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The Effects on Populations of Exposure to Low Levels of Ionizing Radiation DIVISIOX OF h1EDICAL SCIENCES NAT IO XAL ACADEhIY OF SCIENCES NATIONAL RESEARCH COUNCIL
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In anticipation of t h e widespread increased use of nuclear energy, it is time to think anew about radiation protection. K e need standards for t h e major categories of radiation exposure, based insofar a s possible on risk estimates and on cost-benefit analyses which compare t h e activity involving radiation with t h e alternative options. Such analyses, crude though the). must be a t this time. are needed t o provide a better public understanding of the issues and a sound basis f o r decision. These analyses should seek t o clarify such matters as: (a) the environmental and biological risks of given developments, (b) a comparison of these risks with t h e benefits t o be gained, (c) the feasibility and worth of reducing these environmental and biological risks, (d) the net benefit t o society of a gjven development as compared tothe altertors t o radiation exposure of the population xi11 continue t o be natural background ui than average whole-body dose of about 100 mrem! year, and medical applications which now contribute comparable expo5iires t o varioiis tissues of t h e body. M;.dica! exposures G:’? n.ot under control or guidance b:i regulation or law at present. The use of ionizing radiation in medicine is of tremendous \*slue but it is essential to reduce exposures since this can be actornpljshed without loss of benefit and a t relatively low cost. The aim is not only t o reduce the radiation exposure tothe indiyidual but also t o have procedures carried out with maximum eficiency so that there can be a continuing increase i n medical benefits accompanied by a niinjmuni radiation exposure. arises because of i t s poter:tial magnitude and widespread distribution. Based on experience to date and present engineering judgment, t h e contribution t o radiation exposure averaged over theU. S. population from the developing nuclear power industry can remain les: than about 1 nlrem per year (about 1% of natural background) and t h e exposure of any individual kept t o a small fraction of background provided t h a t there is: (a) attainment and lonpterm maintenance of anticipated engineering oactive wastes, (c) control of sabotage and diversion of fissionable material, (d, a\Toidance of The present Radiation Protection Guide €or t h e general population was based on genetic considerations and conforms tothe BE.4PI Committee recommendations that the average individual exposure be less than 10 R (Roentgens) before the mean age of reproduction (30 years). The FRC did not include medical radiation in its liniits and set 5 rem a s t h e 30-year limit (0.17 rem per year). Present estimates of genetic risk a r e expressed in four ways: (a) Risk Rel a tiw to Natural Background Radiation. Ex-posure t o manmade radiation below t h e level of background radiation will produce additional effects that a r e less in quantity and no different in kind from those which man has experienced and has been able t o tolerate throughout his history. (h) Risk Estimates for Specific Genetic Conditions. The expected effect of radiation can be compared with current incidence of genetic effects by use of t h e concept of doubling dose (the dose required t o produce a number of mutations equal t o those which occur naturally). Based mainly on experimental studies in t h e mouse and Drosophila and with some support from observations of human populations i n Hiroshima and Kagasaki, t h e doubling dose for chronic radiation in man is estimated t o fall in the range of 20-200 rem. It is calculated that t h e effect of 170 mrem per year (or 5 rem per 30-year reproduction generation) would cause in t h e first generation between 100 and 1800 cases of serious, dominant or );-linked diseases and defects per year (assuming 3.6 million births annually in the U.S.). This isan incit e r several generations) these numbers would
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be about fivefold larger. Added t o these would be a smaller number caused by chromosomal defects and recessive diseases. (c) Risk ReIati\-e to Current Prevalence of Serious D s a biljties. In addition t o those in (b) caused by single-gene defects and chromosome aberrations a r e congenital abnormalities and constitutional diseases which a r e partly genetic. It is estimated that the totalincidence from all these including those in (b) above, would be between 1100 and 27,000 per year a t equilibrium (again, based on 3.6 million births). This would be about 0 . 7 5 5 a t equilibrium, or 0.15 in the first generation. (d) The Risk in Terms of Overall Ill-Health. The most tangible measure of total genetic damage is probably “ill-health” which includes but is not limited tothe above categories. It is thought t h a t between 5% and 50‘3 of ill-health is proportional tothe mutation rate. Using a value of 20R and a doubling dose of 20 rem, we can calculate that5 rem per generation would eventually lead toan increase of 5% in the illhealth of t h e population. Using estimates of t h e financial costs of ill-health. such effects can be measured in dollars if this is needed for costUntil recently, ithas been taken for granted that genetic risks from exposure of populations t o ionizing radiation near background levels were of much greater import than were somatic risks. However, this assumption can no longer be made if linear non-threshold relationships are accepted asa basis for estimating cancer risks. Based on knowledge of mechanisms (admittedly incomplete) it muct be stated that tumor inductior: as a result cf radiation injury t o one or a few cells of the body cannot be excluded. Risk estimates have been made based on this premise and using linear extrapolation from the data from the A-bomb survivors of Hiroshima and Nagasaki, from certain groups of patients irr a di a t ed therapeutic a11y , and from groups occupationally exposed. Such calculations based on these data from irradiated humans lead tothe prediction that additional exposure cd t h e U. s. population of 5 rem per 30 years cou!d cauce from rough!:; 2,000 t o the assumptions used in the calculations. The Committee considers t h e most likely estimate t o be approximately 6,000 cancer deaths a n - nually, a n increase of about 2(7( in the spontaneous cancer death rate which is a n increase of about 0.35 in the overall death rate fron: 2: Given the estimctes for genetic and somz:; risk, the question arises as t o how this in?(,: matior; can be used a s a basi. for radia:ic.: protection guidance. Logically t h e guidance o: standards should be related tcr risk. R’he:i-lr: we regard a risk a s acceptable or not depenfon h o u avoidable it is. and, t o the ex<en! n.,: avoidable. how it compares with t h e risks (,! alternative options and those normally accept - ed by society. There is reason t o expect that over the nes; few decades, t h e dose commitments for all mal;- made sources of radiation except medic,: should not exceed more thana few rniIlire~~~.- average annual dose tothe entire U.S. populhtion. The present guides of 170 mrem/yr grev: out of a n effort t o balance societal need: against genetic risks. I t appears that the:$ needs can be met ui th far lower average exposures and lower genetic and somatic risk th2.n permitted by t h e current Radiation Protectio:.. Guide. To this extent, the current Guide is U I - i - necessa ri 1y high. The exposures from medical and dental uses should be subject tothe same rationale. To the extent that such exposures can be reduced without impairing benefits, they a r e also unneces sari 1y high. It is not within the scope of this Committee t o propose numerical limits of radiation exposure. It is apparent that sound decisions require technic aI. economic and so cio1ogica1 con c i der a. tions of a complex nature. However, we cai? state some general principles, many of uhicf-, a r e well-recognized and in use, and some of which may represent a departure from present a)No exposure t o ionizing radiation should be permitted without the expectation of a b) The public must be protected from radiation but not tothe extent that the degree of protection provided results in the substitution of a worse hazard for the radiation avoided. Additionally there should not be attempted t h e reduction of small risks even further atthe cost of large sums of money that spent otherwise. would clearly producegreater benefit.
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c) There should be a n upper limit of manals in the general population such that the risk of serious injury from somatic effects in such individuals is very small relative to risks that are normally accepted. Exceptions t o this. limit in specific case5 ~ h o u l dbe allnwable only if it can be demonstrated that meeting it would cause individuals t o he exposed to other risks greater than those from the radiation av oi ded. d ) There should be a n upper limit of manmade non-medical exposure for t h e general population. The average exposure permitted for t h e population should be considerably lower than the upper limit permitted for individuals. e) Medical radiation exposure c a n and should be reduced considerably by limiting its use t o clinically indicated procedures utilizinR efficient exposure techniques and optimal operation of radiation equipment. Consideration should be given to t h e fol1) Restriction of t h e use of radiation for public health survey purposes, unless there is a reasonable probability of significant detection of disease. 2) Inspection and licensing of radiation 3) Appropriate training and certiftcation of involved personnel. Gonad shjelding (especially shielding t h e testis) is strongly recommended a s a simple and highly efficient way t o reduce theGe net ica 11 y Significant Dose. f ) Guidance for the nuclear power industrj. should be established on t h e basis of costbenefit analysis, particularly taking into account t h e total biological and environable and thecost-effectiveness of reducing these risks. The quantifying of the “as l o w a5pra ct icab 1e” concept and cons ider ati on of t h e net effect on the welfare of society should be encouraged. g ) In addition t o normal operating conditions in t h e nuclear power industry, careful consideration should be given tothe probabilities and estimated effects of uncontrolled releases. It has been estimated that a catastrophic accident leading t o melting of t h e core of a large nuclear reactor could result i n mortalit>-comparable to that of a severe natural disaster. Hence ex-traordinary efforts t o minimize this risk are clearly called for. h) Occupational and emergency exposure limits have not been specifically considered but should be based on those sections of the report relating t o somatic risk tothe individual. i) In regard t o possible effects of radiation on the enyironment. i t is felt that if the guidelines and standards are accepted a s adequate for man then it is highly unlikely that populations of other living organisms would be perceptibly harmed. Nevertheless, ecological studies should be improved and strengthened and programs put in force t o answer the following questions about release of radioactivity tothe environment: (1) how much, where, and what type of radioactivity is released; (2) how a r e these materials moved through the environment; (3) where a r e they concentrated in natural systems; (4)how long might it take for them t o move through these systems t o a position of contact with man; ( 5 ) what is their effect on t h e environment itself; (6) how c a n this information be used asan early warning system t o prevent potential problems franm j) Every effort should be made t o assure accurate estimates and predictions of radiation equivalent dosages from all existing and planned sources. This requires use of present knowledge on transport in the environment, on meta boli m, end on relative biological efficiencies of radiation as well as further research on many aspects.