PERMISSIBLE DOSES CONFERENCE held at Chalk River, Ontario, September 1949. Atomic Energy or Canada Limited AECL Research Scientific Document Distribution Office Chalk River Laboratories Chalk River, Ontario, Canada KOJ 1J0 Chalk River, May 1950. The following is based on the "Report of the United Kingdom Delegation" NP/P/TD/122. The report has been modified in accordance with the numerous suggestions and criticisms that have bene received from the various delegates. It was not always possible to reconcile differences of opinion or conflicting information and in such cases only one version is given which is not necessarily the correct one. In other instances all parties agree that the information is unacceptable but cannot offer any alternative. It is anticipated that this report is justified as a basis for future discussion. G.E. McMurtrie, Secretary Chalk River, Ontario. May 1950. AGENDA FOR THE CONFERENCE 1. Standard Man a. Mass of organs b. Chemical composition c. Physiology d. Working time e. Retention of particulate matter in the lungs. 2. Relative Biological Efficiency a. Beta Particles b. Protons c. Alpha Particles d. Fast Neutrons e. Slow Neutrons. 3. Permissible Exposure to External Radiation X and Gamma Rays, Beta Particles, Fast Neutrons, Slow Neutrons. a. Whole body, long continued exposure. b. Whole body, single exposure. c. Hands, long continued exposure d. Hoad, long continued exposure. 4. Internal Irradiation by: Group I Radium, Radon, Uranium, Plutonium, Thorium, Polonium. Group II Fission Products Group III H3, C14, Na24, P32, S35, A41, Sr89, I131, Co60. a. Amount of radioisotope permitted fixed in the body. b. Concentration of radioisotope permitted in inspired air. c. Concentration of radioisotope permitted in drinking water. - 4 - I. Standard Man The desirability of adopting a common set of values for the basic anatomical and physiological data required in calculation of permissible levels was generally agreed, even though some of these values might have to be a provisional character owing to lack of complete scientific information. It was agreed that the figures submitted by Dr. Cipriani should form the basis for the discussion. (a) Mass of Organs The figures presented were based on those of H. Lisco (ANL Report 4253, and "Science and Engineering of Nuclear Power", Vol. 2, 1949), which include some earlier estimates by Lisco and Parker (MDDC Report 783). there was considerable discussion of the importance of the variability of the masses of organs and tissues. Of various proposals, it was agreed that the most useful and practicable course was to give the average figures, and if necessary to consider the variability along with other factors of uncertainty when assessing the safety factor for any particular calculation. Furthermore, it was agreed that while some rounding off of figures was desirable, a reasonably close approximation to the original values should be maintained. Particular items in the table were next discussed in detail. The figure of 700g. adopted for lymphoid tissues was felt to be one of the more uncertain quantities. It was recognized that the thyroid is a very a variable organ, its size showing an inverse correlation with the proximity to the sea. While the general opinion that the mast lay between 20 and 30 g., Dr. Morgan quoted figures obtained by Dr. Keating at the Mayo Clinic given an average of 15g. Dr. Shields warren said that the number of cases in this series was probably ten to twelve thousand. A figure of 2g. was finally adopted. The mass of the egastro-intestinal tract was also recognized to be a very variable quantity, about which there was not very extensive evidence. The figure of 2000g. which was adopted was understood to include the oesophague. It was felt that since the average age of the population is 30 to 33 years, a figure of 10g. for the mass of the thymus gland would be reasonable, about two thirds of this being made up of fat and fibrous tissue. It was generally agreed that there was considerable uncertainty in the figures 1500g. each for red and for yellow bone marrow, and that further data were needed. The figure of 5000g. adopted for the total blood excludes that small residual amount of blood which cannot be extracted from the organs, and which is understood to be included in the figures for the masses of the organs themselves. Dr. Morgan quoted figures for the mass of the heart given by dr. Keating from a series at the Mayo Clinic, namely, 294g. in the male and 250g. in the female. Dr. Shields Warren pointed out that at necropsies any mass in excess of 350g. is generally regarded as abnormal. A value of 300g. was adopted. Dr. Cipriani's proposed values for the adrenal and prostate glands represent rounded off figures somewhat higher than Dr. Lisco's original, a procedure felt to be justifiable in view of the increase in mass of these glands with age. Finally it was agreed that eh tabulation of the Standard Man could be confined to the male at present. The following values were then formally agreed. - 5 - I. Standard Man. (a) Mass of Organs Organs Grammes Muscles 30,000 Skeleton, Bones 7,000 Red Marrow 1,500 Yellow Marrow 1,500 Blood 5,000 Gastro-intestinal Tract 2,000 Lungs 1,000 Liver 1,700 Kidneys 300 Spleen 160 Pancreas 70 Thyroid 20 Testes 40 Heart 300 Lymphoid Tissue 700 Brain 1,500 Spinal Cord 30 Bladder 150 Salivary Glands 50 Eyes 30 Teeth 20 Prostate 20 Adrenals 20 Thymus 10 Skin and subcutaneous tissues 8,500 Other tissues and organs not separately defined 8,390 Total Body Weight 70,000 - 6 - (b) Chemical Composition The figures for the chemical composition of the body presented for discussion by Dr. Cipriani are taken from "Practical Physiological Chemistry" by Hawk, Oser and Summerson (12th Edn. 1947). Professor Mitchell stated that the figure were in reasonable agreement with figures which he had proposed previously for neutron dosage calculations, but pointed out that the nitrogen content of dividing cells of the basal layer of the skin might be nearer 6%, rather than 3%. Dr. Brues commented that the normal carbon content of organs was about 12%, so that he figure of 18% present more here must include the fatty tissue of the body. Both these speakers and other emphasized the importance of the difference between the chemical composition of a given organ and the average for the whole body, and stressed the fact that very little detailed information exists. much of that information was obtained a considerable time ago, before the development of modern analytical techniques, with a considerable possibility of error. Data on some of the trace elements, of importance for whole body slow neutron exposure, appear to be entirely lacking. Accordingly, it was agreed that each delegation should recommend that a large scale effort be made to obtain accurate data for the mass and the chemical composition of the organs and tissues of the human body, having regard to all the elements of possible importance in radiation effects. Normal organs, preferably from cases of sudden death should be examined, and analyses should be made on homogenized tissue and not merely on slices of organs. Full details of the geographic, occupational and medical history of each subject are desirable. It was felt that in addition ont eh organs and tissues tabulated above for the Standard Man, data on ovaries should be obtained. Dr. Morgan, Dr. Cipriani and Prof. Mitchell agreed to be responsible for liaison on these problems, and Dr. Morgan agreed to circulate the data he has already accumulated. Standard Man. Chemical Composition of the Human Body. Proportion Approximate amount in Element Percent grammes in a 70kg. man Oxygen 66.0 45,500 Carbon 18.0 12,600 Hydrogen 10.0 7,000 Nitrogen 3.0 2,100 Calcium 1.5 1,050 Phosphorus 1.0 700 Potassium 0.35 245 Sulphur 0.25 175 Sodium 0.15 105 Chlorine 0.15 105 Magnesium 0.05 35 Iron 0.004 3 Manganese 0.0003 0.2 Copper 0.0002 0.1 Iodine 0.00004 0.03 - 7 - (c) Applied Physiology It was formally agreed to adopt the data below as broad averages for normal activity in the temperate zone, recognizing that changes caused by abnormal circumstances or environment are not allowed for: (i) Water Balance Average daily water intake In food 1.0 litre (including water of oxidation) In fluids 1.5 litres Total 2.5 litres It was agreed to base calculations of maximum permissible levels for radioactive isotopes in water on the 2.5 litres per day figure, recognizing that in some cases a separate safety factor might be required to allow for the concentration of an isotope in the food from cooking water. Average daily water output. Sweat 0.5 litres From Lungs 0.4 " In faecos 0.1 " Urine 1.5 " Total 2.5 litres Overall water content of body 50 litres (ii) Respiration Area of respiratory tract Respiratory interchange area 50 sq. metres. Non-respiratory area 20 " " (upper tract, trachea to bronchioles) Total 70 sq. metres Respiratory exchange Physical Hours/day Tidal air Resp./min. m3 air/8 hrs. Total Activity At work 8 1 litre 20 10 20 Not at work 16 0.5 litre 20 5 m3/day - 8 - Carbon dioxide content of air Inhaled air - 0.03% (dry air, sea level) Alveclar air - 5.5% Exhaled air - 4.0% (depends on activity) (iii) Duration of Occupational Exposure It was formally agreed to adopt the following figures for the duration of occupational exposures: 8 Hours/day 40 Hours/week 50 Weeks/year (thus, 2000 hours/year) The view was widely held that the adoption of a definite figure for the working lifetime for continuous occupational exposure to radiation might have undesirable psychological repercussions. At the same time, it was generally recognized that the proportion of workers receiving continuous occupational exposure to radiation for more than twenty years is likely to be small. (iv) Duration of "Lifetime" for Non-occupational Exposure. It was formally agreed that for the purposes of calculation the duration of life for the standard man be assumed to be 70 years. (d) Standardized Terminology It was agreed that all formal statements of permissible values should be expressed in microcuries per cubic centrimetre (mc/cc) or microcuries per gramme (mc/g) whether in air, water, or other medium. Where other units are now in used, the value in microcuries per cubic centimetre or microcuries per gramme should be followed in brackets by its equivalent. (Note: it was general understood that for the present the term "microcurie" would be interpreted as 3.7 x 104 disintegration per second) (e) Retention of Particulate Matter in the Lungs. There was considerable discussion of several aspects of the general problem of hazard from inhalation of radioactive particulates. On the question of the retention of such particulates, the influence of size, surface area, and solubility were emphasized. Dr. Brues mentioned that, in general, retention was high for lu particles, but considerably lower for 10U. Dr. Wolf mentioned the report UR67 from the Rochester group as a course of considerable experimental information. Particles of uranyl nitrate up to 4 in diameter were heavily retained. the work of the Columbia group with radioactive sodium has demonstrated that there may be a bimodal distribution of retention with particle size. Dr. Shields warren pointed out that in cases of silicosis ar asbestosis, very few particles greater than 1.5u in diameter are found; it is also know that only a very small percentage of the inhaled material is permanently retained. - 9 - Long fibres of material are much loss likely to be eliminated than large spherical particles. For example, particles 45u in length may be found in cases of asbestosis or in the sugar cane disease bagassosis. Dr. Hamilton mentioned that insoluble particles of yttrium oxide (counted with gold) in the size range 0.1 to 1u gave 95% immediate retention but after 24 hours, 75% of the material was eliminated in the faeces. After further discussion, it was formally agreed that if specific data were lacking, the convention be adopted that 50% of any aerosol reaches the alveoli of the lungs. If the particles are soluble they are considered to be totally absorbed; if insoluble then the 50% amount is to be regarded as retained for 24 hours, after which only half of it, i.e., 25% of the inhaled amount, is retained in situ indefinitely; further that the particles be assumed spherical. In relation to the possible pathological effects of radioactive particulates in the lungs, Dr. Hamilton pointed out that the cells in the immediate neighborhood of a dust particle containing 1 or 2% of plutonium would be subjected to a dose of about 400r/day. The general opinion which emerged from the discussion was that the carcinogenic effect per unit volume is probably considerably less for the irradiation of small masses of tissue than for large. Dr. Shields Warren pointed out that alveolar tumors do not normally occur. The lung tumors in the Joachimstahl minors are bronchogenic, and the mean latent period appears to be 17 years. The majority of investigators have regarded radon as the principal causative factor. Professor Mitchell mentioned that he had calculated that the radon level in the mines would, by the deposition of active decay products on the bronchial epitheliu, result in a dose of several roentgens per day to the latter. A brief discussion of the proportion of insoluble particulate material transported from the lung to the lymph nodes merely served to indicate that this factor is rather dependent on the nature and size of the particles. 2.Relative Biological Efficiency There was some initial discussion of the relative advantages of adopting 200kV X-rays for the radium gamma rays as the reference radiation for purposes of comparison. It was pointed out that there is probably a greater number of experimental comparisons of other radiation relative to X-rays, but on the other hand, the radium gamma rays provide a reference point of more definite character, and probably none of the radiations to be considered has a lower absolute biological efficiency than that for these rays. Moreover, a radium standard is more easily reproducible in the laboratory than an X-ray standard. It was therefore formally agreed that the relative biological efficiency of any given radiation be defined by comparison with the gamma radiation from radium filtered by 0.5 millimetres of platinum, and measured as the inverse ratio of the doses in ergs per gramme of tissue required to produce equal biological effects of specified character. The need for considering relative biological efficiencies for a variety of different effects wa mentioned, but it was not considered practicable to do more than consider chronic bone marrow and skin damage, to which a common set of relative efficiencies were regarded as applicable for purposes of calculation. - 10 - The relative biological efficiency of X-rays in the normal deep therapy region (i.e. around 200kV) was next considered, and Professor Mitchell pointed out that the production of skin erythema and the healing of malignant tumors, a value of 1.5 applied. For the production of leukemia, it might well be that previous gross damage to the blood forming organs was necessary and that integral dose rather than dose to the bone marrow alone was involved. In view of the uncertainties, it was therefore formally agreed to take the relative biological efficiency of 200kV X-rays as unity. It was also agreed that this same value applied to beta radiation. In the discussion on heavy particle effects on the liver with plutonium and gamma radiation, a relative biological efficiency of 4 1/2 had been found. Professor Mitchell pointed out that for the densely ionizing particles, there was every reasons to expect that the relative biological efficiencies would be considerably higher for chronic than for acute effects if these depended on the production of chromosome aberrations, and there was some experimental evidence for this expected rise in efficiency; he proposed a figure of 20 for alpha particles. Dr. Hamilton commented that values for alpha particles and fast neutrons in the ratio of 20 to 10 would correspond with the data of Zirkel. When U235 in colloidal form wa injected, it was deducted that the fission recoil particles were five times as efficient as fast neutrons. The practical and economic aspects of a figure of 20 for alpha particles were briefly considered after which it was formally agreed to adopt the value of 20 for the relative biological efficiency of alpha particles. Protons were next discussed and it was pointed out by Dr. Laurence that there was some reason for taking a lower figure than for fast neutrons because for a given dose, there would be a greater number of protons involved in the second case and so, a greater effect from the densely ionizing regions at the end of the tracks. It was, however, finally agreed to take a value of 10 for the relative biological efficiency of protons. Since thermal neutrons produce their effect by a mixture of gamma and proton radiation, it was agreed to take a value of 5 for the relative biological efficiency. These decisions on relative biological efficiency may be summarized thus: Relative Biological Efficiencies Type of Radiation Bone Marrow Skin Alpha 20 Protons 10 Fast Neutrons < 20 MeV 10 Thermal Neutrons <0.025 ev 5 Beta 1 Gamma (Radium) 1 1 X-Ray (~200 Kv) 1 1.5 - 11 - 3. Permissible Exposure to External Radiation Dr. Failla offered as a basis for discussion a preliminary draft of a report which he had prepared for submission to this Sub-committee on Permissible Dose from External Radiation of the National committee on Radiation Protection and this was accepted. The question of the unit in terms of which the dose should be expressed was discussed, and it was formally agreed to adopt the "rep", defined as 93 ergs per gramme of tissue, subject to subsequent revision by international agreement. It was further agreed that chronic exposure be expressed on a weekly basis. It was also agreed that any recommendation on maximum permissible exposure for external radiation made at this conference was not intended to apply to diagnostic and therapeutic irradiation. The question of what constituted "whole body" exposure was carefully considered. Professor Mitchell stated that in radiotherapeutic experience, irradiation of more than half the trunk is effectively whole body irradiation. Dr. Hamilton pointed out that the therapeutic irradiation of a fairly limited area, even to high doses, does not have consequences at all comparable to those of whole body irradiation. In this connection, Dr. Shields Warren remarked that this was still true for very protracted irradiations, for example, the treatment of carcinoma of the breast may extend of 3 1/2 years. there was general agreement that irradiation o the hands and forearms does not constitute a whole-body irradiation, and several other examples were cited. It was felt, however, to be impracticable to include in a definition all the various possibilities and so it was formally agreed that for the purpose of health monitoring whole body exposure should normally be assumed for exposure other than on the hands and forearms. It was clearly understood, however, that this was merely a working convention adopted without prejudice to the medico-legal aspects of any particular instance which might arise. Dr. Shields Warren expressed the opinion that if an irradiation is know to involve less than one third of the body, it need not be regarded as "whole-body". It was understood that in such circumstances, the dose could not be increased pro rata. Following Dr. Failla's proposal, it was formally agreed that for any exposure to external penetrating radiation, the blood forming organs should be regarded as the critical tissue, the principal hazard probably being leukemia. The estimated depth of the blood-forming organs for purposes of calculation was next discussed. Dr. Shields Warren stated that leukemia probably arise out of changes in the marrow and spleen and rarely in the lymph nodes primarily; Professor Mitchell and R. Cipriani concurred. The question of the importance of variation of the dose to different parts of the marrow was discussed. Dr. Shield warren stated that informally there is not much functioning red marrow in the long bones. He proposed, and it was formally agreed that for purpose of calculation, the blood forming organs be assumed to lie at an effective depth of 5 centimetres. Dr. Failla then proposed that the maximum permissible exposure for external penetrating radiation should correspond to a dose of 0.3 rep/week to the critical tissue (bone marrow). In the discussion it was noted that the existing U.S. recommendation of a weekly surface dose of 0.3 measured in air is approximately equivalent to the British figure of 0.5r measured at the surface of the body (i.e. with backscatter) and that both are approximately equivalent to a dose of 0.3 rep to the bone marrow. It was therefore formally agreed that the maximum permissible dose for exposure to external penetrating radiation be 0.3 rep. per week to the critical tissue, corresponding approximately to 0.5 rep per week to the surface of the body when measured with backscatter and 0.3 rep. per week when measured in air. It was also agreed that the maximum permissible dose for internal radiation should be 0.3 per week to the critical tissue, except in cases where experimental evidence. - 12 - It was emphasized that the permissible dose referred to the energy absorption in the critical tissue itself, and that in principle, the measurement of the dose would be made in a small cavity ionization chamber with walls equivalent in composition to the tissue in question. The question of lifetime dose was considered and Dr. Failla proposed a figure of 300 rep. to the blood forming organs: he stated that the desirability of some such figure depended partly on genetical consideration. dr. Shield warren suggested that the genetic argument is already met by the fact that by the time a dose of 300 rep. has been accumulated, most people will have passed the active reproductive period. Dr. Hamilton mentioned the difficulty of the large numbers of existing workers who may have already approached or even exceeded the dose in question. Dr. Parker asked about the possible shortening of life span in relation to total accumulated dose. Dr. Shields Warren pointed out that the application of the data from animal experiments on this subject to the problem of t permissible dose for man involved a considerable extrapolation. The more direct evidence is the fact that the average life of a radiologists is equal to that of other medical specialists. It was finally agreed to leave open the question of a maximum lifetime dose. It was formally agreed that for external irradiation by beta rays, the skin be regarded as the critical tissue. It was also agreed that for purposes of calculation the depth of the critical tissue, namely the basal layer of the epidermis be assumed to correspond to 7 mg/cm2. It was further agreed that for external whole body irradiation by beta rays, the maximum permissible dose be 0.3 rep per week to the bone marrow or critical tissue. It was agreed to be unnecessary at the Conference to work out explicit figures for maximum permissible fluxes for neutron exposures in view of the figures for relative biological efficiencies. The terms "fast" and "thermal" as applied to neutrons in the table "Relative Biological Efficiencies" were agreed on by the Committee to mean neutrons whose biological effect are primarily by proton recoil and by nuclear reaction respectively. The exposure of the hands alone was discussed and Dr. Failla proposed a maximum weekly dose of 1.5 rep. stating that doses of less and 10 or 15 r/week produce observable changes int eh fingers. Professor Mitchell mentioned that radium surgeons commonly exceed the figure of 1.5 rep. per week, but that they accepted the risk consciously. Dr. Braaton doubted whether radium factories could be operated at this level, in view of the fact that the wrist films at present indicate 10 rep. per week. The possibility that t some of this apparent dose might be due to blackening of the film by contamination was mentioned. dr. Parker sated that there is definite evidence of damage to the hands at a dose level of 4 rep. per week. Finally it was agreed that for external radiation of the hands by X, gamma or beta radiation the maximum permissible dose be 1.5 rep. per week. Arising out of the same discussion it was agreed that no relaxation of the standard permissible dose could be allowed in the case of irradiation of the head alone in view of the risk of cataract formation. The maximum permissible dose in a single exposure was next discussed. Dr. Failla proposed the figures of 25 rep for a person under the age of 45 and 50 rep over this age. Professor Mitchell sated the vie of the Medical Research Councils Tolerances Doses Panel that the single emergency dose be not greater than 10r. and that the total dose in any period of six months should not exceed the normal maximum permissible average of 13r. Dr. Lewis sated that in the Chalk River Establishment, a single dose of 10r. was the limit in extreme emergencies. Dr. Parker felt doubtful whether any relaxation of the normal restrictions was needed for atomic energy plants. Dr. Shields Warren said it was clear that all were agreed that single high exposures should be limited to grave emergencies. A variety of - 13 - different eventualities might arise, but as the problems were largely administrative, he felt that the details need not be discussed at the Conference. On the other hand, a problem of general interest, for example to Civil Defence Authorities, was the assessment of the single dose which will produce no permanent harm. The N.E.P.A. report summarizes some of the basic information. the LD 50 dose for man is around 400r, and at this dose a person would be rapidly incapacitated. Professor Mitchell sated that a consideration of mortality probits as a function of the logarithm of the dose and of the integral dose, and certain clinical data, would lead to a figure of 25r, in agreement with dr. Failla's proposal. In the ensuing discussion there was some question as to whether a smaller unit was needed for children and women, and it was noted that in the U.S. atomic energy plants no person under the age of 18 is employed. Dr. Morgan mentioned the observation of Jacobson that a dose of 50r. given in 4 hours to mice aged 5 moths was followed by a significant increase in ovarian tumors. The general opinion seemed to be that mice are probably peculiar among mammals in this respect, and that the observation had no significance for the present problem of the permissible single dose for women. After some further discussion it was agreed that in the light of present knowledge, no manifest permanent injury is to be expected for a single exposure, of the whole body to 25 rep. or less, with a possible exception in the case of pregnant women. It was generally understood but not specifically sated that such exposure was contemplated once only in a lifetime. 4. Permissible Exposure to Internal Irradiation Dr. Hamilton introduced the subject of hazards from internal irradiation with a brief survey of his results on the metabolism of fission products and radioactive elements in animals, reported in Reviews of Modern Physics, Volume 20, No. 4., from which he reproduced the following table. Radio Element Oral Absorption Organ and Percentage Biological deposition Half-life Sr89, Sr90, Ba140 5-60% Bono, 60-70% > 200 days Y91, Zr95, La140 Ce144, Pr143 Nd147, Pm147 <0.05% Bone, 25-70% > 100 days La140, Co144, Pr143, Nd174, Pm147 <0.05% Liver, 50-70% 10 days Cb95 <0.05% Bone, 30% 30 days Ru106 0.05% Kidney 3.5% 20 days I131 100% Thyroid 20% > 30 days Ca135 100% Muscle 45% 15 days Xe133 Fat content 2 hours U233 < 0.05% Bone, 20% 60 days Pu239 0.007% Bone, 75% < 2 years Note: The biological half life is calculated independently of the radioactive decay. - 14 - Some 80% of the total energy of mixed fission products is associated with the group of elements which concentrate in the skeleton, Sr, Ba, Y, Zr, Cb, La, Ce, Pr, Nd, Pm, Sm, Eu. The deposition in bone is about 70% for Y and Zr, and about 25% for the other elements in this group. The alpha emitting isotopes, Ra, Ac, Th, Pa, U, Np, Pu, Am, Cm, are also concentrated in the skeleton. All the bone seeking elements have long biological half lives. A number of these isotopes are also concentrated in the liver, including La, Ce, Pr, Nd, Pm, Sm, Eu, among the beta emitters and Ac, Am, Cm, among the alpha emitter. Ruthenium and uranium are concentrated in the kidney, and iodine in they thyroid. The absorption of Sr following the inhalation of soluble compounds is probably about 50%. In young animals 16 days after the administration of Sr, the distribution in the skeleton was found to be uniform, but in older animals a much longer period is necessary for transport of the element from the periosteal and endosteal surfaces into the mineral bone. The elements Ac, Am and Cm, which are trivalent show a spotty distribution in bone like that of compact bone; there is deposition in travecular bone also. The radioautographs for thorium are indistinguishable from those for plutonium. Radium D in adult female rates gave similar distribution, as did radium itself but with th latter transference into miner bone was commencing. The absorption of plutonium from the gastro-intestinal tract was next discussed. The experiments of Dr. Wright Langham gave a figure of 0.01% while those of Dr. Brues gave th same order. Dr. Parker said that his experiments had indicated an increased absorption at low ionic concentrations. For example, rate given a dose of 4 x 10-5 mg. per day, of Pu238 in 400 solution with 0.5 ml. of wash water for 20 days showed an absorption of 0.02%, while at 1 x 10-5 per day the absorption was 0.1%. Dr. Wright Langham stated that one person drank a solution of Pu239 at about the "tolerance" concentration and found all the material in the faeces. This suggests that absorption in man is less than in the rat. Dr. Hamilton gave some data obtained by the intravenous injection into a man of a mixture of Pu239 and Pu238 equivalent in activity to 50 of Pu239. The excretion in the urine and also in the faeces were each about 0.01% per day at 16 days, while at 256 days the daily urinary excretion was slightly more than 0.001% and the faecal excretion 0.0004%. In rats, the faecal excretion alone at 256 days was 0.01 to 0.02%. "ILLEGIBLE" so that the human figures are about one tenth the rat figures. Dr. Wright Langham confirmed that he had found comparable figures. Dr. Parker mentioned the case of a man exposed 4 years ago, with a dose of 2 ug. The present excretion rate agrees with Dr. Langham's formula and corresponds to a biological half-life of 105 years. Dr. Hamilton then referred to data on radium indicating 5~20% oral absorption and 30 or 40% of the absorbed amount retained in the skeleton with a very long biological half life. Dr. Hemelmann stated that one year after intravenous injection 90 to 95% was excreted. dr. Hamilton showed radioautographs of the thyroid tissue of animals injected with astatine, the short lived alpha emitter which is chemically a member of the halogen group of elements. One third to one fifth of the material concentrates in the thyroid. Profound histological changes are apparent one month after doses between 2000 and 7000 rep. while complete destruction follows doses between 10,000 and 35,000 rep. Such data might give interesting evidence on the relative biological efficiency of alpha radiation. Dr. Shields Warren next proposed that the elements set down in the agenda be considered individually. - 15 - Group I Ra226, Ur(natural) Ur233, Pu239, Th232(natural), Th234 (UX7) Po210 The discussion was initiated by Professor Mitchell who outlined the standpoint taken u by the Medical Research Councilþs Tolerance Doses Panel. For the evaluation of the hazards from bone seeking isotopes, radium is regarded as the crucial isotope, because clinical data exists only for radium, while an estimate for several of the other isotopes may be made from the experimental data on the relative toxicities of these isotopes and radium. If 1 ug of Ra fixed in the skeleton leads to serious disease in 1% of cases, an estimate compatible with the available clinical data, then at the level of 0.1 ug of often proposed as a þtoleranceþ figure, the incidence of disease might be of the order of 0.1% though it would probably be lower. this level of 0.1 ugm had in fact been adopted provisionally by the Medical Research council for plant operatives and other specific workers under continuous medical supervision. In the case of a large population, such as the ten million persons in London dependent for drinking water on the river Thams which takes a limited amount of effluent from the establishment at Harwell, it seemed essential to introduce a further factor of 100 to provide reasonable safety. A short discussion followed of the general implications of such a viewpoint. Dr. Brues felt that a factor of the order of 100 was reasonable for large populations. The practical difficulties of monitoring at the low levels of activity involved was mentioned and it was pointed out by Mr. Chamberlain that it was necessary to monitor the effluent before dilution in the river, etc. Dr. Morgan gave examples of certain springs with concentrations of activity a billion times higher than those proposed by the Medical Research Councilþs Panel. Dr. Failla drew attention to a feature of the data from the luminizing workers which was not normally considered, namely, that the radium content was in many cases measured when symptoms of injury had appeared. The quantity of radium held in the body would have been considerably higher initially and tit was not certain whether the pathological effects were to be correlated with the radium content at some previous epoch. Dr. Brues stated that about 1% of cases with 1 ug of radium fixed in the body would be likely to show some bone damage on radiological examination. It was agreed that 1 ug might reasonably be regarded as the minimum amount of radium know to produce damage, and it was decided that before the question of safety factors was considered, the equivalent amounts for the other isotopes should be evaluated. Dr. Hamilton stated that the hazard from natural uranium is mainly chemical; there is some evidence the U233 concentrates in cancerous bone and so it might be reasonable to consider the irradiation of the red bone marrow, since the yellow marrow is mainly in the hollow portion of the shaft. Alternatively, it might be argued that the U233 hazard should be evaluated from a consideration of the data on acute toxicities and equivalence to radium. Dr. Brues stated that for acute lethal effects, Pu239 is 5 times and Po210 is 20 times as toxic as radium measured in terms of activity. Dr. Hamilton pointed out that for chronic effects, the radium is more uniformly distributed in the bone and so even higher equivalence figure might be expected. Dr. Wright Langham pointed out on the other hand that for sub-acute lethal effects, plutonium had appeared relatively less effective. Dr. Brues said that it was not certain how the chronic toxicity would vary from acute toxicity but for the production of tumors, the evidence did not warrant the adoption of a lower relative efficiency. dr. Wright Langham was in favor of calculating the permissible levels for all the elements on the basis of radiation energy absorption, but it was pointed out that this gives results at variance with the experimentally determined equivalence for radium and strontium. Mr. Chamberlain stated that a dose of 0.3 biologically equivalent roentgens would be produced by 2 x 10-2 ug of radium distributed throughout the 7 kg. skeleton assuming an effective energy per disintegration of 15 MaV and a relative biological efficiency for alpha particles of 20. - 16 - Dr. Failla remarked that the distribution of radium in bone is not uniform even 10 years after ceasing radium work, while Dr. Wright Langham mentioned a case examined by Professor Robley Evans in which seven bone samples gave an average concentration twice that for the whole skeleton. He therefore urged that in assessing the equivalent quantities of isotopes know to be deposited non-uniformly, care be taken to avoid overestimating the safety factors. He knew of 12 cases with 5 ug of plutonium, 12 with 1ug and 26 with 0.3 ug, all without ill effects, while Dr. Hamiltonþs case had the equivalent of 50 ug. Dr. Shields Warren summed up this discussion by saying that at present there is insufficient direct evidence for man, and that the data from animal experiments has still to be relied on. It should therefore be assumed that he minimum damaging amount of material fixed in the body is 1 ug for plutonium just as for radium, the risk being of the order of 1 per cent. In the discussion on polonium it emerged that there was no known case of damage in man. Dr. Lewis proposed that 0.05 ug be adopted as the minimum damaging amount, but Dr. Wright Langham observed that the kidney has a higher concentration than the bone and the maximum histological damage occur in that organ; it was not clear what the chronic effects from polonium would be. Dr. Hamilton proposed that U233 be considered as equivalent to plutonium, so that allowing for the difference in the radioactive half lives, the minimum damaging amount would be about 6 ug. Dr. Wolf observed that for chronic exposure to natural uranium, the concentration in bone is about three times that in the kidney; the biological half-life is about 300 days. Dr. Hamilton doubted whether those data would apply exactly to U233, where the gross chemical quantities involved are so much smaller. Dr. Shields Warren summed up this phase of the discussion, saying that there appeared to be no choice but to accept figures of 0.05 nc for Po210 and 6 ug for U233. Professor Mitchell said that in the United Kingdom there are no known cases for occupational disease attributable to natural thorium dioxide. Dr. Wolf confirmed that a survey in the U.S. had revealed no cases. Dr. Shields Warren stated that observations with thorotract in animals were of doubtful applicability to the present problem: injections of an apparently inert substance like Indian ink may produce tumors. It seem best to leave thorium aside for the present. Dr. Hamilton pointed out that Th234 (i.e. UX1) is exactly similar metabolically to Pu, he therefore proposed that, allowing for the average energy of the beta irradiation which is 0.8 MeV. as against 5.5 MeV for the alpha radiation from plutonium, and taking a relative biological efficiency of 20 for the alpha radiation, the minim damaging amount of Th234 be taken as 8 ug. Dr. Wolf stated that for soluble compounds of natural uranium the principal hazard is chemical, and 120 ug fixed in the kidney appears to be the minimum injurious amount; for insoluble compounds the principal hazard would appear to be radiation effects from particles fixed in the lung. A dose of 0.3 biologically equivalent roentgens per week would be produced by 105 mg. of uranium in the lungs. It was then formally agreed to adopt the following figures for the minimum damaging amount of material continuously present in the body: Ra226 1 microgramme Pu239 1 þ U233 6 þ Po210 0.06 microcurie Th234 8 þ - 17 - The meeting then proceeded to consider how maximum permissible amounts of material should be deduced from the foregoing minimum damaging amounts. Dr. Parker said he was not in favor of having two different sets of figures, one for plant workers and the other for the general population, and after some general discussion it was decided to consider the form class first. The present figure for radium of 0.1 ug corresponds to a safety factor of 10 and it was suggested by dr. Hamilton that by comparison with the practice for other noxious agents, such a factor was reasonable. Dr. Lewis on the other hand felt that if the relation between damage and þdoseþ were indeed linear for radium, then the factor of 10 does not represent a very great margin of safety; after further discussion, Dr. Edson proposed that a factor of 10 be adopted, and this was formally agreed, leading to the figures: Maximum Permissible Amounts of Isotopes continuously present in the Body, for Plant and other special workers. Ra226 0.1 microgramme Pu239 0.1 þ U233 0.6 þ Po210 0.005 microcurie Th234 0.8 þ The question of the general population was now reconsidered, and Dr. Shield Warren point out that the Columbia River and the Clinch River presented problems comparable with those of the River Thames. Dr. Brues said that he magnitude of the safety factor required for the general population, depended on the avoidance of a statistically significant increase of pathological effects. Consequently, since it could not be ruled out that such effects were linearly related to the dose, he would propose a safety factor of 10 for populations of the order of 105 and 100 for populations of the order of 107 on the figures accepted for plant and other special workers. He considered that it would be unsafe to draw conclusions from the existence of natural water with very high radioactive content, since the populations exposed to such waters were very limited. He felt it was undesirable to double the natural radium content of the skeleton. Professor Mitchell referred to the recent work of Dr. Hale and Dr. Hursh indicating a radium content of the skeleton of the order of 2 x 10-4 ug. He felt that an increase of 10 on that figure was the absolute limit of what was justifiable. The question of a possible correlation between the natural incidence of osteogenic sarcoma and the natural radium content of the skeleton was mentioned, and Dr. Hamilton suggested that he idea was not in accord with the fact that the disease occurred predominantly in young people. Dr. Brues quoted Swedish statistics, and Dr. Neary quoted British statistics showing that the peak of incidence in youth is followed by a greater rise in meddle and old age. Dr. Warren pointed out that this latter effect is related to the appearance of Pagetþs disease of bone (osteitis deformas) in later life, which appears to be associated with metabolic changes unconnected with radioactive effects. The practicability of proposal based on the large safety factor, and the difficulty of publish opinion on the existence of difference levels for different sections of the population were mentioned. At this point, Dr. Brues reminded the meeting that the Medical Research Councilþs proposals for the River Thames were confidential. Mr. Chamberlain stated tha the proposal for the river Thames are practicable. He pointed out that in the neighborhood of a plant, it is impossible to draw a hard hand fast defining line between the plant population and the general population. A proposal to apply a safety factor of 100 to say the gaseous effluent from a plant would certainly not be practicable. - 18 - Finally, it was formally agreed that a safety factor of 100 on the permissible levels for plant and other special workers should be applied to all elements when dealing with large centers of population. The following figures were therefore adopted for Group I: Maximum Permissible Amounts of Isotopes continuously Present in the Body, for all large centres of population. Ra226 0.001 microgramme Pu239 0.001 þ U233 0.006 þ Po210 0.00006 microcurie Th234 0.008 þ Group II Fission Products In view of the limitations of time, it was decided to consider only the most dangerous of the fission products, namely, strontium. Dr. Hamilton observed that the mean energy per disintegration of þILLEGIBLEþ and Y90 is 1.1 MeV, or about one fifteenth of the effective energy for radium in the skeleton. Then the relative biological efficiencies are also taken into account, the ratio of biologically equivalent amounts ( in curies) of Sr90 and radium would appear to be 300. Dr. Brues pointed out however that acute toxic effect and the carcinogenic effect in animals gave a ratio of only 10 for Sr89 which would indicate a ratio of approximately 20 for Sr90 and Y90. Mr. Chamberlain said that some of the alpha radiation is effectually expended in mineral bone and so if the harmful effect of radiation is dependent on integral dose as well as on the dose at particular points an extrapolation on energy grounds is unsound. Dr. Shields Warren stated that the mean diameter of the traveculae in man is 30 to 36. Dr. Hamilton then observed that it would have been better to consider þILLEGIBLEþ in relation to the strontium data rather than the plutonium data. (Secretaryþs note: if this were done, then the minimum damaging amount of Th234 would be six tenths of that for Sr89). He calculated that 7 uc of Sr90 in the bone and marrow gives a dose of 0.3r per week; the range of the beta rays is sufficient to average out the effect of any non-uniform distribution of the strontium. Dr. Bues stated that the figure of 7 uc as a permissible amount seemed too high on the basis of the experimental comparisons. Dr. Hamilton observed that some of the beta rays in man will be ineffectually absorbed in mineral bone so that the efficiency relative to radium will be smaller than in mice. Professor Mitchell remarked that the problem was related to the discrepancy between the empirically determined permissible radium burden in the body and the permissible dose for external penetrating radiation. Dr. Brues pointed out tha the disagreement could be reduced if a smaller relative biological efficiency for alpha radiation were adopted; for example, a value of 5 would lead to a permissible level (plant workers) of 2uc for Sr89, of 1uc for Sr90 and Y90, values which he was prepared to accept. After some further discussion, it was formally agreed to adopt 10 uc of Sr89 to 4uc of Sr90 (in equilibrium with Y90) as the minimum damaging amount. The foregoing conclusions for the isotopes in Group I and II may therefore be summarized as follows: - 19 - Minimum damaging Maximum Maximum amount continuous- permissible permissible ly present amount for amount for Plant and all large other special centres of Element workers population. Ra226 1 microgramme 0.1 microgramme0.001 microgramme Pu239 1 þ 0.1 þ 0.001 þ U233 6 þ 0.6 þ 0.006 þ Po210 0.05 microcurie 0.005micrcurie 0.00005 microcurie Th234 8.0 þ 0.8 þ 0.008 þ Sr89 10 þ 1.0 þ 0.01 þ Sr90(+Y90) 5 þ 0.5 þ 0.005 þ At this point, the time officially allotted for the Conference had expired. Dr. Shields Warren proposed that in view of the importance of the common isotopes in Group III, a further informal session to be held to consider them, and this was agreed. Before concluding this session, Dr. Shields Warren suggested that each national delegate might prepare draft notes on the Conference which could then be circulated with a view to getting a final agreed statement of the proceedings. He further suggested that the necessary liaison might be effected through Dr. Cipriani and Mr. McMurtrie. These proposals were agreed. Group III H3, C14, Na24, P32, S35, A41, Sr89, Sr90, I131, Co60 C14 The problem of C14 as carbon dioxide in the atmosphere was discussed by Dr. Brues. A dose rate of 0.3 rep. per week would be produced by 0.014 uc of C14 per gramme of tissue. If the highest proportion of carbon in tissue is 10%, as in bone carbonate, then the maximum permissible concentration of C14 in carbon in the body is 0.14 uc per gramme of carbon. The postulated route of entry of C14 into the body is via the alveoli of the lungs, and the isotopic concentration of C14 in the carbon of the body can never be greater than the concentration in the alveolar air, which must therefore be limited to 0.14 uc per gramme of carbon. Since alveolar air contains 5.5% by volume of carbon dioxide, the maximum permissible concentration of C14 in alveolar air is 0.14 x 0.055 x 12 2.24 x 104 uc/cc. or 4.1 x 10-6 uc/cc. Hence the maximum permissible concentration of C14 in the atmosphere may be taken as 4 x 10-6 uc/cc. with a small safety factor since the actual concentration of C14 in the alveolar air may be expected to be somewhat less than that prevailing in the atmosphere. On the other hand, the concentration of carbon in some tissues may be higher than 10%, perhaps up to 50% fat. Dr. Brues, therefore, proposed the adoption of a figure of 10-6 uc/cc. for maximum permissible concentration of C14 as carbon dioxide in the atmosphere for continuous breathing. A dose rate of 0.3 rep per week is produced by 0.014 uc of C14 per gramme of tissue. If the carbon content of tissue can be as high as 50%, then 0.028 uc of C14 per gramme of carbon is the maximum concentration that can be permitted. If we take 18% by weight of the standard man as carbon then 0.028 x 12,600 = 350uc. Dr. Brues went on to describe some of the actual experimental evidence on the metabolism of C14. Mice were exposed for two months to an atmosphere containing C14 as carbon dioxide. The concentration of C14 in the tissues rose during the first week and then soon reached limiting equilibrium values. - 20 - The maximum local concentration found in bone was not more than five times the average concentration in bone. In Shubertþs experiments, the isotopic concentration in one carbonate reached a value one seventh of that in expired air. H3 The question of tritium as a gas rather than as water vapor was first briefly mentioned; Dr. Lewis stated that he exchange of the gas with water seem to be fairly rapid. Dr. Brues stated that since the energy of the tritium radiation is about one tenth that of the C14 radiation, it follows that a concentration of 0.23 uc per gramme of tissue will result in a dose of 0.3 rep/week. Therefore the total amount in a 70 kg. man would be 16mc. Following the same argument as for C14, and taking the water vapor content of alveolar air as 50 mg/litre, the maximum permissible concentration of H3 in alveolar air (and therefore in inhaled air) is 0.23 x 5 x 10-5 uc/cc or 12 x 10-6 uc/cc where the body has been regarded as entirely composed of water. An alternative approach is to consider the 50 kg. of water in the body as turned over at the rate of 2 1/2 kg per day (Standard Man), that is with the mean life of 20 days. Since the maximum permissible amount of tritium in the body was calculated above as 16 mc, the concentration in inhaled air (assuming complete absorption) would be 40 x 10-6 uc/cc Experimentally, the half life of tritium water in the body is about 5 days. Dr. Parker stated that a value of 6 or 7 days had been found. Dr. Morgan said that he calculated the maximum permissible amount of H3 in the body as 6mc. Dr. Brues considered the question of the relative biological efficiency of the soft beta radiation from tritium. It is observed that the lethal dose of tritium water for mice is such as to give a dose of about 300 rep. per day initially, falling with a five day half life. Thus the dose is comparable to that for X or gamma radiation, and the relative biological efficiency is not greater than 2. Maximum permissible amount in the body 10mc. þ concentration in air 40 x 10-6 uc/cc þ þ in drinking water 1 x 10-2 uc/cc Na24 Dr. Hamilton remarked that experimentally 25% of ht gamma radiation of Na24 is absorbed in passing through the body. Dr. Morgan calculated the effective energy (beta and gamma) per disintegration as 2.7 MeV. This lends to a figure of 15uc as the maximum permissible amount in the body. Dr. Hamilton pointed out that the biological half life corresponds to about 5% loss per day; this may be ignored in comparison with the radioactive decay for which the mean life is approximately 1 day. Therefore the maximum permissible concentrations in air and drinking water are approximately 10-6 uc/cc and 5 x 10-3 uc/cc. respectively. these figures were formally agreed. (Secretaryþs note: the immersion tolerance for Na24 in water is approximately 10-4 uc/cc, which is lower than the drinking water figure). - 21 - P32 Dr. Neary stated that the Medical Research Councilþs Panel recommends a figure of 10-4 uc/cc for P32 in drinking water. The calculation is based on the assumption that half of all the absorbed phosphorus is concentrated in the red bone marrow where the equilibrium amount is controlled by radioactive decay only, and is required not to exceed 3 uc. Dr. Hamilton said that the LD.50 is 20mc, while Dr. Shields Warren sated that 250 cu gives no perceptible hematological effect. Drs. Failla and Morgan were in favor of calculating the dose of the equilibrium distribution of phosphorus, i.e., with 98% in the skeleton, which they said would lead to a maximum permissible amount of 5 uc. Dr. Hamilton pointed out that months are required for equilibrium to be reached. Dr. Parker quoted a recommendation from the National Research Councilþs Maximum Permissible Internal Dose Committee that he permissible amount P32 in the skeleton, be limited to 1.2 x 10-3 uc/g, corresponding to 8.4 uc in a 7 kg. skeleton, which is supposed to contain 90% of the body phosphorus. At this point, Dr. Shield Warren suggested that in view of the lateness of the hour, the meeting should break up into separate groups to consider the remaining isotopes, and this was agreed. Dr. Brues, Dr. Hamilton and Mr. Chamberlain reported on P32 as follows: It is known both experimentally and clinically that at times of the order of the mean life of P32 in red bone marrow reaches a value only about three times the average concentration for the whole body, and so most of the P32 is diffused throughout the body. In order to allow for the threefold relative concentration in red bone marrow, the total phosphorus may be regarded as contained in a hypothetical critical tissue of mass 70 kg. This argument would suggest a permissible amount of P32 in the body of approximately 25 uc. It may be that somewhat higher concentrations of P32 in bone marrow may occur and so a figure of 10 uc as maximum permissible amount was proposed. The corresponding figure for air was 5 x 10-8 uc/cc, and for water 2 x 10-4 uc/cc assuming radioactive decay only. These values were formally agreed. S35 Drs. Branton, Hempelmann, Wright Langham, Morgan and Wolf reported on S35 as follows: Since the mean energy of the S35 beta radiation is 0.173 Mev; the concentration in the critical tissue to give 0.3 rep/week is 14 uc/kg. If the critical tissue were bone, then the maximum permissible amount would be approximately 100 uc; if liver, then 17 uc. (Note: 23 uc would appear to be a more precise figure); if skin, then 89 uc. It was therefore assumed that the total permissible body content would be 200 uc. for equal concentrations in the three tissues mentioned. The figures proposed for the permissible concentration in air and water were 2 x 10-6 uc/cc. and from 10-5 to 10-2 uc/cc respectively. (Note: If 100% uptake, and radioactive decay only are assumed, the figures would be 10-7 uc/cc and 6 x 10-4 uc/cc.) A41 and Xe133 It was agreed that the hazard from these isotopes was from external irradiation so that the figures for the maximum permissible concentrations in air are 10-6 uc/cc for A41 and 10-5 uc/cc for Xe133. (Note: the concentration of Xe133 in fat would need to be about 104 times that in water before internal irradiation became a comparable hazard. - 22 - I131 Dr. Edson, Dr. Failla, Prof. Mitchell and Dr. Parker report on I131. The effective total energy absorbed in the thyroid gland is estimated to be 0.22 MeV per disintegration, so that a dose of 0.3 rep/week is produced by 3.5 x 10-3 uc per gramme of tissue; the maximum permissible amount of I131 in the thyroid is therefor 0.06 uc. It was stated that the biological half life of iodine in the thyroid is very much longer than in the rest of the body and it was estimated that the total amount of iodine in the body corresponding to 0.07 uc in the thyroid alone is 0.1 uc. The absorption of iodine into the body is assumed to be 100%, and 20% of the absorbed amount is assumed to be deposited in the thyroid gland. On this basis the maximum permissible concentration of iodine in air is 10-9 uc/cc and in water is 10-5 uc/cc. These figures were formally agreed. It was noted that a calculation based on isotopic dilution of the I131 with the normal daily intake of iodine would give unduly low permissible concentration because the radioactive decay is in fact much more rapid than the rate of biological turnover in the gland. The isotopic ratio of I131 for the stationary amount of iodine in the gland is therefore lower than for the daily intake of iodine. Co60 Drs. Carmichael, Laurence and Hoary reported on Co60. It is assumed that the cobalt which is absorbed is all deposited in the liver, following the indications of some animal experiments. The effective energy absorbed in the liver is assumed to be 1.3 MeV per disintegration, so tha the amount to give 0.3 rep/week is 1 uc. The half life in the liver is taken to be 20 days. The maximum absorption of 50% for a soluble aerosol is assumed, leading to a maximum permissible concentration in air of 2 x 10-9 uc/cc. The absorption from drinking water is assumed to be 100% so that the maximum permissible concentration in water is 1 x 10-5 uc/cc. These figures were formally agreed. U (natural) The maximum permissible concentration for uranium compounds in air is taken to be 50 ug/m3 on the basis of the chemical toxicity of the soluble compounds. This value was taken to be equivalent to 3.3 x 10-11 uc/cc, which was formally agreed. (Note: the latter figure is in error; it should be 8.7 x 10-12 uc/cc.) Ra226 The maximum permissible amount fixed in the body was accepted above on clinical evidence to be 0.1 ug. Dr. Hamilton suggested that 25% of the absorbed material was finally retained with a half-life of the order of 104 days (27 years). Allowing 50% absorption for a soluble aerosol, the maximum permissible concentration in air for soluble compounds is 4 x 10-12 uc/cc. Mr. Chamberlain proposed that inhalation the permissible levels should be adjusted proportionally in circumstances where less than the full 24 hours exposure occurred, and this was agreed. For oral ingestion, Dr. Hamilton suggested a final overall retention of 10%, with a mean life greater than 10 years. A value of 104 days would lead to a maximum permissible level in drinking water of 4 x 10-8 uc/cc. It was formally agreed to adopt these values. - 23 - Pu239 The relative permissible values for plutonium and radium fixed in the body (0.1 microgram of each) were agreed upon at Chalk River on the basis of evidence presented by Dr. Brues that the toxicity ratio between equal microcurie amounts of plutonium and radium is approximately 15 to 1. Further facts now presented by Dr. Brues lead to a greater permissible amount of Pu as compared to Ra, as follows: 1. The toxicity ratio of 15:1 was based on injected dose. But 75% of the plutonium is retained in the body of the rat, and 25% of the radium. This consideration leads to a factor of 3, making the toxicity ratio actually 5:1. 2. Further, about 50% of the radon is retained in the human, but only 15-20% in the rodent, leading to another factor of about 2 in favour of plutonium. That is, the alpha energy delivered from the radium chain to the human is 4.8 + 0.5 (5.5 + 6.0 + 7.7) = 14.4 MeV and to the rodent is 4.8 + 0.15(5.5 + 6.0 + 7.7) - 7.7 MeV Hence, the "estimated fixed minimal damaging dose" of Pu for the human, based on 1 microgram of Ra is 1 mg Ra x 1 mg Pu x 24000 x 3 x 2 = 6 mg Pu ----- 1600 The value of 0.5 mg Pu for the maximum permissible dose, as compared with 0.1 mg Ra, is therefore a conservative. On the basis of 0.5 mg. Pu permissible dose, 10 4 days mean life, 10% retention and 20 cubic metres of air breathed per day, permissible air concentration becomes: 0.5 mg Pu = cone x 10 4 days x 10 6 cc/day x 10% cone = 2.5 x 10-11 mg Pu/cc. = 1.5 x 10 -12 mg Pu/cc. On the basis of 2.5 litres of drinking water per day for 10 4 days, and assuming 0.1% absorption at low concentrations, the permissible concentration of Pu in drinking water is 0.5 mg Pu = cone x 10 4 days x 2.5 x 10 3 cc/day x .001 cone = 2 x 10 -5 mg Pu/cc = 1.2 x 10 -6 mg/cc SR90 (+ Y 90) Dr. Hamilton stated that he calculated the maximum permissible levels for Sr 90(+ Y 90) to be 2 x 10 - 10 mc/cc in air and 4 x 10 -6 mc/cc in drinking water. (Note: these figures would appear to correspond to a final retention of approximately 1%). U 233 Dr. Hamilton also mentioned figures for U 233. For soluble compounds in air, he calculated a maximum permissible level of 6 x 10 -9 mg/cc, or 6 x 10 -11 mc/cc while for insoluble compounds, the figure is 2.5 x 10 -11 mg/cc or 2.5 x 10 -13 mc/cc. For ingestion, a figure 2 x 10 -3 mg/cc or 2 x 10 -5 mc/cc was obtained. - 24 - These recommendations are summarised: Maximum permissible levels for Plant and Other Workers under Medical Supervision Element Maximum Permiss- Maximum Permissible Maxium Permissible ible Amount in Concentration in Concentration in Body air (24 hour day) Drinking Water _______________________________________________________________________ H 3 10 mc 40 x 10 -6 mc/cc 1 x 10 -2 mc/cc C 14 (as CO2) 300 mc. 1 x 10 -6 " Na 24 15 mc. 1 x 10 -5 " 5 x 10 -3 " P 32 10 mc. 5 x 10 -8 " 2 x 10 -4 " S 35 200 mc. 2 x 10 -6 " 1 x 10 -2 " I 131 (0.07 mc. 2 x 10 -9 " 1 x 10-5 " (thyroid) (0.1 mc. (body) Co 60 1 mc. 2 x 10 -9 " 1 x 10-5 " A 41 1 x 10 -6 " Xe 133 1 x 10 -5 " Xe 135 3 x 10 -6 " U (Natural) 8.6 x 10 -12 " (25 mgm/m 3) Ra 266 0.1 mg. 4 x 10 - 12 mgm/cc for soluble compounds 4 x 10 -8 mc/cc Pu 239 0.5 mg. 1.5 x 10 -12 mc/cc 1.2 x 10 -6 " 2.5 x 10 -11 mg/cc (2 x 10 -5 mg/cc) for soluble & insoluble compounds. U 233 0.6 mg. 6 x 10 - 11 mc/cc. 2 x 10 -5 mc/cc (6 x 10 -9 mg/cc) (2 x 10 -5 mg/cc) for soluble compounds 2.5 x 10 -13 mc/cc. (2.5 x 10 -11 mg/cc) for insoluble compounds Sr 90 (+Y 90) 1.0 mc. 2 x 10 -10 mc/cc 4 x 10 -6 mc/cc Sr 89 2.0 mc. Po 210 0.005 mc. Th 234 (UX 1) 0.8 mc. ______________________________________________________________________ Dr. Sheilds Warren reminded members of the Conference that there would be an opportunity for further comments on the foregoing figures when the notes on the Conference were circulated according to the procedure already agreed. The business of the Conference then terminated.