/ . Soc. Occup. Med. (1976) 26, 43-49
Health Care of People at Work Safety in the Handling of Radionuclides NORMAN SPOOR Research and Development Division, National Radiological Protection Board
According to a recent estimate (Webb, 1974) about 78,000 workers in the United Kingdom are exposed or potentially exposed to ionizing radiation. Most of them are employed either in the National Health Service or in a field of industry—construction, manufacturing or nuclear power. The occupational risks associated with the use of radiation on this scale are in general far outweighed by the social benefits. However, the safety of these workers and their progeny may be assured only by strict control of radioactive sources and regular monitoring of the working environment. The aim of this paper is to outline the principles by which radiation hazards are assessed and controlled. Types of Radiation There are several kinds of radiation, of which the most common are the following: 1. Alpha particles. These are swiftly moving particles of high energy, which carry a positive electrical charge. Because of their relatively large mass they have little penetrating power and are stopped by a sheet of paper or thin metal foil. From outside the body alpha particles are not harmful; from inside the body they cause considerable cellular damage. 2. Beta particles. These are fast-moving electrons and are much more penetrating than alpha particles. From outside the body and from inside they can cause biological injury. 3. Gamma rays. These electromagnetic radiations of high energy have properties similar to those of X-rays. They may traverse several centimetres of tissue with little absorption. From outside the body and from inside they are a potential danger to all internal organs.
4. Neutrons. These are normal constituents of atomic nuclei which may be released in nuclear reactions with considerable energy. They can penetrate several centimetres into tissue and deposit energy by collision with other nuclei. A single substance may emit one or more of these types of radiation and may itself change into another radionuclide and emit further radiations. The higher the specific activity of an isotope (Ci/g) the more rapidly does the activity die away. This decay is an exponential phenomenon which is most conveniently expressed in terms of half-life, that is, the time required for the activity to decay to half its initial value. The specific activity has a bearing on the maximum permissible concentrations in air of radionuclides, which are usually expressed in terms of activity per unit volume of air (Ci/m3). When these concentrations are expressed in terms of mass (g/m3) it is clear that the maximum
Table I. Maximum permissible air concentrations (occupational exposure) for some stable and some radioactive substances: a comparison
(MPQ
Inactive substance (mg/m») Carbon dioxide Ethyl bromide Benzene Hydrogen cyanide Iodine Cobalt Silver Beryllium
10* 10» 10l 10 1 10"' 10"1 10" 10" 10" 10" 10"
Active substance
Uranium 238
Carbon 14 Tritium Caesium 137 Plutonium 239 Strontium 90 , 0 -io Phosphorus 32 10"" Californium 252
10"" 10"" Radon 220
43
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Summary This paper distinguishes between various kinds of occupational radiation hazard and describes principles by which they may be understood and controlled.
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OCCUPATIONAL MEDICINE
permissible concentrations of most radionuclides are much lower than those of most chemical contaminants (Table I). There are a few compounds (natural uranium, for example) that are of low specific activity for which the toxicological hazard and the radiological hazard are of comparable seriousness.
External and Internal Radiation Sources Radionuclides are widely used in industry and medicine. In industry, gamma radiation from iridium 192 for example is used in radiography, and beta particles from strontium 90 are used in thickness gauges. In medicine, cobalt 60 sources are used in radiotherapy. In normal use the hazards from such sealed sources are only external. In industrial processes, such as the manufacture of nuclear fuel and the extraction of plutonium from irradiated fuel rods, workers may be inadvertently exposed to airborne radioactive materials. In hospitals and universities where radionuclides are often handled as unsealed sources, accidental spillages may occur; in such situations a radionuclide may enter the body by inhalation, ingestion, or open wound, and present a potential internal radiation hazard.
Table II. The relative contributions from different sources to the annual genetically significant dose (GSD) in the United Kingdom* Source Natural background Medical irradiation Fall-out (annual) Miscellaneous sources Occupational exposure Nuclear power industry (waste disposal) Total * Webb (1974).
UK collective gonaddose {man rady'1) 4,800,000 780,000 121,000 38,500 32,500 500
5,800,000
Annual GSD Percentage (mrady~l) 87 14 2-2 0-3 0-4 001 104
83-7 13 5 21 0-3 0-4 001
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Biological Effects of Radiation When the radiations described above are absorbed in tissue they deposit their energy at random along tracks of ionization. Some of this energy breaks bonds and causes chemical changes at the molecular level and this leads to biological damage. The most important biological effect arises from damage to the DNA molecules in cell nuclei. This damage may deprive the cell of its ability to pass through cell division, and most of the observed acute effects of high doses of radiation result from a depletion in the numbers of dividing cells in the body. The effects resulting from the depletion of dividing cell populations are not observed in workers exposed at the lower levels of radiation dose. At these low levels the biological risk is from cancer induction, which probably originates from small viable changes induced in the DNA. These changes occur at random and there is a finite chance of cancer induction even at very low doses. However, this risk is very small, and not more than a few in a million for doses of 1 rad. The purpose of radiological protection is to reduce the risk of late effects to an acceptable minimum. There are two types of biological effect: the somatic effect, which eventually injures the irradiated person, and the genetic effect, which
eventually injures some of the descendants of the irradiated person. Evaluations of the damage from exposure to radiation indicate that a substantial part of the whole damage is that which is expressed in the descendants of exposed individuals. Because the genetic dose influences the whole population it is usually expressed in statistical terms. One of the most useful criteria, the annual genetically significant dose, is an estimated average dose to the whole population (ICRJP, 1966; Webb, 1974). The results of a recent survey have shown that in the United Kingdom the main contribution to the annual genetically significant dose is from natural background (84 per cent); 13 per cent results from medical irradiation and 0-4 per cent from occupational exposure (Table II).
HANDLING RADIONUCLIDES
The International Commission on Radiological Protection The International Commission on Radiological Protection (ICRP) was established in 1928 under the name of International X-ray and Radium Protection Commission, with the purpose of advising users of ionizing radiations on the safe levels of exposure. Over the years ICRP recommendations have become accepted throughout the world. At present there are four ICRP Committees
making comprehensive recommendations which are published in a series of reports. Committee 2 deals with permissible levels from internally incorporated radionuclides. It published a report in 1959 which contains recommendations of permissible exposure to radionuclides in air and water {Tables ///and IV). In making recommendations for maximum permissible doses the ICRP emphasize that they are maximum values and further recommend that 'all doses be kept as low as practicable and that any unnecessary exposure be avoided'. In other publications the ICRP also advise on the implementation of basic principles, e.g. Radiation Protection in Schools for Pupils up to the Age of 18 Years (ICRP, 1970) and Protection of the Patient in Radionuclide Investigations (ICRP, 1971).
Table III. Dose limits in a year*
Organ or tissue
Gonads and red bone marrow Skin, bone and thyroid Hands and forearms; feet and ankles Other single organs
Exposure Exposure experienced experienced by adults in by members the course of ofthe their work public {rents) {rems) 5 30
0-5 30
75 15
7-5 1-5
* ICRP (1966).
Chemical vis-a-vis Radiological Criteria At exposure levels ten times the threshold limit value (TLV) the effects of chemical toxins are immediate or early, and usually identifiable. Accordingly, the aim of toxicological protection is to control exposure to an extent which prevents adverse immediate and chronic effects or reduces them to an acceptable level. At exposure levels ten times the maximum permissible concentration (MPC) the effects of radiation are, by contrast, latent and unidentifiable. The aim of radiological protection is therefore to control exposure to an extent which effectively prevents those delayed effects (e.g. cancer, life-shortening and cataract) known to be related to the cumulative dose. At low exposure levels these effects are presumed to be minutely small and statistically so elusive that their seriousness may be assessed only by theoretical
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In medicine, radionuclides are administered for diagnostic or therapeutic purposes. Iodine 131labelled triolein is given in gastro-intestinal function tests; iodine 131-labelled human serum albumin is injected for determining cardiac output, and sodium iodide 131 is used for thyroid function tests. Phosphorus 32 is given, orally or intravenously, for the treatment of polycythemia vera, and gold 198 is used in the treatment of cancer of the prostate. About 25 nuclides are currently used in medical practice or research and many scientists, physicians and nurses are employed in the preparation and administration of labelled compounds. When an insoluble substance like plutonium dioxide is deposited in the lung after inhalation, it is likely to remain there for years, exposing the lung tissues to continuous radiation. Such materials also enter the gastro-intestinal tract from which they are absorbed only to a minute extent; consequently the concentrations in the blood are rarely significant. A soluble substance, like tritiated water vapour, sodium iodide 131, or caesium 137 carbonate, is rapidly absorbed into the blood from both lungs and the gastro-intestinal tract. The radionuclide then moves out of the blood into organs and tissues; tritium permeates the whole body, iodine concentrates in the thyroid gland and caesium concentrates in muscle. Once in the blood each element follows a metabolic pathway characteristic of that element and its chemical analogues; e.g. caesium behaves like potassium, and strontium like calcium. Metabolic extrapolations based on chemical affinity may be misleading, however (cf. fluorine and iodine), and some elements (e.g. thallium, plutonium) have no useful analogue. If the radionuclide enters the body as part of a complex ion or an organic molecule, it will follow the metabolic pathways of the ion or molecule and the assessment of the radiological hazard will depend on the identification of those pathways.
45
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OCCUPATIONAL MEDICINE
Table IV. Maximum permissible body burdens and maximum permissible concentrations (MPC) of some radionuclides in air and water for occupational exposure* Nuclide
Body burden
Critical organ'f
UiCi) »"Pu
"•Ra 131 t0
I Sr
"Sr
004 01 0-7 2 4
6 7 30 300 1000
187
Cs
"C 3 H
MPC in water OiO/cm')
2x10"" 3x10"" 9x10"
io-« 4x10-'5
3x10" 10 3x10" 7x10" 10"
6x10" 4x10" 5xl0"«
6xlO" 4x10"' 3X10"'
5x10"'
6xlO" 3 4x10-' 2x10-' 10"x
• ICRP (1959). t The critical organ is the organ which receives the highest radiation dose.
estimates on large populations. In toxicological protection it is assumed that there is a 'threshold', i.e. a level of exposure which can be accepted indefinitely without risk of injury or discomfort. In radiological protection it is assumed, in statistical calculations concerning large groups, that there is no threshold. Associated with these differences of principle are the different assessments of the individual exposure. For instance, a personal inhalation of 2 mg iodine in the course of one hour would be an unpleasant experience involving an appreciable physiological insult to the body. The experience might be transient and harmless, however, and of no long-term significance. In contrast, a personal exposure to 3 rem radiation in the course of one hour would not be accompanied by any personal sensation or physiological disturbance. The significance of such an exposure is notional, and is assessed entirely in the context of long-term radiation risks. The distinction between a rapid unpleasant insult of no clinical consequence on the one hand, and on the other an unfelt insult associated with a definite but unknown risk which can be assessed only within a wide time perspective, is very relevant to the appreciation of the problems of radiological protection. Control of Hazards Laboratory Design Special facilities are required for the handling of large amounts of the more toxic radionuclides, and
the most suitable laboratories are purpose-built. The aim is to prevent the spread of radioactivity and to ensure that radiation levels are kept as low as possible. 'High-level' laboratories are separated from 'low-level' laboratories and both are rigidly separated from change-rooms, washing facilities and counting rooms. All laboratories are built with floors amenable to regular polishing and with gloss-painted walls, and are equipped with fume cupboards. Shielding Shielding requirements are often determined by the amount of space available around the source. Given a weak source or considerable space the exposed person may withdraw to a distance at which the dose rate falls to an acceptable level. The dose rate falls off roughly according to the inverse square law. In other situations some form of shielding is required to reduce the radiation level. Beta rays are stopped by plastic plates of a few centimetres thickness. Gamma rays of an energy exceeding 0-5 MeV are very penetrating and a considerable thickness of shielding material (lead for example, or concrete) may be needed to stop them. One of the principal requirements in neutron shielding is the reduction of their energy, as the permissible flux of neutrons increases as the energy decreases. Materials of low atomic mass, such as water and paraffin wax, slow down neutrons by elastic collisions; cadmium or boron shields, which absorb neutrons, also reduce the dose.
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"P »Na
Bone Bone Thyroid Bone Bone Bone Total body Total body Fat Body tissue
MPC in air OiQ/cm3)
HANDLING RADIONUCLIDES
Surface Control Wherever unsealed sources are handled there are risks associated with the possible dispersal of activity into the environment; in these situations both surfaces and air may be regularly monitored. SURFACE MONITORING
Table V. Limits for surface contamination
Type of surface Inactive areas Active areas Personal clothing Skin
Principal alpha emitters 10~5 10"* 10"5
Low toxicity alpha emitters
3
10"
10- 4 10"5
Beta emitters 10" 4 10" 3 lO" 4 10-'
AIR MONITORING
Regular air sampling is a means of checking that the precautions taken to prevent the dispersal of activity have been effective. The contamination usually consists of fine particles of the active material either in its pure form or associated with dust particles. The air is sampled by drawing it through a high-grade filter paper which traps and retains a high proportion of the particulate matter. The volume of filtered air and the amount of activity on the filter paper are measured. There are three kinds of air-sampling equipment:
fixed, portable and personal. Fixed monitors are used in laboratory and nuclear reactor areas, where individual samples may be taken over periods of up to a week. Portable monitors, requiring 230 V a.c. supply for operation and weighing 7 kg, are recommended for special intermittent operations. The small personal air sampler includes a 2-cm sampling head which is pinned against the worker's coat lapel; the complete 0-7 kg unit is powered by rechargeable 6 V batteries which give a life of 10 hours before recharging is necessary. The purpose of the air-sampling programme is to ensure that the contamination level is as low as practicable and within the permitted maximum. If this requirement is not met, steps are taken either to lower the contamination level or, when this is not possible, to provide personal protection for the workers. This is available in a variety of forms, from the high-efficiency respirator, with a plastic hood and plastic overalls, to the complete rubber suit with boots, gloves and helmet, into which fresh air is supplied through an air-line. From the resemblance to the suit worn by underwater frogmen, the latter is usually known as a frogsuit. Personal Monitoring The first line of defence in radiological protection is the implementation of measures required to ensure that exposure levels are reduced to the lowest practicable level. The second is the measurement of the levels of radiation to which individual workers are exposed. The film badge is the most widely used method of measuring the radiation dose received by the individual. Its advantages are its cheapness, its small size, its capacity for integrating exposure over long periods, and the provision of a durable record. Organizations with a substantial use of radiations, such as nuclear establishments and hospitals, have their own film badge service. The National Radiological Protection Board (NRPB) provides a film badge service for many smaller organizations in the United Kingdom and currently issues 544,000 film badges a year. Classified workers (as defined by the regulations) and designated persons (as defined by the code of practice) are required to wear a personal dosimeter of an approved type. During the last decade there has been an increasing use of the radiation-induced thermoluminescent effect in substances like lithium fluoride
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Control of contamination is required not only in the interest of safety but also to avoid spurious measurements resulting from the contamination of apparatus or counting equipment. Surfaces are checked in one of two ways: either directly by means of a contamination monitor, or indirectly by means of a smear test. The contamination monitor detects surface activity whether loose or fixed without differentiating between them, whereas a smear test detects only that which is readily removable. The method is to rub a piece of filter paper lightly but firmly over the surface; when an area of about 300 cm2 has been wiped, simple counting equipment is used to measure any activity collected by the paper. The accepted limits for surface contamination are based on levels recommended in the current legislation and codes of practice {Table V).
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OCCUPATIONAL MEDICINE
examination. A count is made of the number of dicentric chromosome aberrations in 200-500 cells and from the result the dose is estimated using a dose/response calibration curve for the type and rate of overexposure suspected. At the cytogenetics laboratory, NRPB, Harwell, 180 cases of known or suspected overexposure have been examined by this technique during the last 4 years (Purrott et al., 1974). In the large majority of cases the evidence has supported the belief that the worker concerned was not seriously overexposed. Measurements of this kind have served to remove anomalies in conventional dose estimation and to avoid the severe disruption of the individual's work which would otherwise be required following a suspected overexposure. Urine Analysis One of the most useful procedures in biological monitoring is the analysis of urine samples. Clearly the rate of urinary excretion of a radionuclide (uCi/day) reflects only the rate at which that nuclide is leaving the body through the kidneys, and this may or may not reflect the amount retained in the body. When the inhaled material is insoluble (e.g. thorium dioxide, plutonium dioxide) most of it is deposited in the nasopharyngeal or bronchial regions and most of that which is not retained in the lungs is excreted via the gut. For this reason urine analysis is not used for measuring intakes of very insoluble materials. When the inhaled material is soluble it is normally absorbed via the lungs and the gut into the body fluids, and its fate is determined by the rate of radioactive decay and by the mobility of the element in the body. Some nuclides (e.g. strontium 90) are deposited in particular organs from which they are slowly released back into body fluids (and into urine and faeces) over many years. Other nuclides (e.g. iodine 131) concentrate in one or two organs or tissues from which they are released within a few weeks. The relationship between the amount of radioactivity in the body (uCi/kg) and the amount excreted in the urine varies according to the metabolism of the nuclide, the size of the intake and the solubility of the material. In a situation in which workers have been exposed to the same nuclide over a long period, the rate of excretion in the urine is a function of the amount of activity deposited in the body and of the amount recently assimilated. For this reason
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and lithium borate to measure radiation dose. These compounds are now widely used for monitoring doses to fingers and wrists. This type of dosimeter has the advantages of close resemblance to tissue as far as radiation absorption is concerned, and amenability to automation. The NRPB is currently using this dosimeter for studying doses to patients from the medical and dental uses of radiation, and intends to introduce an automated dosimeter service based on the lithium fluoride dosimeter in 1976. It is sometimes possible to measure radiation outside the body from radionuclides present inside the body. The measurement can be made to detect radionuclides in a particular organ or in the whole body. For instance, by measuring gamma radiation over the neck the amount of iodine 131 in the thyroid may be estimated. By measuring gamma radiation from the chest the level of retained cobalt 60 in the lungs may be estimated. In the whole-body monitor a number of detectors are arranged above and below the person, who normally lies in a horizontal position inside a heavily shielded chamber. The shielding required to reduce the background count from wide-angle radiation to an acceptably low level is massive. The shield and the large crystal detector are expensive. However, the whole-body monitor has proved a useful tool both in cases of accidental contamination and in research investigations, and for this reason it has been installed at several hospitals and research establishments in the United Kingdom. The whole-body monitor has been used to measure the amount and the distribution of fallout caesium 137 in the body. Among plutonium workers it has been used routinely to measure plutonium dioxide retained in the chest. Mobile gamma spectrometers are available at a number of centres and these have proved useful for screening in the event of radiation accident. When lymphocytes are exposed to radiation the number of structural chromosome aberrations increases, and the yield is related to dose. For many years this relationship has been used in radiological protection. The method is especially convenient after a radiation accident in which data from conventional physical dosimetry are suspect or absent. Venepuncture blood samples of 5—10 ml are conveyed in prepared tubes to a laboratory where a lymphocyte fraction is cultured, from which cells are fixed and slides prepared for microscopic
HANDLING RADIONUCLIDES
49
the Universities of the United Kingdom, 1966; Department of Employment and Productivity, 1968; Department of Health and Social Security, 1972).
Legislation: Codes of Practice The United Nations, the World Health Organization, the International Atomic Energy Agency and the International Labour Organization are all bodies actively interested in radiological health and safety. The ICRP, on which the United Kingdom is well represented, is the recognized international authority. Its latest recommendations on maximum doses were published in 1966 and, with minor changes, these are the basis of most of the legislation. The Medical Research Council is responsible for advising the Government on the biological implications of standards and their acceptability for application in the United Kingdom. The Acts of Parliament and the regulations made under them which refer specifically to work with radioactive materials are The Radioactive Substances Act of 1948 and 1960, and The Nuclear Installations Act of 1965 and 1969. The Radioactive Substances Act I960 requires all persons keeping or using radioactive materials to register with the Radiochemical Inspector of the Department of the Environment. The Ionizing Radiations {Sealed Sources) Regulations 1969 and The Ionizing Radiations (Unsealed Radioactive Substances) Regulations 1968 made under The Factories Act 1961 impose requirements for the protection of persons employed in factories and other places to which the Act applies. The regulations require (inter alia) that District Inspectors of Factories be notified of the use of sources and of accidents involving sources; that one or more 'competent persons' be appointed to supervise the work and assist in enforcing the regulations; that exposure be restricted; and that no employed person be exposed to radiations unless he has been instructed in the nature of the hazards involved and the precautions required. The regulations provide also for medical supervision, for the organization of work and the maintenance and use of monitoring instruments. Several interpretative codes have been written for the guidance of management and workers (the Committee of Vice-Chancellors and Principals of
Conclusion When management introduce radioactive substances into their factory or laboratory they commit themselves to a new responsibility. The hazards associated with radiation require more attention than most conventional hazards because they are more elusive. At the same time they are readily measurable and amenable to control. In order to establish and maintain safe working conditions it if necessary to provide appropriate facilities and proper supervision. The staff chosen for the work should be carefully selected bearing in mind medical requirements as well as ability and aptitude. Radiation workers must be fully informed about the nature of their duties and any associated risks.
REFERENCES Committee of Vice-Chancellors and Principals of the Universities of the United Kingdom (1966) Radiological Protection in the Universities. London, Association of Commonwealth Universities. Department of Employment and Productivity (1968) Code of Practice for the Protection of Persons Exposed to Ionizing Radiations in Research and Teaching. London, HMSO. Department of Health and Social Security (1972) Code of Practice for the Protection of Persons against Ionizing Radiations arising from Medical and Dental Use. London, HMSO. ICRP (1959) Permissible Dose for Internal Radiation. Publication 2. Oxford, Pergamon. ICRP (1966) Recommendations of the International Commission on Radiological Protection. Publication 9. Oxford, Pergamon. ICRP (1968) General Principles of Monitoring for Radiation Protection of Workers. Publication 12. Oxford, Pergamon. ICRP (1970) Radiation Protection in Schools for Pupils up to the Age of 18 Years. Publication 13. Oxford, Pergamon. ICRP (1971) Protection of the Patient in Radionuclide Investigations. Publication 17. Oxford, Pergamon. Purrott R. J., Lloyd D. C , Prosser J. S., Dolphin G. W., Eltham Elaine J., Tipper Patricia A., White Carolyn M. and Cooper Susan J. (1974) The Study of Chromosome Aberration Yield in Human Lymphocytes as an Indicator of Radiation Dose. IV. A Review of Cases Investigated: 1973. Harwell, NRPB, Report NRPB R 23. Webb G. A. M. (1974) Radiation Exposure of the Public— the Current Levels in the United Kingdom. Harwell, NRPB, Report NRPB R 24.
Requests for reprints should be addressed to: Dr N. L. Spoor, Research and Development Division, National Radiological Protection Board, Harwell, Didcot, Oxon., OX11 ORQ.
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urine analysis data are interpreted in terms of dose commitment, taking into account whole-body monitoring data.
Health Care of People at Work Safety in the Handling of Radionuclides NORMAN SPOOR Research and Development Division, National Radiological Protection Board
According to a recent estimate (Webb, 1974) about 78,000 workers in the United Kingdom are exposed or potentially exposed to ionizing radiation. Most of them are employed either in the National Health Service or in a field of industry—construction, manufacturing or nuclear power. The occupational risks associated with the use of radiation on this scale are in general far outweighed by the social benefits. However, the safety of these workers and their progeny may be assured only by strict control of radioactive sources and regular monitoring of the working environment. The aim of this paper is to outline the principles by which radiation hazards are assessed and controlled. Types of Radiation There are several kinds of radiation, of which the most common are the following: 1. Alpha particles. These are swiftly moving particles of high energy, which carry a positive electrical charge. Because of their relatively large mass they have little penetrating power and are stopped by a sheet of paper or thin metal foil. From outside the body alpha particles are not harmful; from inside the body they cause considerable cellular damage. 2. Beta particles. These are fast-moving electrons and are much more penetrating than alpha particles. From outside the body and from inside they can cause biological injury. 3. Gamma rays. These electromagnetic radiations of high energy have properties similar to those of X-rays. They may traverse several centimetres of tissue with little absorption. From outside the body and from inside they are a potential danger to all internal organs.
4. Neutrons. These are normal constituents of atomic nuclei which may be released in nuclear reactions with considerable energy. They can penetrate several centimetres into tissue and deposit energy by collision with other nuclei. A single substance may emit one or more of these types of radiation and may itself change into another radionuclide and emit further radiations. The higher the specific activity of an isotope (Ci/g) the more rapidly does the activity die away. This decay is an exponential phenomenon which is most conveniently expressed in terms of half-life, that is, the time required for the activity to decay to half its initial value. The specific activity has a bearing on the maximum permissible concentrations in air of radionuclides, which are usually expressed in terms of activity per unit volume of air (Ci/m3). When these concentrations are expressed in terms of mass (g/m3) it is clear that the maximum
Table I. Maximum permissible air concentrations (occupational exposure) for some stable and some radioactive substances: a comparison
(MPQ
Inactive substance (mg/m») Carbon dioxide Ethyl bromide Benzene Hydrogen cyanide Iodine Cobalt Silver Beryllium
10* 10» 10l 10 1 10"' 10"1 10" 10" 10" 10" 10"
Active substance
Uranium 238
Carbon 14 Tritium Caesium 137 Plutonium 239 Strontium 90 , 0 -io Phosphorus 32 10"" Californium 252
10"" 10"" Radon 220
43
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Summary This paper distinguishes between various kinds of occupational radiation hazard and describes principles by which they may be understood and controlled.
44
OCCUPATIONAL MEDICINE
permissible concentrations of most radionuclides are much lower than those of most chemical contaminants (Table I). There are a few compounds (natural uranium, for example) that are of low specific activity for which the toxicological hazard and the radiological hazard are of comparable seriousness.
External and Internal Radiation Sources Radionuclides are widely used in industry and medicine. In industry, gamma radiation from iridium 192 for example is used in radiography, and beta particles from strontium 90 are used in thickness gauges. In medicine, cobalt 60 sources are used in radiotherapy. In normal use the hazards from such sealed sources are only external. In industrial processes, such as the manufacture of nuclear fuel and the extraction of plutonium from irradiated fuel rods, workers may be inadvertently exposed to airborne radioactive materials. In hospitals and universities where radionuclides are often handled as unsealed sources, accidental spillages may occur; in such situations a radionuclide may enter the body by inhalation, ingestion, or open wound, and present a potential internal radiation hazard.
Table II. The relative contributions from different sources to the annual genetically significant dose (GSD) in the United Kingdom* Source Natural background Medical irradiation Fall-out (annual) Miscellaneous sources Occupational exposure Nuclear power industry (waste disposal) Total * Webb (1974).
UK collective gonaddose {man rady'1) 4,800,000 780,000 121,000 38,500 32,500 500
5,800,000
Annual GSD Percentage (mrady~l) 87 14 2-2 0-3 0-4 001 104
83-7 13 5 21 0-3 0-4 001
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Biological Effects of Radiation When the radiations described above are absorbed in tissue they deposit their energy at random along tracks of ionization. Some of this energy breaks bonds and causes chemical changes at the molecular level and this leads to biological damage. The most important biological effect arises from damage to the DNA molecules in cell nuclei. This damage may deprive the cell of its ability to pass through cell division, and most of the observed acute effects of high doses of radiation result from a depletion in the numbers of dividing cells in the body. The effects resulting from the depletion of dividing cell populations are not observed in workers exposed at the lower levels of radiation dose. At these low levels the biological risk is from cancer induction, which probably originates from small viable changes induced in the DNA. These changes occur at random and there is a finite chance of cancer induction even at very low doses. However, this risk is very small, and not more than a few in a million for doses of 1 rad. The purpose of radiological protection is to reduce the risk of late effects to an acceptable minimum. There are two types of biological effect: the somatic effect, which eventually injures the irradiated person, and the genetic effect, which
eventually injures some of the descendants of the irradiated person. Evaluations of the damage from exposure to radiation indicate that a substantial part of the whole damage is that which is expressed in the descendants of exposed individuals. Because the genetic dose influences the whole population it is usually expressed in statistical terms. One of the most useful criteria, the annual genetically significant dose, is an estimated average dose to the whole population (ICRJP, 1966; Webb, 1974). The results of a recent survey have shown that in the United Kingdom the main contribution to the annual genetically significant dose is from natural background (84 per cent); 13 per cent results from medical irradiation and 0-4 per cent from occupational exposure (Table II).
HANDLING RADIONUCLIDES
The International Commission on Radiological Protection The International Commission on Radiological Protection (ICRP) was established in 1928 under the name of International X-ray and Radium Protection Commission, with the purpose of advising users of ionizing radiations on the safe levels of exposure. Over the years ICRP recommendations have become accepted throughout the world. At present there are four ICRP Committees
making comprehensive recommendations which are published in a series of reports. Committee 2 deals with permissible levels from internally incorporated radionuclides. It published a report in 1959 which contains recommendations of permissible exposure to radionuclides in air and water {Tables ///and IV). In making recommendations for maximum permissible doses the ICRP emphasize that they are maximum values and further recommend that 'all doses be kept as low as practicable and that any unnecessary exposure be avoided'. In other publications the ICRP also advise on the implementation of basic principles, e.g. Radiation Protection in Schools for Pupils up to the Age of 18 Years (ICRP, 1970) and Protection of the Patient in Radionuclide Investigations (ICRP, 1971).
Table III. Dose limits in a year*
Organ or tissue
Gonads and red bone marrow Skin, bone and thyroid Hands and forearms; feet and ankles Other single organs
Exposure Exposure experienced experienced by adults in by members the course of ofthe their work public {rents) {rems) 5 30
0-5 30
75 15
7-5 1-5
* ICRP (1966).
Chemical vis-a-vis Radiological Criteria At exposure levels ten times the threshold limit value (TLV) the effects of chemical toxins are immediate or early, and usually identifiable. Accordingly, the aim of toxicological protection is to control exposure to an extent which prevents adverse immediate and chronic effects or reduces them to an acceptable level. At exposure levels ten times the maximum permissible concentration (MPC) the effects of radiation are, by contrast, latent and unidentifiable. The aim of radiological protection is therefore to control exposure to an extent which effectively prevents those delayed effects (e.g. cancer, life-shortening and cataract) known to be related to the cumulative dose. At low exposure levels these effects are presumed to be minutely small and statistically so elusive that their seriousness may be assessed only by theoretical
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In medicine, radionuclides are administered for diagnostic or therapeutic purposes. Iodine 131labelled triolein is given in gastro-intestinal function tests; iodine 131-labelled human serum albumin is injected for determining cardiac output, and sodium iodide 131 is used for thyroid function tests. Phosphorus 32 is given, orally or intravenously, for the treatment of polycythemia vera, and gold 198 is used in the treatment of cancer of the prostate. About 25 nuclides are currently used in medical practice or research and many scientists, physicians and nurses are employed in the preparation and administration of labelled compounds. When an insoluble substance like plutonium dioxide is deposited in the lung after inhalation, it is likely to remain there for years, exposing the lung tissues to continuous radiation. Such materials also enter the gastro-intestinal tract from which they are absorbed only to a minute extent; consequently the concentrations in the blood are rarely significant. A soluble substance, like tritiated water vapour, sodium iodide 131, or caesium 137 carbonate, is rapidly absorbed into the blood from both lungs and the gastro-intestinal tract. The radionuclide then moves out of the blood into organs and tissues; tritium permeates the whole body, iodine concentrates in the thyroid gland and caesium concentrates in muscle. Once in the blood each element follows a metabolic pathway characteristic of that element and its chemical analogues; e.g. caesium behaves like potassium, and strontium like calcium. Metabolic extrapolations based on chemical affinity may be misleading, however (cf. fluorine and iodine), and some elements (e.g. thallium, plutonium) have no useful analogue. If the radionuclide enters the body as part of a complex ion or an organic molecule, it will follow the metabolic pathways of the ion or molecule and the assessment of the radiological hazard will depend on the identification of those pathways.
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Table IV. Maximum permissible body burdens and maximum permissible concentrations (MPC) of some radionuclides in air and water for occupational exposure* Nuclide
Body burden
Critical organ'f
UiCi) »"Pu
"•Ra 131 t0
I Sr
"Sr
004 01 0-7 2 4
6 7 30 300 1000
187
Cs
"C 3 H
MPC in water OiO/cm')
2x10"" 3x10"" 9x10"
io-« 4x10-'5
3x10" 10 3x10" 7x10" 10"
6x10" 4x10" 5xl0"«
6xlO" 4x10"' 3X10"'
5x10"'
6xlO" 3 4x10-' 2x10-' 10"x
• ICRP (1959). t The critical organ is the organ which receives the highest radiation dose.
estimates on large populations. In toxicological protection it is assumed that there is a 'threshold', i.e. a level of exposure which can be accepted indefinitely without risk of injury or discomfort. In radiological protection it is assumed, in statistical calculations concerning large groups, that there is no threshold. Associated with these differences of principle are the different assessments of the individual exposure. For instance, a personal inhalation of 2 mg iodine in the course of one hour would be an unpleasant experience involving an appreciable physiological insult to the body. The experience might be transient and harmless, however, and of no long-term significance. In contrast, a personal exposure to 3 rem radiation in the course of one hour would not be accompanied by any personal sensation or physiological disturbance. The significance of such an exposure is notional, and is assessed entirely in the context of long-term radiation risks. The distinction between a rapid unpleasant insult of no clinical consequence on the one hand, and on the other an unfelt insult associated with a definite but unknown risk which can be assessed only within a wide time perspective, is very relevant to the appreciation of the problems of radiological protection. Control of Hazards Laboratory Design Special facilities are required for the handling of large amounts of the more toxic radionuclides, and
the most suitable laboratories are purpose-built. The aim is to prevent the spread of radioactivity and to ensure that radiation levels are kept as low as possible. 'High-level' laboratories are separated from 'low-level' laboratories and both are rigidly separated from change-rooms, washing facilities and counting rooms. All laboratories are built with floors amenable to regular polishing and with gloss-painted walls, and are equipped with fume cupboards. Shielding Shielding requirements are often determined by the amount of space available around the source. Given a weak source or considerable space the exposed person may withdraw to a distance at which the dose rate falls to an acceptable level. The dose rate falls off roughly according to the inverse square law. In other situations some form of shielding is required to reduce the radiation level. Beta rays are stopped by plastic plates of a few centimetres thickness. Gamma rays of an energy exceeding 0-5 MeV are very penetrating and a considerable thickness of shielding material (lead for example, or concrete) may be needed to stop them. One of the principal requirements in neutron shielding is the reduction of their energy, as the permissible flux of neutrons increases as the energy decreases. Materials of low atomic mass, such as water and paraffin wax, slow down neutrons by elastic collisions; cadmium or boron shields, which absorb neutrons, also reduce the dose.
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"P »Na
Bone Bone Thyroid Bone Bone Bone Total body Total body Fat Body tissue
MPC in air OiQ/cm3)
HANDLING RADIONUCLIDES
Surface Control Wherever unsealed sources are handled there are risks associated with the possible dispersal of activity into the environment; in these situations both surfaces and air may be regularly monitored. SURFACE MONITORING
Table V. Limits for surface contamination
Type of surface Inactive areas Active areas Personal clothing Skin
Principal alpha emitters 10~5 10"* 10"5
Low toxicity alpha emitters
3
10"
10- 4 10"5
Beta emitters 10" 4 10" 3 lO" 4 10-'
AIR MONITORING
Regular air sampling is a means of checking that the precautions taken to prevent the dispersal of activity have been effective. The contamination usually consists of fine particles of the active material either in its pure form or associated with dust particles. The air is sampled by drawing it through a high-grade filter paper which traps and retains a high proportion of the particulate matter. The volume of filtered air and the amount of activity on the filter paper are measured. There are three kinds of air-sampling equipment:
fixed, portable and personal. Fixed monitors are used in laboratory and nuclear reactor areas, where individual samples may be taken over periods of up to a week. Portable monitors, requiring 230 V a.c. supply for operation and weighing 7 kg, are recommended for special intermittent operations. The small personal air sampler includes a 2-cm sampling head which is pinned against the worker's coat lapel; the complete 0-7 kg unit is powered by rechargeable 6 V batteries which give a life of 10 hours before recharging is necessary. The purpose of the air-sampling programme is to ensure that the contamination level is as low as practicable and within the permitted maximum. If this requirement is not met, steps are taken either to lower the contamination level or, when this is not possible, to provide personal protection for the workers. This is available in a variety of forms, from the high-efficiency respirator, with a plastic hood and plastic overalls, to the complete rubber suit with boots, gloves and helmet, into which fresh air is supplied through an air-line. From the resemblance to the suit worn by underwater frogmen, the latter is usually known as a frogsuit. Personal Monitoring The first line of defence in radiological protection is the implementation of measures required to ensure that exposure levels are reduced to the lowest practicable level. The second is the measurement of the levels of radiation to which individual workers are exposed. The film badge is the most widely used method of measuring the radiation dose received by the individual. Its advantages are its cheapness, its small size, its capacity for integrating exposure over long periods, and the provision of a durable record. Organizations with a substantial use of radiations, such as nuclear establishments and hospitals, have their own film badge service. The National Radiological Protection Board (NRPB) provides a film badge service for many smaller organizations in the United Kingdom and currently issues 544,000 film badges a year. Classified workers (as defined by the regulations) and designated persons (as defined by the code of practice) are required to wear a personal dosimeter of an approved type. During the last decade there has been an increasing use of the radiation-induced thermoluminescent effect in substances like lithium fluoride
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Control of contamination is required not only in the interest of safety but also to avoid spurious measurements resulting from the contamination of apparatus or counting equipment. Surfaces are checked in one of two ways: either directly by means of a contamination monitor, or indirectly by means of a smear test. The contamination monitor detects surface activity whether loose or fixed without differentiating between them, whereas a smear test detects only that which is readily removable. The method is to rub a piece of filter paper lightly but firmly over the surface; when an area of about 300 cm2 has been wiped, simple counting equipment is used to measure any activity collected by the paper. The accepted limits for surface contamination are based on levels recommended in the current legislation and codes of practice {Table V).
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examination. A count is made of the number of dicentric chromosome aberrations in 200-500 cells and from the result the dose is estimated using a dose/response calibration curve for the type and rate of overexposure suspected. At the cytogenetics laboratory, NRPB, Harwell, 180 cases of known or suspected overexposure have been examined by this technique during the last 4 years (Purrott et al., 1974). In the large majority of cases the evidence has supported the belief that the worker concerned was not seriously overexposed. Measurements of this kind have served to remove anomalies in conventional dose estimation and to avoid the severe disruption of the individual's work which would otherwise be required following a suspected overexposure. Urine Analysis One of the most useful procedures in biological monitoring is the analysis of urine samples. Clearly the rate of urinary excretion of a radionuclide (uCi/day) reflects only the rate at which that nuclide is leaving the body through the kidneys, and this may or may not reflect the amount retained in the body. When the inhaled material is insoluble (e.g. thorium dioxide, plutonium dioxide) most of it is deposited in the nasopharyngeal or bronchial regions and most of that which is not retained in the lungs is excreted via the gut. For this reason urine analysis is not used for measuring intakes of very insoluble materials. When the inhaled material is soluble it is normally absorbed via the lungs and the gut into the body fluids, and its fate is determined by the rate of radioactive decay and by the mobility of the element in the body. Some nuclides (e.g. strontium 90) are deposited in particular organs from which they are slowly released back into body fluids (and into urine and faeces) over many years. Other nuclides (e.g. iodine 131) concentrate in one or two organs or tissues from which they are released within a few weeks. The relationship between the amount of radioactivity in the body (uCi/kg) and the amount excreted in the urine varies according to the metabolism of the nuclide, the size of the intake and the solubility of the material. In a situation in which workers have been exposed to the same nuclide over a long period, the rate of excretion in the urine is a function of the amount of activity deposited in the body and of the amount recently assimilated. For this reason
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and lithium borate to measure radiation dose. These compounds are now widely used for monitoring doses to fingers and wrists. This type of dosimeter has the advantages of close resemblance to tissue as far as radiation absorption is concerned, and amenability to automation. The NRPB is currently using this dosimeter for studying doses to patients from the medical and dental uses of radiation, and intends to introduce an automated dosimeter service based on the lithium fluoride dosimeter in 1976. It is sometimes possible to measure radiation outside the body from radionuclides present inside the body. The measurement can be made to detect radionuclides in a particular organ or in the whole body. For instance, by measuring gamma radiation over the neck the amount of iodine 131 in the thyroid may be estimated. By measuring gamma radiation from the chest the level of retained cobalt 60 in the lungs may be estimated. In the whole-body monitor a number of detectors are arranged above and below the person, who normally lies in a horizontal position inside a heavily shielded chamber. The shielding required to reduce the background count from wide-angle radiation to an acceptably low level is massive. The shield and the large crystal detector are expensive. However, the whole-body monitor has proved a useful tool both in cases of accidental contamination and in research investigations, and for this reason it has been installed at several hospitals and research establishments in the United Kingdom. The whole-body monitor has been used to measure the amount and the distribution of fallout caesium 137 in the body. Among plutonium workers it has been used routinely to measure plutonium dioxide retained in the chest. Mobile gamma spectrometers are available at a number of centres and these have proved useful for screening in the event of radiation accident. When lymphocytes are exposed to radiation the number of structural chromosome aberrations increases, and the yield is related to dose. For many years this relationship has been used in radiological protection. The method is especially convenient after a radiation accident in which data from conventional physical dosimetry are suspect or absent. Venepuncture blood samples of 5—10 ml are conveyed in prepared tubes to a laboratory where a lymphocyte fraction is cultured, from which cells are fixed and slides prepared for microscopic
HANDLING RADIONUCLIDES
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the Universities of the United Kingdom, 1966; Department of Employment and Productivity, 1968; Department of Health and Social Security, 1972).
Legislation: Codes of Practice The United Nations, the World Health Organization, the International Atomic Energy Agency and the International Labour Organization are all bodies actively interested in radiological health and safety. The ICRP, on which the United Kingdom is well represented, is the recognized international authority. Its latest recommendations on maximum doses were published in 1966 and, with minor changes, these are the basis of most of the legislation. The Medical Research Council is responsible for advising the Government on the biological implications of standards and their acceptability for application in the United Kingdom. The Acts of Parliament and the regulations made under them which refer specifically to work with radioactive materials are The Radioactive Substances Act of 1948 and 1960, and The Nuclear Installations Act of 1965 and 1969. The Radioactive Substances Act I960 requires all persons keeping or using radioactive materials to register with the Radiochemical Inspector of the Department of the Environment. The Ionizing Radiations {Sealed Sources) Regulations 1969 and The Ionizing Radiations (Unsealed Radioactive Substances) Regulations 1968 made under The Factories Act 1961 impose requirements for the protection of persons employed in factories and other places to which the Act applies. The regulations require (inter alia) that District Inspectors of Factories be notified of the use of sources and of accidents involving sources; that one or more 'competent persons' be appointed to supervise the work and assist in enforcing the regulations; that exposure be restricted; and that no employed person be exposed to radiations unless he has been instructed in the nature of the hazards involved and the precautions required. The regulations provide also for medical supervision, for the organization of work and the maintenance and use of monitoring instruments. Several interpretative codes have been written for the guidance of management and workers (the Committee of Vice-Chancellors and Principals of
Conclusion When management introduce radioactive substances into their factory or laboratory they commit themselves to a new responsibility. The hazards associated with radiation require more attention than most conventional hazards because they are more elusive. At the same time they are readily measurable and amenable to control. In order to establish and maintain safe working conditions it if necessary to provide appropriate facilities and proper supervision. The staff chosen for the work should be carefully selected bearing in mind medical requirements as well as ability and aptitude. Radiation workers must be fully informed about the nature of their duties and any associated risks.
REFERENCES Committee of Vice-Chancellors and Principals of the Universities of the United Kingdom (1966) Radiological Protection in the Universities. London, Association of Commonwealth Universities. Department of Employment and Productivity (1968) Code of Practice for the Protection of Persons Exposed to Ionizing Radiations in Research and Teaching. London, HMSO. Department of Health and Social Security (1972) Code of Practice for the Protection of Persons against Ionizing Radiations arising from Medical and Dental Use. London, HMSO. ICRP (1959) Permissible Dose for Internal Radiation. Publication 2. Oxford, Pergamon. ICRP (1966) Recommendations of the International Commission on Radiological Protection. Publication 9. Oxford, Pergamon. ICRP (1968) General Principles of Monitoring for Radiation Protection of Workers. Publication 12. Oxford, Pergamon. ICRP (1970) Radiation Protection in Schools for Pupils up to the Age of 18 Years. Publication 13. Oxford, Pergamon. ICRP (1971) Protection of the Patient in Radionuclide Investigations. Publication 17. Oxford, Pergamon. Purrott R. J., Lloyd D. C , Prosser J. S., Dolphin G. W., Eltham Elaine J., Tipper Patricia A., White Carolyn M. and Cooper Susan J. (1974) The Study of Chromosome Aberration Yield in Human Lymphocytes as an Indicator of Radiation Dose. IV. A Review of Cases Investigated: 1973. Harwell, NRPB, Report NRPB R 23. Webb G. A. M. (1974) Radiation Exposure of the Public— the Current Levels in the United Kingdom. Harwell, NRPB, Report NRPB R 24.
Requests for reprints should be addressed to: Dr N. L. Spoor, Research and Development Division, National Radiological Protection Board, Harwell, Didcot, Oxon., OX11 ORQ.
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urine analysis data are interpreted in terms of dose commitment, taking into account whole-body monitoring data.
