damage than, for example, long-lived nerve cells. A foetus is therefore more vulnerable to cell damage than an adult.

What does all this mean for the organism as a whole and, in particular, for human beings? How do we know?

Ionising radiation has been studied intensively since the discovery of X-rays in 1895. � ousands of sources of information include studies of the health of the early radiographers themselves, of hospital patients treated with large doses of radiation, of victims of accidents involving radioactive sources and of the populations of areas such as Ramsar with high natural background radiation. � ere are also groups of military and other personnel caught up in nuclear weapons testing and, more recently, of site workers and the population close to Chernobyl. It appears that no one was signifi cantly irradiated at Fukushima other than the workers already mentioned.

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Our greatest source of information remains irradiated survivors from relatively close to ground zero at Hiroshima and Nagasaki. � e radiation exposures of approximately 100 000 individuals was calculated in the 1950s and their health records have been monitored ever since. Several hundred more than would have been expected have died of various types of cancer over the past seventy years or so. � e incidence of various types of cancer can therefore be related to known levels of radiation exposure.

Children born less than nine months after the bombings have exhibited various problems including lowered IQ. � ere has, however, been no observable increase in genetic defects in the off -spring of the survivors. In fact, genetic defects have never been observed in any human populations exposed to higher than normal radiation levels. Genetic eff ects are therefore considered to be of rather less concern than the induction of cancer and leukaemia.

CONTROLLING RADIATION EXPOSURE� e vast and ever-growing library of radiation information is monitored by national and international organisations such as the United Nations Scientifi c Committee on the Effects of Atomic Radiation (UNSCEAR) and the International Commission on Radiological Protection (ICRP). ICRP makes recommendations for maximum acceptable levels of radiation exposure for radiation workers (in both normal and emergency situations) and on much lower permissible exposure levels for members of the general public.

ICRP recommendations are reviewed by national nuclear safety authorities such as the South African National Nuclear Regulator (NNR) and are usually incorporated into legally binding national regulations.

Trained ‘radiation workers’ wear devices that continuously record their radiation exposure. � ey also undergo pre-employment and thereafter annual medical examinations.

Wilhelm Roentgen, discoverer of X-rays in 1895.

Koeberg Nuclear Power Station

Fact Sheet 4

HEALTH EFFECTSOver-exposure to ionising radiation, as to other forms of electromagnetic radiation including sunlight and intense radio-waves, can be dangerous and even fatal. Fortunately, ionising radiation is easy to detect and measure even at levels far below what could be considered harmful. With due care, therefore, radiation exposure is easy to control.

� e eff ects of acute (short-term) whole-body exposure to ionising radiation are discussed below. � e eff ects of the same doses delivered over long periods, for example over a year, are much less severe.

5000 mSvDepending upon the medical treatment available, 5000 mSv (5 Sv) will cause sickness and death in half the exposed population within a matter of weeks. 8000 mSv (8 Sv) will probably kill everyone exposed.

1000 mSvExposed personnel will become somewhat sick but will not die, certainly in the short-term. � ere will, however, be an increased probability of cancer (including leukaemia) later. If a group of 100 people is exposed to 1000 mSv, about fi ve will probably develop cancer in later life – in addition to the twenty or more who will eventually succumb to the disease in the normal course of events.

250 mSv� is is the lowest exposure known to produce observable clinical symptoms, specifi cally changes in the blood count and visible chromosome damage.

100 mSv� ere will be no short-term symptoms. No health eff ects of any sort have ever been observed in any population exposed to less than 100 mSv.� is is not to say defi nitely that there are no health eff ects below 100 mSv merely that, if there are, they are too few or too slight to show up statistically in major epidemiological studies of exposed populations.

20 mSvAnnual allowable exposure for radiation workers, for example at nuclear power stations (see below).

1 mSvAnnual exposure typically received by radiation workers at nuclear power stations.

Ionising radiation is to be respected but need not be feared.

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There are scientists who maintain that exposure right down to the level of natural background radiation (~ 2.4 mSv) is harmful. This is difficult to prove or disprove. However, populations in areas with much higher levels of exposure, particularly due to radon and radioactive material in the lungs, do not show adverse health effects. Indeed, there is evidence that even higher levels of radiation, possibly by triggering the body’s immune system, are actually beneficial. This possible phenomenon is known as radiation hormesis.

MAXIMUM ALLOWABLE RADIATION EXPOSURERadiation safety authorities base their rules on the deliberately ‘safe’ but unprovable assumption that even the smallest exposure to ionising radiation is harmful. At the same time, professional scientifi c organisations make it clear that this prudent approach does not justify claims that large numbers of cancer cases arise from the exposure of very large populations to very low levels of radiation as, for example, around Chernobyl.

Radiation workers� e internationnaly accepted ICRP recommendation is that radiation workers may not be exposed to more than 100 mSv in any fi ve year period, and to not more than 50 mSv in any one year. � is includes external radiation exposure and any contribution due to radionuclides taken into the body.

General publicNo member of the general public, for example someone living close to a nuclear facility, may receive more than 1 mSv per year in addition to natural background radiation exposure of about 2.4 mSv.

� is aspect is discussed further in NIASA Fact Sheet 7.

Nuclear Industry Association of South Africawww.niasa.co.za

6 August 2012

Pelindaba Nuclear Research Centre