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18 Radionuclides in Foods: American
Perspectives
E.J. Baratta*
US Food and Drug Administration, 109 Holton Street, Winchester, MA 01890, USA
Introduction
The US Food and Drug Administration (FDA) has the responsibility for the whole- someness of the food supply in the USA. In 1961, the FDA initiated a programme for monitoring the radionuclides in foods in the teenage diet (FDA, 1963; Laug et al ., 1963). This was in response to the above- ground weapons testing. At the same time, the US Public Health Service (PHS) insti- tuted a nationwide pasteurized milk net- work (PMN); in cooperation with the FDA (Roecklin et al ., 1970), milk samples were collected from at least one of the largest cities in each state. These samples were forwarded to one of three PHS laboratories for analysis. The radionuclides of interest were 131 I, 137 Cs, 140 La, (^90) Sr and 89 Sr. Various food products were
collected as they were harvested. The PHS also instituted a food programme (Roecklin et al ., 1970) called the Institutional Diet Program (IDP) for collecting samples from various institutions of teenage diets, similar to the FDA programme, except that in the PHS study the total diet sample was homoge- nized, rather than analysing each component. With the signing of the US–Soviet above-
ground weapons testing ban treaty, these programmes were changed. The FDA had stopped sampling and analysing the foods in the teenage diet in 1969 to avoid duplication. In 1973, the FDA decided to resume its programme, due to a governmental reorganization. Two of the PHS laboratories had been transferred to the Envi- ronmental Protection Agency (EPA) and were no longer analysing for radioactivity in food (Anderson and Nelson, 1962). The PMN was put on a standby and EPA in 1973 cancelled the IDP.This new FDA programme included the teenage and infant diet sample and was responsible for analysing radioactivity in food (Food and Drug Administration, 1973). This Teenage and Infant Diet Program was also a part of its Total Diet Program Study, which analyses for various other components. The radionuclides in the food programme also included imported foods that had domestic status. In addition, the FDA would be able maintain a capability for analysing radionuclides in food samples in the event of a release from a nuclear accident. Such incidents occurred, the first in 1979, when there was an accident at the Three Mile Island Nuclear Power Plant near Harrisburg, Pennsylvania. Later, in 1986, there was an
©CAB International 2003. Food Safety: Contaminants and Toxins (ed. J.P.F. D’Mello) 391
- E-mail: EBARATTA@ORA.FDA.GOV
accident at the Chernobyl nuclear power plant near Kiev in the Ukraine, USSR. (see Smith and Beresford, Chapter 17, this volume).
Nature of Radionuclides of Interest
The primary radionuclides found in foods and milk were the short-lived fission prod- ucts 89 Sr, 131 I, 140 Ba and 140 La. In addition, there were several longer-lived fission prod- ucts such as 137 Cs and 90 Sr. 134 Cs was found in products from the Chernobyl incident. The food products contained other short-lived and longer-lived fission products such as (^65) Zn, 95 Zr, 95 Nb, 103 Ru, 106 Ru, 140 Ce and 141 Ce.
These radionuclides were not found in milk. The milk and milk products that were produced from the cow contained only the former, as the cow’s digestive system dis- criminated against the other radionuclides.
Background – nature of radionuclides
The modern epoch in physics may be said to have begun with the discovery of X-rays by Roentgen in 1895. This was followed by the discovery of radioactivity by Becquerel in 1896. These two discoveries led to other developments in the understanding of nuclear structure, reactions and the various types of particles involved. All these investi- gations finally culminated in the discovery of ‘fission’, which resulted in the development of atomic energy, in both peace and war.
Atomic structure
All matter is made up of elements. The small- est part of an element is the atom. The size of a hydrogen atom, which is the smallest, is about 1.5 × 10−^8 cm while its weight is about 1.67 × 10−^24 g. In comparison, a uranium atom would weigh about 4.0 × 10−^22 g. The atom itself consists of a central nucleus surrounded by a cloud of electrons ranging in number from one for hydrogen to 92 for uranium. These electrons are said to move in orbits around the central nucleus. The size of the
nucleus is considerably smaller than that of the atom, the size of the hydrogen nucleus being 1.4 × 10−^13 cm. The nucleus itself con- sists of protons, which carry a unit positive charge ‘e’ (e = 1.6 × 10−^17 coulombs), and neutrons, which carry no charge. The mass of the neutron is only slightly greater than that of the proton. The electron carries a unit neg- ative charge and has a mass approximately 1/1840 of that of a proton or a neutron. Hence, most of the mass of the atom is carried in the nucleus. It is also known that most of the energy of the atom is also stored in the nucleus.
Elements and isotopes
All the elements of the periodic table are made up of protons and neutrons in the nucleus and electrons orbiting around it. The total charge carried by the nucleus is equal to the number of protons in it. This number is called the ‘atomic number’ and is characteris- tic of each element. Since the atom as a whole is electrically neutral, there will be as many electrons around the nucleus as there are protons in it. The number of protons plus neutrons gives the ‘mass number’ of the atom. Thus, the mass number of carbon, 612 C, is 12 and the atomic number is 6. If two nuclei have the same number of protons in the nucleus, but different number of neutrons, they are called isotopes. For example, 47 Be and 410 Be are isotopes of beryllium having the same number of protons and the number of neutrons being three and six, respectively. Isotopes have the same chemical properties, which depend on the number of electrons in the orbits around the nucleus, but different physical properties, which depend on the central nucleus.
Radioactivity
As mentioned before, it was noted early in this century that certain substances are radio- active, i.e. they spontaneously emit different types of radiations and transform into other elements depending on the emitted radia- tions. This process, which was first noted in
392 E.J. Baratta
emphasis as significant sources of environ- mental contamination.
The food chain
The pathways which radionuclides follow in moving from their origin to man constitute the food chain. Radioactive materials are removed from the atmosphere by meteoro- logical processes, primarily precipitation. In general, the most serious food chain contami- nation problem arises from direct deposition of radioactive materials on animal feed crops or on food crops directly consumed by man. Following this initial deposition, various pro- cesses may remove the radioactive materials, such as being washed off by rain or blown off by wind. The extent of this removal is a func- tion of many physical and biological parame- ters. Man’s intake of radioactive material may occur from contaminated food crops, from contaminated meat and meat products and from contaminated milk or milk products. The inhalation route (atmosphere directly to man) may be important under special cir- cumstances and is not discussed here. The relative importance of the various pathways of intake will depend on many factors, among which are the physical half-lives of the radionuclides, the rate and route by which they pass through the food chain and the dietary habits of the population. The immediate and generally most significant pathway is pasture–cow–milk–man for the more significant radionuclides up to approxi- mately 100 days following deposition (one time event). Plant losses are such that, after this time period, adsorption by plants is the most significant pathway for the longer-lived radionuclides. The final step in the food chain (uptake by man) primarily depends on the chemical characteristics of the radionuclide and the metabolism of the concentrating organ. As an example of radionuclide behaviour in the terrestrial food chain, the pathway of the fission product 90 Sr may be cited. It has been shown that the grazing animal popula- tion readily transfers this nuclide to man from atmospheric nuclear testing by deposition on
the earth’s surface, with subsequent plant uptake and transference to milk. In considering the relationships between the food chain and any particular radio- nuclide, it becomes necessary first to consider the source of the contaminant. The source should be considered as it relates to the physi- cal and chemical state of the radionuclide, since these properties relate to the degree of movement in the food chain. Fission products from an atmospheric burst would not be expected to exist in the same physical and chemical state as those from a reactor incident, fuel reprocessing plant or waste treatment facility. Even among the latter operations, a wide variety of physical and chemical states of the fission products would be expected Studies of the movement and effects of radionuclides in the food chain are of value because they:
- make^ possible^ the^ prediction^ of^ the relationships between the kinds of radio- activity in the various steps within the food chain and the resulting levels in the human population;
- provide a means for evaluating ways by which the levels in the human population may be minimized;
- provide^ background^ information^ for setting up environmental sampling programmes and for interpreting the data obtained;
- make^ available^ animal^ data,^ which may be useful for estimating behaviour of radionuclides in humans;
- make possible predictions of the possi- ble effects of environmental contamina- tion, especially in the grazing animal population.
Radioactive materials from atmospheric releases are deposited on the earth’s surface by precipitation or direct deposition. Although the discussion of possible fallout patterns is beyond the scope of this chapter, it should be stated that fallout distribution depends on many parameters. For nuclear weapons test- ing, these include meteorological conditions, fission yield, type of explosion (i.e. ground, water or air) and geographical location. In the case of a nuclear incident, such as an air release from a nuclear reactor, the factors of
394 E.J. Baratta
major concern would be the existing fission product inventory, the micrometeorological conditions and the nature of the surrounding terrain.
Contamination of animals and animal products
Certain radionuclides are readily transferred to the human population via domestic graz- ing animals, which are effective collectors of contamination from various vegetative forms (Eisenbud, 1973). There are many factors which affect the degree to which animals are contaminated. The most important include:
- pasture type used for grazing;
- extent of barn feeding (purchased and stored), miscellaneous feeding practices –age of food and supplemental feeding use of purchased feed, etc.;
- water source; and
- animal housing –degree of sheltering animals from surface contaminants.
Because of the many variables involved, the degree of animal and animal product contamination may be quite variable, even from apparently similar sources. Of particular interest in this pathway of the food chain are the relationships that exist between the quantities of radionuclides ingested by the animal and the subsequent quantities which are deposited in the tissues and secretions that serve as human food. To study these relationships requires knowledge of the metabolic characteristics of the animal and the particular radionuclide. Only those radionuclides which enter the food chain at a significant rate and quantity will be of importance to man. These radionuclides must also possess characteristics that allow for their continued movement through the chain.
Radionuclide Metabolism
Classically, metabolism refers to the bio- logical processes whereby complex cellular elements are synthesized.
General considerations involved in radionuclide metabolism
For practical purposes, only the gastrointesti- nal mode of entry of nuclides into the com- partments of a biological system is important (Thompson, 1960; National Academy of Sciences–National Research Council, 1961). In special situations, the pulmonary and skin routes may be important in permitting assim- ilation of the nuclides. The intravenous route of entry is artificial and only important as an experimental tool. However, it must be noted that it simulates the situation once a nuclide is absorbed into the bloodstream. This method to some extent by-passes the uncertainties involved in a study with natural routes of assimilation. The gastrointestinal system is probably the most important route of entry for solu- ble forms of nuclides. Insoluble forms will be dependent on the degree of solubility and remain in the intestine, with this organ receiving the bulk of the radiation exposure. α Emitters will dissipate about 1% of their energy in the tissues of the gastrointestinal tract. A much greater percentage of the energy from β emitters will be absorbed and dissipated within the gastrointestinal tract. Insoluble γ emitters will effectively radiate the intestine; however, the rest of the body will also be exposed. Pertinent examples of the more important fission products are given in (Table 18.1).
Metabolic classification of radionuclides
No completely satisfactory classification of nuclides is possible. One approach is to group the nuclides according to their positions in the periodic table. It would be expected that nuclides of the same group would behave similarly because of their simi- lar chemical properties. The contamination of the terrestrial food chain from the various uses of nuclear energy constitutes a potential health problem to man. To cope with this problem requires a consideration of the phys- ical, chemical and biological characteristics of the radio-contaminants and the terrestrial
Radionuclides in Foods: American Perspectives 395
essential element iodine, concentrates in the thyroid gland. 90 Sr and 226 Ra are alkaline earths similar to calcium and follow it to the bone. 14 C and 3 H, isotopes of two very essen- tial elements, are distributed throughout all living tissues. The metabolic processes of all plants and animals are similar; radionuclides which concentrate in animal tissues are usu- ally those that pass most readily through the food chain.
SPECIFIC CHARACTERISTICS The relative im- portance of individual radionuclides depends on many factors. Among the most important factors, the following can be cited: the mag- nitude of the hazard from radionuclide
deposition in the body is dependent on the type and energy of radiation. For the deposi- tion of the same activity of a radionuclide in a given organ, the hazard from the type of radiation would be α>β>γ. The hazard from the given emitter would also increase with increasing energy of emission.
CHARACTERISTICS BASED ON PHYSICAL, CHEMICAL AND BIOLOGICAL PROPERTIES An arbitrary separation of the above-listed characteristics into physical and biochemical categories can be made for discussion purposes. Develop- ments in this area are concerned with the retention of the liberated radon. Recent inves- tigation indicates that, in long-standing cases
Radionuclides in Foods: American Perspectives 397
Radioisotopeb^235 U (%) 238 U (%) Type of radiation Physical half-life GI absorption (%)
(^90) Sr/ 90 Y (^137) Cs (^147) Pm (^144) Ce (^106) Ru/ 106 Rh (^95) Zr (^89) Sr (^103) Ru (^95) Nb (^141) Ce (^140) Ba/ 140 La (^131) T
β β, γ β β, γ β, γ β, γ β β, γ β, γ β, γ β, γ β, γ
28 years 29 years 2.6 years 285 days 1.0 years 65 days 51 days 39.7 days 35 days 33 days 12.8 days 8.04 days
30 100
30
5 100
aFood and Agriculture Organization (1960) and Katcoff (1958, 1960). bIsotope pairs are classed according to the chemical and biological characteristics of the parent.
Table 18.2. Characteristics of the more important fission products of food chain significance. a
Chemical character Isotope Fission abundance (%) Half-life
Halogens Oxygenated anions Alkali metals Alkaline earths
Rare earths
Noble metals
(^131) I, 133 I, 135 I (^132) Te, 132 I (^137) Cs, 137 Ba (^89) Sr, 90 Sr, 90 Y (^140) Ba, 140 La (^91) Y, 95 Zr, 95 Nb (^141) Ce, 144 Ce, 144 Pr
(^143) Pr, 147 Nd, 147 Pm
(^103) Rh, 106 Ru, 106 Rh
3.1, 6.3, 6.
4.6, 5.
- c^ , 6. 6.0, 5.
- c^ , 2.9, 2.
3.4, 0.
8.1 days, 22 h, 6.7 h 7.8, 2.4 h 37 years, 2.6 months 51 days, 26 years, 61 h 12.8 days, 40 h 57 days, 65 days, 35 days 33 days, 290 days, 17.5 months 13.7 days, 11.6 days, 3.7 years 40 days, 1.0 year, 30 s
Atom yield > 0.03%; half-life > 10 h. aFood and Agriculture Organization (1960) and Katcoff (1960). bFission abundance values are approximately the same for 233 U, 235 U and 239 Pu. c (^) Data lacking on abundance.
Table 18.3. Fission products important due to fission abundance and half-life. a,b
of radium poisoning, an average of 70% of the radons produced are exhaled.
1. Chemical properties. As examples, ele- ments of group VII, fluorine, chlorine, bro- mine, iodine and astatine, have strikingly different modes of metabolism. Fluorine is deposited in bone; chlorine and bromine are fairly equally extensive within the extracell- ular fluid space; and iodine is concentrated in the thyroid gland. Astatine is also localized in the thyroid gland. 2. Physical properties. The fission process gives rise to a mixture of radionuclides with a wide range of half-lives. Each of these nuclides is produced in a certain proportion (abundance), which is dependent on the fis- sion materials and the energy of the fissioning neutrons. The abundance of the various nuc- lides produced has been found to be approxi- mately the same for the different fissionable materials. Table 18.1 presents the most important fission products, based on fission abundance and half-life, which are of immedi- ate concern in environmental contamination. High fission abundance and a moderate to long half-life when considering parent– daughter relationships characterize these nuclides. Many of these radioisotopes can subsequently be eliminated from food chain consideration because they are not present significantly long enough to be a long-term health hazard. The more important nuclides will be those which are formed in high abundance, with moderate to long half- lives and which are isotopes of, or chemically similar to, essential elements. 3. Biochemical properties. The chemical and biological properties of the various radio- nuclides greatly affect their ability to move through the food chain. Table 18.2 presents the fission products of biological importance grouped according to similar chemical charac- teristics, and shows the relative uptake by the total body and critical organ from the gastrointestinal tract. From Table 18.2, it is seen that the important fission products are those which comprise the rare earths, the zirconium–niobium isotopes, the noble metals, particularly ruthenium and rhodium, the isotopes of iodine, the alkali metal caesium and the alkaline earths, especially strontium
and barium. Examining the fractional uptake from the gut of these various groups of iso- topes biologically can give some indication of their relative importance. This leaves the alka- line earths strontium and barium, the alkali metal caesium and the iodine isotopes as nuclides of primary importance. Depending on the particular situation, these nuclides will assume a greater or lesser degree of importance in the food chain. Additional biological factors which aid in accessing the potential health hazard from the particular nuclide include: (i) the quantity deposited and the residence time of the nuclide in the critical organ; and (ii) the essentialness or indispens- ability of the critical organ to the organism.
Specific radionuclides
The fission products which enter the environ- ment from fallout or from various nuclear facilities include more than 30 radioactive isotopes. From the above discussion, it is evident that all of these radionuclides are not equally harmful to the human population. Intensive study of fission product behaviour in the food chain has revealed that 89 Sr, 90 Sr, (^140) Ba, 131 I and 137 Cs are the radionuclides of major concern. 90 Sr and 137 Cs are radio- nuclides of long physical half-life and are considered long-term hazards. 89 Sr, 140 Ba and (^131) I, due to their shorter physical half-lives are only short-term hazards. This discussion will deal primarily with the environmental behaviour of 90 Sr, 131 I and 137 Cs.
(^89) Sr and 90 Sr
The radionuclides of major importance in the alkaline earth series are 89 Sr, 90 Sr and 140 Ba. Like calcium, these alkaline earth radionuc- lides are deposited in large amounts in the skeleton (Food and Agriculture Organiza- tion, 1960, 1964; Comar et al ., 1961; Comar, 1963; Russell, 1963; Comar and Bronner, 1964; Kahn et al ., 1965). All three radionuclides are produced in relatively large abundance during nuclear fission (see Table 18.2) and assume a greater or lesser degree of impor- tance in food chain contamination depending
398 E.J. Baratta
Radiocaesium
In contrast to 90 Sr, 137 Cs does not readily follow the soil uptake route (Food and Agri- culture Organization, 1964). This nuclide is fixed in forms largely unavailable to plants as a result of entrapment in the lattice structure of certain clays. Plant contamination, there- fore, occurs primarily by direct deposition. Since 137 Cs is capable of concentrating in soft tissues (e.g. muscle), the step from animal to animal products (other than dairy products) assumes importance for this radionuclide. A long-term genetic dose is thus possible from ingestion of foods contaminated with (^137) Cs. Since a significant quantity of 137 Cs
is excreted into animal’s milk, the path- way atmosphere–plants–animals–milk–man is most important for diets containing aver- age amounts of milk. In general, the biologi- cal significance of 137 Cs is somewhat less than that of 90 Sr because of its shorter effective half-life in the body.
Natural radioactivity
There are a number of naturally occurring radionuclides in the biosphere. They enter and are transferred through the food chain to varying extents. A major portion of the work with natural occurring radionuclides has been undertaken in order to establish baseline levels in the environment. Such levels have been used for comparison in assessing the degree of contamination of the environment with artificial radioactivity. The most important naturally occurring radionuclides belong to the uranium, thorium and actinium series. The actinium series is the least abundant of the three. These natural series are composed of a number of the heavier elements of varying half-lives that exhibit extremely complex decay schemes. Studies of the transfer of these radionuclides to man via the food chain involves measuring the activity levels in plants which are con- sumed directly by man, and plants such as grasses which form the principal food of animals, which in turn become the principal food of man. Dietary surveys indicate that there is a wide range of natural activities in vegetation and there appears to be no simple
correlation with the activities found in the soil, which have a much smaller range. Also, there is not much information available concerning the discrimination factors for soil–food and man–food processes. It appears that plants and animals absorb the majority of these radionuclides to only a very small extent when compared with the most pertinent artificial radioisotopes. Few quantitative data are available on absorption phenomena. The greatest attention has been paid to radium since this element appears to be the principal one absorbed by plants. However, dietary surveys have indicated that the occurrence of this element in the majority of foods is well below that of man-made 90 Sr. The following naturally occurring radio- nuclides are present in a singular form in the environment: rubidium, lanthanum, samar- ium and ruthenium. Little information is avai- lable on plant and animal absorption of these natural radionuclides. They are known to be present in the environment at considerably lower concentrations than the most abundant naturally occurring radioisotope, 40 K. Consid- eration of several of their characteristics such as their chemical nature, the behaviour of their man-made counterparts and their essential- ness to plants would tend to indicate that these radionuclides are little absorbed by plants. The remaining fission products, activation products and naturally occurring radionuclides that are found in the terrestrial environment are of lesser hazard to man as a result of their limited movement in the food chain. However, depending on special circumstances some of these radionuclides may assume a greater importance than normally expected.
Monitoring
Surveillance
The Total Diet Program was reinitiated in 1973 (Food and Drug Administration, 1973). This programme was necessary to:
- monitor the foodstuffs in the USA. It was part of an overall programme to ensure
400 E.J. Baratta
that the food products were safe and wholesome; and
- maintain a capability in the event that: (i) above-ground testing was initiated; (ii) leaks from around nuclear facilities occurred; (iii) a nuclear accident occurred; and (iv) there was increased use by medical facilities and increased commercial radioactive materials. The data collected for this programme can be found in the reports that have been generated in the literature (Simpson et al ., 1974, 1977, 1981; Tanner and Baratta, 1980; Strobe et al ., 1985; Cunningham et al ., 1989, 1994; Caper and Cunningham, 2000). The data have shown that the radioactive content of the food grown and consumed in the USA meets the requirements of the Federal Radia- tion Council (FRC) as promulgated in 1961 (Table 18.5). These results are also below the requirements of newer regulations issued in 1998 (US Department of Health and Human Services, 1998). This capability was soon to be put to use. In 1979, an incident occurred at the reactor at Three Mile Island (Unit II) in Pennsylvania, resulting in some releases of radioactive gases. However, the containment vessel was not breached. This resulted in the sampling and analyses of thousands of samples during the first month. These were mainly milk and produce samples from the farms and dairies in
the immediate area. Also included were food- stuffs processed and manufactured from that area. Sampling from that area continued for another year at a less vigorous pace. The only radionuclide found was 131 I, and the concen- trations were in range II of the FRC’s radiation protection guidelines (FRC, 1961). Intake in this range calls for active surveillance and routine control. The limits for range II are 0.37–3.7 Bq l−^1 (Table 18.5). This radionuclide ( 131 I) was only detected during the first week. The limit of detection for this method at the 2 sigma level is ± 0.37 Bq l−^1. The 90 Sr content in all the samples tested was similar to its content due to the worldwide fallout. No 89 Sr was detected, which confirmed this. The second occasion that required exten- sive sampling and analyses was when an incident occurred at the Chernobyl nuclear power plant in the Ukraine, of the then USSR. The radionuclides from the incident at Chernobyl did not increase the dietary intake of foods grown and processed in the USA. However, the foods imported into the USA did contain ‘fallout’. In the early period, the major radionuclide of concern was iodine. Several mushroom samples and soft cheese samples were detained as they exceed the US Department of Agriculture (USDA)–FDA levels of concern (FDA, 1986) (Table 18.6). Other samples, particularly spices and herbs, contained high amounts of fission products. Later, samples of cheese, apple juice and pasta
Radionuclides in Foods: American Perspectives 401
Radionuclide Target
RPG dose (mSv year−^1 ) I c^ II III
(^131) I (^137) Cs d (^90) Sr
Tritium ( 3 H) d
Thyroid Whole body Bone Whole body
0–0. 0–54. 0–0. 0–7,
0.37–3. 54– 0.74–7. 7,400–74,
3.7–37. 540–5, 7.4–74. 74,000–740,
aDerived concentrations were calculated on the basis of an average contaminated food intake of 1 kg
day−^1 (includes water and other beverages). bFederal Radiation Council (1965). c (^) Range I requires no specific action; range II requires surveillance and routine control of upward trends
towards range III; range III requires surveillance and controls to reduce exposure to range II (1); the range II–range III transition corresponds to the RPG dose 1 Bq = approximately 27 pCi. d137Cs and tritium were not considered by the FRC. The ranges were derived by using the radionuclide
concentrations in water, tabulated by the National Council on Radiation Protection for occupational exposure, × 1/30 to apply to the general population.
Table 18.5. Radiation protection guides (RPGs) and derived concentration action ranges (Bq kg−^1 ) a^ for selected radionuclides for the general population.b
Risk Assessments
Philosophy of radiation protection
The setting and execution of guidelines for radiation protection are based on an under- lying philosophy in which two factors are of prime importance. First is the assumption that radiation effects follow a linear or non- threshold dose–response relationship. There is convincing evidence, particularly in so far as the genetic effects of radiation are con- cerned, that there exists a non-threshold phe- nomenon. Although positive proof is lacking thus far, it has been deemed prudent to adopt this more conservative hypothesis in setting protection standards for large numbers of people. According to the non-threshold con- cept, there is no radiation dose so small that it does not involve some degree of risk. The non-threshold relationship, therefore, implies that there is no radiation protection guide- line, no matter how low, which can ensure absolute safety to every individual in a large population receiving the guideline dosage. However, since the magnitude of the risk is proportional to the dose received, untoward effects would become manifest at very low dose levels only if extremely large numbers of exposed individuals were observed. The radiation protection guide (RPG) (FRC, 1965) may be defined as the radiation dose which should not be exceeded without careful consideration of the reasons for so doing. In light of the non-threshold phenome- non, every effort should be made to encourage the maintenance of radiation exposures as far below the guide as practicable. Methods of estimating guides are experiments, which have contributed greatly to the study of the effects of radiation. From this combined knowledge and from an understanding of the relative biological damage produced by various types of radiation, protection guides for whole-body exposure and for various organs have been recommended. These guides, of course, represent doses far below those at which any effects have thus far been observed.
Basis for radiation protection guides
Establishment of ‘safe’ levels of a long-term radiation dose requires knowledge of the cause–effect relationship between radiation dose and biological damage. Such damage may appear many years after initial exposure and is usually indistinguishable from the normal diseases and impairments of man. Information accumulated on this subject is, therefore, difficult to evaluate and is often controversial. Nevertheless, observations involving man and animal life have resulted in the accumulation of significant data.
Factors influencing radioactivity concentration guides
The radioactivity concentration guide (RCG) is the concentration of radioactivity in the environment that is determined to result in whole-body or organ doses equal to the RPG. In calculating RCG values for a given radionuclide, the following factors must be taken into consideration.
INITIAL BODY UPTAKE Large fractions of some elements are absorbed when taken into the body. In the case of certain other elements, only small fractions are absorbed in passage through the gastrointestinal tract. Therefore, the greater retention would increase the hazard from the first group as compared with the second, other factors being equal. When radionuclides are inhaled, unless information specific to the radionuclide is available, it is assumed, in the case of soluble compounds, that 25% is retained in the lower respiratory tract. From here, the nuclides move into the bloodstream and a portion of each is deposited in its critical tissue within a few days. Approximately 50% is held in the upper respiratory tract and swallowed. In the case of insoluble compounds, it is assumed that 12% is retained in the lower respiratory tract, which is usually taken as a critical organ. The remainder is eliminated by exhalation and swallowing.
Radionuclides in Foods: American Perspectives 403
FRACTION RETAINED IN THE BODY The rate of elimination from the blood and tissues of the body varies considerably for different elements or compounds. The time required for one-half of the original quantity of radio- active material to be removed from the body by biological processes is called the biological half-life. Some materials in the bloodstream are eliminated rapidly from the body, whereas large fractions of others remain in the body organs. For example, radium, plutonium and strontium are deposited in the bone where the rate of turnover is very slow, i.e. the biological half-life is many years. Radioisotopes of these elements are much more hazardous than those of carbon, sodium, sulphur and those that have biological half-lives of a few days or weeks. The principal biological methods of elimination of radionuclides from the body are the urine, faeces, exhalation and perspira- tion. Usually, elimination is much more rapid before the radionuclide is translocated from the body to a more permanent area, such as the bone. This time is usually from a few days to a few weeks. After the initial period, the elimination rate becomes more nearly exponential, and the application of the term ‘biological half-life’ has more meaning.
Regulatory Issues
The US FRC issued its Report No. 2 that set up the RPGs in September 1961 (FRC, 1961). These guides were set up to ensure that the public in the USA would be protected from the radioactive ‘fallout’ (Table 18.5). During the ‘fallout’ from Chernobyl, imported foods were regulated using a guide approved by both the USDA and the FDA. These were known as the USDA–FDA ‘levels of concern’ (Food and Drug Administration, 1986) (Table 18.6). They were primarily for 131 I, 137 Cs and (^134) Cs. In 1998, the FDA published in the
US Federal Register new guides: Accidental Radioactive Contamination of Human Food and Animal Feeds: Recommendations for State and Local Agencies (US Department of Health and Human Services, FDA, CDHR, 1998). This replaces the previous US Federal Radiation Council Report No. 2. (FRC, 1961, 1965). The
USDA–FDA ‘levels of concern’, became the ‘derived intervention levels’ or ‘DILs’. The values remain the same. Table 18.7 shows the new levels recommended in the Acciden- tal Radioactive Contamination of Human Food and Animal Feeds: Recommendation for State and Local Agencies (US Department of Health and Human Services, FDA, CDHR, 1998). The previous guidelines (FRC, 1965) were predicated upon the more or less constant release of low level radioactivity into the envi- ronment from the routine uses of radiation, and assumes continuous radionuclide intake by the population. Control for the population is based on the source of the release. There are cases, however, in which the contamination of the environment might be accidental or unforeseen, producing contamination that is transient and not likely to recur; these might include, for example, reactor incidents that result in relatively high but temporary local radioactivity levels. In cases of this kind, the ‘contaminating event’ would not occur on a regular basis, and control of the population might base protective action upon limiting or changing the uptake of certain contaminated foods. However, the impact of such measures
404 E.J. Baratta
Radionuclide group Bq kg−^1 (^90) Sr (^131) I (^134) Cs + 137 Cs (^238) Pu + 239 Pu + 241 Am (^103) Ru + 106 Ru
160 170 1200 2 [C3/6800 + C6/450] < 1 c aThe DIL for each radionuclide group (except for (^103) Ru + 106 Ru) is applied independently. Applicable to foods as prepared for consumption (for dried or concentrated food products, such as powdered milk or concentrated juices, adjust by a factor appropriate to reconstituted product). For spices, which are consumed in small quantities, use a dilution factor of 10. bUS Department of Health and Human Services (1998). c (^) Due to the large differences in DILs for 103 Ru and (^106) Ru, their individual concentrations are divided by their respective DILs and then summed. The sum must be less than 1. The C3 and C6 must be the concentrations at the time of measurement.
Table 18.7. Recommended derived intervention level (DIL) a^ or criterion for each radionuclide group.b
Baratta, E.J. and Reavey, T.C. (1969) Rapid determination of strontium-90 in tissues, food, biota and other environmental media by tributi-phosphate. Journal of Agricultural and Food Chemistry 17, 1337–1339. Caper, G.C. and Cunningham, W.C. (2000) Element and radionuclide concentrations in foods: FDA Total Diet Study 1991–1999. Journal of the Association of Official Analytical Chemists 83, 157–177. Comar, C.L. (1963) Factors influencing the bio- logical availability of fallout radionuclides for animals and man. Federal Proceedings 22, 1402–1409. Comar, C.L. and Bronner, F. (1964) Mineral Metabolism. Academic Press, New York, pp. 523–572. Comar, C.L., Wassermann, R.H. and Twardock, O.R. (1961) Secretion of strontium and calcium into milk. Health Physics 7, 69–80. Cunningham, W.C., Strobe, W.B. and Baratta, E.J. (1989) Radionuclides in domestic and imported foods in the United States, 1983–1986. Journal of the Association of Official Analytical Chemists 72, 15–18. Cunningham, W.C., Anderson, D.L. and Baratta, E.J. (1994) Radionuclides in domestic and imported foods in the United States, 1987–1992. Journal of the Association of Official Analytical Chemists 77, 1422–1427. Eisenbud, M. (ed.) (1973) Environmental Radio- activity. Academic Press, New York. Federal Radiation Council (1961) Background Material for the Development of Radiation Protection Standards. Federal Radiation Council Report No. 2. US Government Printing Office, Superintendent of Documents, Washington, DC. Federal Radiation Council (1965) Background Material for the Development of Radiation Protection Standards. Federal Radiation Council Report No. 7. US Government Printing Office, Superintendent of Documents, Washington, DC. Food and Agriculture Organization of the United Nations (1960) Radioactive Materials in Food and Agriculture. FAO Atomic Energy Series No. 2, Rome. Food and Agriculture Organization of the United Nations (1964) Agricultural and Public Health Aspects of Radioactive Contamination in Normal and Emergency Situations. FAO Atomic Energy Series No. 5, Rome. Food and Drug Administration (1963) Teenage Diet Survey. Radiological Health Data Reports 4. FDA, Rockville, Maryland, pp. 18–22.
Food and Drug Administration (1973) Total Diet Studies, Radionuclides in Food. FDA Compliance Program. F-10 7320.08A. FDA, Rockville, Maryland. Food and Drug Administration (1986) Imported Foods. Compliance Program. FDA, Rockville, Maryland. Interlaboratory Technical Advisory Committee (ITAC), Subcommittee on Surveillance (1965) Routine Surveillance of Radioactivity Around Nuclear Facilities. Division of Radiological Health, US Public Health Service, Department of Health, Education, and Welfare, Rockville, Maryland. Kahn, B., Jones, J.R., Porter, C.R. and Straub, C.P. (1965) Transfer of radiostrontium from cow’s feed to milk. Journal of Dairy Science 48, 1023–1030. Katcoff, S. (1958) Fission yields from thorium, uranium and plutonium. Nucleonics 16, 78–85. Katcoff, S. (1960) Fission-product yields from neutron-induced fission. Nucleonics 18, 201–208. Laug, E.P., Mikalis, A., Billinger, H.M., Dimitroff, J.M., Deutsch, W.J., Duffy, D., Pillsbury, H.E., Loy, H.W. and Mills, P.A. (1963) Total Diet Study. Journal of the Association of Agricultural Chemistry 46, 749–767. National Academy of Sciences–National Research Council (1961) Internal Emitters. Publication No. 88. National Academy Press, Washington, DC. Roecklin, P.D., Smedely, C.E. and Simpson, R.E. (1970) Strontium-90 and cesium-137 in total diet samples. Radiological Heath Data Reports 11, 47–64. Russell, S.R. (1963) The extent and consequences of the uptake by plants of radioactive nuclides. Annual Review of Plant Physiology 14, 271–294. Simpson, R.E., Baratta, E.J. and Jelinek, C.F. (1974) Radionuclides in foods: monitoring program. Radiation Data and Reports 15, 647–655. Simpson, R.E., Baratta, E.J. and Jelinek, C.E. (1977) Radionuclides in foods. Journal of the Association of Official Analytical Chemists 60, 1364–1368. Simpson, R.E., Shuman, F., Baratta, E.J. and Tanner, J.T. (1981) Survey of radionuclides in food, 1961–77. Health Physics 40, 529–534. Strobe, W.B., Jelinek, J.C. and Baratta, E.J. (1985) Survey of radionuclides in foods, 1978–1982. Health Physics 49, 731–735. Tanner, J.T. and Baratta, E.J. (1980) Radionuclides in foods. In: Goss, B.L. (ed.) Factors Affecting Power Plant Waste Heat Utilization. Pergamon Press, Elmswood, New York, pp. 140–151.
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