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Recent estimates of cancer risk from low-let ionizing radiation and radiation protection limits
Adv. Space Res. Vol. 12, No. 2—3, pp. (2)375—(2)378, 1992 Printed in Great Britain. All rights reserved.
0273—1177/92 $15.00 Copyright e 1991 COSPAR
RECENT ESTIMATES OF CANCER RISK FROM LOW-LET IONIZING RADIATION AND RADIATION PROTECTION LIMITS Warren K. Sinclair National Council on Radiation Protection and Measurements, 7910 Woodmont Avenue, Suite 800, Bethesda, MD 20814, U.S.A.
ABSTRACT
Estimates of the risk of cancer induction, formerly about 1Z/Sv, formed the basis of ICRP radiation protection limits in 1977. They have now increased to about 4—5%/Sv for low doses. These increases are based mainly on new data for the Japanese survivors of the A— bombs of 1945. They result from the accumulation of 11 years more of data on solid tumors, the revisions in the dosimetry of those exposed and improvement in statistical methods and projections. The application of a dose rate effectiveness factor between effects at high dose rate and those at low dose and dose rate is also an important consideration. Not only has the total risk changed but also the distribution of risk among organs. the effective dose equivalent may require modification. These changes are modifying ICRP and NCRP thinking limits, especially for radiation workers.
about
recommendations
on
Thus
protection
INTRODUCTION In 1977, when radiation protection recommendations were last made by the International Commissior~on R~diological Protection (ICR.P) risk estimates for cancer were believed to be about 10 Sv /1/. Furthermore this value was broadly consistent with estimates from three other national and international bodies, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) of 1977, the Committee on the Biological Effects of Ionizing Radiations (BEIR) of 1980 and a committee of the National Council on Radiation Protection and Measurements (NCRP) in 1983, see /2/. I have drawn attention, in previous reports to t~0SPARin 1984 and 1986 /2,3/, to the fact that estimates of radiation induced cancer risk were on the increase based on new information resulting from the ongoing aurvey of the Japanese population exposed to the atomic bombs at Hiroshima and Nagasaki in 1945. However it was not possible until recently to determine fully the changes in the estimates of cancer risk in the Japanese studies. This paper will address these changes and the impact they are having on radiation protection recommendations. NEW EPIDEMIOLOGICAL INFORMATION New epidemiological information since 1977 includes a further 11 years of accumulated data and evaluation in the Japanese studies (1974 to 1985); an update in clinical studies such as those involving patients treated with x rays for ankylosing spondylitis in the U.K.; information on the international cervix series which has only recently become available and updates in additional clinical studies such as on the breast and thyroid. Among this new material, the developments in the study of the Japanese exposed to the A bombs in 1945 have assumed much the greatest import~ance for quantitative risk estimates of cancer induced by radiation although the studies on ankylosing spondylitis patients and on cervix patients broadly confirm the estimates from the Japanese studies /4/. The Japanese studies include three new cycles of data, 1975—78, 1979—8% and 1983—85; an increase in the excess solid tumors available for analysis of about a factor of 2; additional information on the age dependence of the cancer risk; additional information on the time course for the expression of the cancer risk and finally revisions in the dosimetry of those exposed. REVISIONS IN THE DOSIMETRY OF THE SURVIVORS OF THE JAPANESE A BOMBS Until recently the estimates of dose to the individuals exposed at Hiroshima and Nagasaki were based on a dosimetry system known as the T65D for “tentative 1965 dose” /5,6/. Revisions in the output spectrum of neutron and gamma radiation from both the Hiroshima and Nagasaki weapons were made in 1977, revisions which were extensive for the Hiroshima weapon. This together with revised calculations by means of new sophisticated codes used to JASR 12:2/3-Y (2)375
(2)376
W. K. Sinclair
describe the transport of neutrons and gamma rays through moist air over ground yielded new dosimetry results which were considerably different from those of the T65D, especially for Hiroshima, during 1980—81 /7,8,9,10/. Subsequently a broad scale investigation of all aspects of the dose to survivors, including new calculations of neutron and gamma ray transport, tests of measurement vs. calculation in a subcritical assembly using a Hiroshima type bomb case, and actual measurements of gamma ray doses in ceramic materials such as bricks and tiles using thermoluminescent techniques, plus the re—evaluation of the effect of shielding of both houses and tissues, resulted in a new dosimetry system, the DS86 /11/. This system was applied to the survivor information available at the Radiation Effects Research Foundation (RERF) in 1986 and subsequently and has enabled new estimates of organ dose to be determined for the majority of those survivors in the Life Span Study. These new estimates of dose were invariably less than those of the T65D system, the extent of the difference depending mainly on the depth of the organ in the body. At most, the dose was about a factor of 2 less and at least a factor of 1 (i.e., no change). Overall, therefore, risk estimates were increased by the dosimetry change by up to 2 times depending on the organ considered, i.e., on the average, about 1.5 times for all organs /12,13/. ESTIMATES OF CANCER RISK The changes in the dosimetry, plus the increased follow—up and change in projection methods (to a multiplicative projection model) were taken into account in the new evaluations of cancer risk made by Preston & Pierce at the RERF during 1987—88 /13/. They concluded that for the high dose, high dos~ ratf exposure resulting from the A bombs, that the risk of leukemia alone was 1 about Sv ofand ad~litio~ial to all solid tumors for 10 a total 12 the x 1O~ Sv~ forrisk all due cancer for the a population of was all about ii. x io2 Sv ages. The cancer risk was about 7—8 x 1O~ Sv~ for an adult (working) population. They noted that others had divided these results by about 2.5 to convert to expected 5espo?ses at low dose and low dose rate. If tl9.s werr done the result would be about 5 x 10 Sv for a population of all ages and 3 x 1O~ Sv — for an adult population /13/. In 1988, the UNSCEAR reviewed all the epidemiological information available in the world to that time and concluded that while many of the clinical and other studies resulting in risk estimates were broadly supportive of the results from the studies of the Japanese survivors, the data base from the latter was so much more complete and the range of doses so broad that only that source would be used in deriving quantitative risk estimates /4/. They examined the raw data again via the work of Shimizu et al /14,15/ and used two projection models, the additive and the multiplicative to obtain lifetime risk estimates. Since the multiplicative model is preferred tody on]1y results for that model will be cited. It yield~da yisk for all cancer of 11 x 11 Sv in a population of all ages (Japan) and 8 x 1O~ Sv for an adult or working population (Japan) for high dose, high dose—rate exposure /4/. These results are very similar to those of Preston & Pierce. The UNSCEAR recommended that to convert the risk from high dose, high dose—rate exposure to low dose, low dose—rate exposure a factor of between 2 and 10 should be applied, as NCRP had indicated earlier /16/. Subsequently, in a third evaluation of the Japanese data up to 1985, the BEIR V committee in the USA considered all the epidemiological information anew /17/. They again concluded that the Japanese data was superior to all other sources. They reexamined the entire Japanese data set, applying a modified multiplicative model for projection allowing for some decrement in the response with time which was adjusted for each of the organs considered. They obtained a result 2for the U.S. population after transfer from the Japanese which corresponded to 9 x 1O Sv~ for a U.S. population of all ages for high dose, high dose rate exposure /17/. For low dose, low dose—rate exposure the BEIR V committee stated that this should be divided by “2 or more”. THE DOSE RATE EFFECTIVENESS FACTOR The ratio between the effects of radiation following high dose, high dose—rate exposure vs. low dose, low dose—rate exposure was designated by NCRP as the dose—rate effectiveness factor /16/. NCRP pointed out that this factor was a function of dose and examined all the laboratory animal and cellular information that bore on this question. They concluded the ratio was between 2 and 10 depending on the dose and on the endpoint under consideration /16/. Human data then and since are very limited but tend to be at the low end of this range. Some studies on breast and thyroid show no effect of fractionation while in others a ratio of about 3 or perhaps 4 is indicated. Importantly, the data on solid tumors at Hiroshima and Nagasaki seem to fit best a linear dose response although the statistics of the data are such thst other responses with s DREF up to about 2 are possible. In actual application various committees since 1977 have used values of about 2.5 or less for this factor, e.g., /18,19/. Protection bodies such as the ICRP, after considering all the available information, have decided to use 2 for this ratio, recognizing the somewhat arbitrary nature of the choice given the limited information available /20/.
Cancer Risk Estimates
(2)377
ESTIMATES OF CANCER RISK FOR RADIATION PROTECTION PURPOSES Applying a DREF of 2 to an average of UNSCEAR multiplicative projection for the Japanese population and the BEIR V result for the U.S. population yields the following estimates of risk for low dose, low dose rate exposure. 1_~or a population of all ages 5 4x x io—2 10 ~vSv for an adult population
(1)
RADIATION PROTECTION LIMITS ICR.? has recently made available a draft report in which they propose, in view of the increased risk estimates, to lower the effective annual limit for occupational exposure to no more than 100 mSv in 5y with no more than 50 mSv in any one year. This is reduced from the present simple limit of 50 mSv in a year /20/. The public limit was lowered to expression is a values contained
of 1 mSv/y expressed as no more than an average of 1 mSv/y in any 5 years, this value in 1985 and will remain the same except that the form of little different. The ICRP has now confirmed (June, 1990) that the limit in the draft report will be maintained.
The NCRP is in a somewhat different position, having provided general recommendations in 1987 /21/. They concluded at that time, on the basis of suspected but not fully quantitated higher risk estimates, that in addition to the existing annual limit for occupational exposure of 50 mSv/y, a cumulative guidance should be followed which limited the cumulative exposure of a worker to no more than 10 mSv x his or her age in years. This guidance is already being widely followed in, for example, the nuclear industry in the USA. Again the NCRP has maintained their limit for exposure of the public from man—made sources other than medical to 1 mSv/y which they first introduced in 1984 /22/ and reaffirmed in 1987 /21/. NCRP will be considering in the near future whether or not the changes made in 1987 are fully adequate in the light of the new quantitative changes in risk estimates. EFFECTIVE DOSE The effective dose equivalent was defined by ICR? in 1977 and apportioned the effect of whole body exposure among the various organs of the body thus enabling partial body or single organ exposure to be equated with the same effect as a lower exposure to the whole body. The system depended on weights assigned to the more important contributory organs. New estimates of the relative contribution of individual organs are being made by both ICRP and NCRP and are likely to result in a somewhat different mix of organ weights to compose the quantity now to be called the effective dose by ICRP. Final values for these weighting factors should be available when the ICRP completes their recommendations and a report by NCRP on risk for radiation protection is finalized. Evidently, higher risk estimates for cancer realized within the last few years, have already caused a lowering of recommended limits of exposure for workers. It seems quite likely that in time this will be followed by legislation in countries round the world following the lead of the professional radiation protection bodies. REFERENCES 1.
ICRP. Recommendations of the International Commission on Radiological Protection, Publication 26, Annals of the ICRP 1, #3. Pergamon Press, New York, (1977).
2.
W.K. Sinclair. Radiation Risk Estimates and Space. Mv. Space Res., 4, 115—120, (1984).
3.
W.K. Sinclair. (1986).
4.
UNSCEAR. Sources, Effects and Risks of Ionizing Radiation, 1988 Report, United Nations Publication No. E.88.IX.7. United Nations, New York, New York, (1988).
5.
J.A. Auxier, J.S. ~heka, F.F. Haywood, T.D. Jones and J.H. Thorngate, Free—Field Radiation—Dose Distributions for the Hiroshima and Nagasaki Bombings. Health Phys 12, 425—429, (1966).
Its Application to Human Beings
Radiation Protection Standards in Space.
in
Adv. Space Res. 6, 335—343,
(2)378
W. K. Sinclair
6.
J.A. Auxier. ICHIBAN: Radiation Dosimetry for the Survivors of the Bombings at Hiroshima and Nagasaki. National Technical Information Service, Springfield, Virginia, (1977).
7.
W.K. Sinclair and P. Failla.
Dosimetry of the Atomic Bomb Survivors.
Radiat. Res.
88, 437—447, (1981). 8.
W.E. Loewe and E. Mendelsohn. Health Phys. 41, 663—666, (1981).
Revised Dose Estimates
at Hiroshima
and Nagasaki.
9.
G.D. Kerr. Review of Dosimetry of the Atomic Bomb Survivors, in: Fourth Symposium on Neutron Dosimetry, Vol. I, Luxembourg, Commission of the European Communities, 1981. Report EUR 7448 EN, 501—513.
10.
V.P. Bond and J.W, Thjessen, Eds. Reevaluations of Dosimetric Factors: Nagasaki. Washington, Department of Energy, Report 1~ONF—81O928 (1982).
11.
W.C. Roesch.
12.
W.K. Sinclair and D.L. Preston. Revisions in the Dosimetry of the A—Bomb Survivors at Hiroshima and Nagasaki and the Consequences. In: Radiation Research. Proceedings of the 8th International Congress of Radiation Research, eds. E.M. Fielden, J.F. Fouler, J.H. Hendry and 0. Scott, Edinburgh, July 1987. Taylor & Francis, London (1987).
13.
D.L. Preston and D.A. Pierce. The Effect of Changes in Dosimetry on cancer Mortality Risk Estimates in the Atomic Bomb Survivors. Radiat. Rae. 114, 437—466, (1988).
14.
Y. Shimizu, H. Kato and W.J. Schull. Life Span Study Report 11. Part 2. Cancer Mortality in the Years 1950—85 Based on the Recently Revised Doses (DS86). Technical Report RERF TR—5—88. Radiation Effects Research Fou~dation, Hiroshima, Japan, (1988).
15.
Y. Shimizu, H. Kato and W.J. Schull. Studies of the Mortality of A—Bomb Survivors 9. Mortality, 1950—85: Part 2. Cancer Mortality Based on the Recently Revised Doses (DS86). Radiat. Res. 121, 120—141, (1990).
16.
NCRP. Influence of Dose and its Distribution in Time on Dose—Response Relationships for Low—LET Radiation. 64, NCRP, Bethesda, Maryland, (1980).
17.
NAS—NRC. Health Effects of Exposure to Low Levels of Ionizing Radiation. National Academy Press, Washington, D.C., (1990).
18.
UNSCEAR. Sources and Effects of Ionizing Radiation, 1977 Report, United Nations Publication No. E.77.IX.1. United Nations, New York, New York, (1977).
19.
NIH. Report of the National Institutes of Health Ad Hoc Working Group to Develop Radioepidemiological Tables. Publication 85—2748. U.S. Government Printing Office, Washington, D.C., (1985).
20.
ICR?.
21.
NCR?. Recommendations on Limits for Exposure to Ionizing Radiation. Bethesda, Maryland, (1987).
22.
NCR?. Exposures from the Uranium Series with Emphasis on Radon and Its Daughters. NCR?, Bethesda, Maryland, (1984).
Hiroshima and
U.S.—Japan Joint Reassessment of Atomic Bomb Radiation dosimetry in Hiroshima and Nagasaki. Final Report. Radiation Effects Research Foundation, Hiroshima, Japan, Vol. I & II, (1987).
Recommendations of the Commission, 1990.
Draft G/O1.
ICR?,
BEIR V,
(1990). 91.
NCR?,
77,
0273—1177/92 $15.00 Copyright e 1991 COSPAR
RECENT ESTIMATES OF CANCER RISK FROM LOW-LET IONIZING RADIATION AND RADIATION PROTECTION LIMITS Warren K. Sinclair National Council on Radiation Protection and Measurements, 7910 Woodmont Avenue, Suite 800, Bethesda, MD 20814, U.S.A.
ABSTRACT
Estimates of the risk of cancer induction, formerly about 1Z/Sv, formed the basis of ICRP radiation protection limits in 1977. They have now increased to about 4—5%/Sv for low doses. These increases are based mainly on new data for the Japanese survivors of the A— bombs of 1945. They result from the accumulation of 11 years more of data on solid tumors, the revisions in the dosimetry of those exposed and improvement in statistical methods and projections. The application of a dose rate effectiveness factor between effects at high dose rate and those at low dose and dose rate is also an important consideration. Not only has the total risk changed but also the distribution of risk among organs. the effective dose equivalent may require modification. These changes are modifying ICRP and NCRP thinking limits, especially for radiation workers.
about
recommendations
on
Thus
protection
INTRODUCTION In 1977, when radiation protection recommendations were last made by the International Commissior~on R~diological Protection (ICR.P) risk estimates for cancer were believed to be about 10 Sv /1/. Furthermore this value was broadly consistent with estimates from three other national and international bodies, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) of 1977, the Committee on the Biological Effects of Ionizing Radiations (BEIR) of 1980 and a committee of the National Council on Radiation Protection and Measurements (NCRP) in 1983, see /2/. I have drawn attention, in previous reports to t~0SPARin 1984 and 1986 /2,3/, to the fact that estimates of radiation induced cancer risk were on the increase based on new information resulting from the ongoing aurvey of the Japanese population exposed to the atomic bombs at Hiroshima and Nagasaki in 1945. However it was not possible until recently to determine fully the changes in the estimates of cancer risk in the Japanese studies. This paper will address these changes and the impact they are having on radiation protection recommendations. NEW EPIDEMIOLOGICAL INFORMATION New epidemiological information since 1977 includes a further 11 years of accumulated data and evaluation in the Japanese studies (1974 to 1985); an update in clinical studies such as those involving patients treated with x rays for ankylosing spondylitis in the U.K.; information on the international cervix series which has only recently become available and updates in additional clinical studies such as on the breast and thyroid. Among this new material, the developments in the study of the Japanese exposed to the A bombs in 1945 have assumed much the greatest import~ance for quantitative risk estimates of cancer induced by radiation although the studies on ankylosing spondylitis patients and on cervix patients broadly confirm the estimates from the Japanese studies /4/. The Japanese studies include three new cycles of data, 1975—78, 1979—8% and 1983—85; an increase in the excess solid tumors available for analysis of about a factor of 2; additional information on the age dependence of the cancer risk; additional information on the time course for the expression of the cancer risk and finally revisions in the dosimetry of those exposed. REVISIONS IN THE DOSIMETRY OF THE SURVIVORS OF THE JAPANESE A BOMBS Until recently the estimates of dose to the individuals exposed at Hiroshima and Nagasaki were based on a dosimetry system known as the T65D for “tentative 1965 dose” /5,6/. Revisions in the output spectrum of neutron and gamma radiation from both the Hiroshima and Nagasaki weapons were made in 1977, revisions which were extensive for the Hiroshima weapon. This together with revised calculations by means of new sophisticated codes used to JASR 12:2/3-Y (2)375
(2)376
W. K. Sinclair
describe the transport of neutrons and gamma rays through moist air over ground yielded new dosimetry results which were considerably different from those of the T65D, especially for Hiroshima, during 1980—81 /7,8,9,10/. Subsequently a broad scale investigation of all aspects of the dose to survivors, including new calculations of neutron and gamma ray transport, tests of measurement vs. calculation in a subcritical assembly using a Hiroshima type bomb case, and actual measurements of gamma ray doses in ceramic materials such as bricks and tiles using thermoluminescent techniques, plus the re—evaluation of the effect of shielding of both houses and tissues, resulted in a new dosimetry system, the DS86 /11/. This system was applied to the survivor information available at the Radiation Effects Research Foundation (RERF) in 1986 and subsequently and has enabled new estimates of organ dose to be determined for the majority of those survivors in the Life Span Study. These new estimates of dose were invariably less than those of the T65D system, the extent of the difference depending mainly on the depth of the organ in the body. At most, the dose was about a factor of 2 less and at least a factor of 1 (i.e., no change). Overall, therefore, risk estimates were increased by the dosimetry change by up to 2 times depending on the organ considered, i.e., on the average, about 1.5 times for all organs /12,13/. ESTIMATES OF CANCER RISK The changes in the dosimetry, plus the increased follow—up and change in projection methods (to a multiplicative projection model) were taken into account in the new evaluations of cancer risk made by Preston & Pierce at the RERF during 1987—88 /13/. They concluded that for the high dose, high dos~ ratf exposure resulting from the A bombs, that the risk of leukemia alone was 1 about Sv ofand ad~litio~ial to all solid tumors for 10 a total 12 the x 1O~ Sv~ forrisk all due cancer for the a population of was all about ii. x io2 Sv ages. The cancer risk was about 7—8 x 1O~ Sv~ for an adult (working) population. They noted that others had divided these results by about 2.5 to convert to expected 5espo?ses at low dose and low dose rate. If tl9.s werr done the result would be about 5 x 10 Sv for a population of all ages and 3 x 1O~ Sv — for an adult population /13/. In 1988, the UNSCEAR reviewed all the epidemiological information available in the world to that time and concluded that while many of the clinical and other studies resulting in risk estimates were broadly supportive of the results from the studies of the Japanese survivors, the data base from the latter was so much more complete and the range of doses so broad that only that source would be used in deriving quantitative risk estimates /4/. They examined the raw data again via the work of Shimizu et al /14,15/ and used two projection models, the additive and the multiplicative to obtain lifetime risk estimates. Since the multiplicative model is preferred tody on]1y results for that model will be cited. It yield~da yisk for all cancer of 11 x 11 Sv in a population of all ages (Japan) and 8 x 1O~ Sv for an adult or working population (Japan) for high dose, high dose—rate exposure /4/. These results are very similar to those of Preston & Pierce. The UNSCEAR recommended that to convert the risk from high dose, high dose—rate exposure to low dose, low dose—rate exposure a factor of between 2 and 10 should be applied, as NCRP had indicated earlier /16/. Subsequently, in a third evaluation of the Japanese data up to 1985, the BEIR V committee in the USA considered all the epidemiological information anew /17/. They again concluded that the Japanese data was superior to all other sources. They reexamined the entire Japanese data set, applying a modified multiplicative model for projection allowing for some decrement in the response with time which was adjusted for each of the organs considered. They obtained a result 2for the U.S. population after transfer from the Japanese which corresponded to 9 x 1O Sv~ for a U.S. population of all ages for high dose, high dose rate exposure /17/. For low dose, low dose—rate exposure the BEIR V committee stated that this should be divided by “2 or more”. THE DOSE RATE EFFECTIVENESS FACTOR The ratio between the effects of radiation following high dose, high dose—rate exposure vs. low dose, low dose—rate exposure was designated by NCRP as the dose—rate effectiveness factor /16/. NCRP pointed out that this factor was a function of dose and examined all the laboratory animal and cellular information that bore on this question. They concluded the ratio was between 2 and 10 depending on the dose and on the endpoint under consideration /16/. Human data then and since are very limited but tend to be at the low end of this range. Some studies on breast and thyroid show no effect of fractionation while in others a ratio of about 3 or perhaps 4 is indicated. Importantly, the data on solid tumors at Hiroshima and Nagasaki seem to fit best a linear dose response although the statistics of the data are such thst other responses with s DREF up to about 2 are possible. In actual application various committees since 1977 have used values of about 2.5 or less for this factor, e.g., /18,19/. Protection bodies such as the ICRP, after considering all the available information, have decided to use 2 for this ratio, recognizing the somewhat arbitrary nature of the choice given the limited information available /20/.
Cancer Risk Estimates
(2)377
ESTIMATES OF CANCER RISK FOR RADIATION PROTECTION PURPOSES Applying a DREF of 2 to an average of UNSCEAR multiplicative projection for the Japanese population and the BEIR V result for the U.S. population yields the following estimates of risk for low dose, low dose rate exposure. 1_~or a population of all ages 5 4x x io—2 10 ~vSv for an adult population
(1)
RADIATION PROTECTION LIMITS ICR.? has recently made available a draft report in which they propose, in view of the increased risk estimates, to lower the effective annual limit for occupational exposure to no more than 100 mSv in 5y with no more than 50 mSv in any one year. This is reduced from the present simple limit of 50 mSv in a year /20/. The public limit was lowered to expression is a values contained
of 1 mSv/y expressed as no more than an average of 1 mSv/y in any 5 years, this value in 1985 and will remain the same except that the form of little different. The ICRP has now confirmed (June, 1990) that the limit in the draft report will be maintained.
The NCRP is in a somewhat different position, having provided general recommendations in 1987 /21/. They concluded at that time, on the basis of suspected but not fully quantitated higher risk estimates, that in addition to the existing annual limit for occupational exposure of 50 mSv/y, a cumulative guidance should be followed which limited the cumulative exposure of a worker to no more than 10 mSv x his or her age in years. This guidance is already being widely followed in, for example, the nuclear industry in the USA. Again the NCRP has maintained their limit for exposure of the public from man—made sources other than medical to 1 mSv/y which they first introduced in 1984 /22/ and reaffirmed in 1987 /21/. NCRP will be considering in the near future whether or not the changes made in 1987 are fully adequate in the light of the new quantitative changes in risk estimates. EFFECTIVE DOSE The effective dose equivalent was defined by ICR? in 1977 and apportioned the effect of whole body exposure among the various organs of the body thus enabling partial body or single organ exposure to be equated with the same effect as a lower exposure to the whole body. The system depended on weights assigned to the more important contributory organs. New estimates of the relative contribution of individual organs are being made by both ICRP and NCRP and are likely to result in a somewhat different mix of organ weights to compose the quantity now to be called the effective dose by ICRP. Final values for these weighting factors should be available when the ICRP completes their recommendations and a report by NCRP on risk for radiation protection is finalized. Evidently, higher risk estimates for cancer realized within the last few years, have already caused a lowering of recommended limits of exposure for workers. It seems quite likely that in time this will be followed by legislation in countries round the world following the lead of the professional radiation protection bodies. REFERENCES 1.
ICRP. Recommendations of the International Commission on Radiological Protection, Publication 26, Annals of the ICRP 1, #3. Pergamon Press, New York, (1977).
2.
W.K. Sinclair. Radiation Risk Estimates and Space. Mv. Space Res., 4, 115—120, (1984).
3.
W.K. Sinclair. (1986).
4.
UNSCEAR. Sources, Effects and Risks of Ionizing Radiation, 1988 Report, United Nations Publication No. E.88.IX.7. United Nations, New York, New York, (1988).
5.
J.A. Auxier, J.S. ~heka, F.F. Haywood, T.D. Jones and J.H. Thorngate, Free—Field Radiation—Dose Distributions for the Hiroshima and Nagasaki Bombings. Health Phys 12, 425—429, (1966).
Its Application to Human Beings
Radiation Protection Standards in Space.
in
Adv. Space Res. 6, 335—343,
(2)378
W. K. Sinclair
6.
J.A. Auxier. ICHIBAN: Radiation Dosimetry for the Survivors of the Bombings at Hiroshima and Nagasaki. National Technical Information Service, Springfield, Virginia, (1977).
7.
W.K. Sinclair and P. Failla.
Dosimetry of the Atomic Bomb Survivors.
Radiat. Res.
88, 437—447, (1981). 8.
W.E. Loewe and E. Mendelsohn. Health Phys. 41, 663—666, (1981).
Revised Dose Estimates
at Hiroshima
and Nagasaki.
9.
G.D. Kerr. Review of Dosimetry of the Atomic Bomb Survivors, in: Fourth Symposium on Neutron Dosimetry, Vol. I, Luxembourg, Commission of the European Communities, 1981. Report EUR 7448 EN, 501—513.
10.
V.P. Bond and J.W, Thjessen, Eds. Reevaluations of Dosimetric Factors: Nagasaki. Washington, Department of Energy, Report 1~ONF—81O928 (1982).
11.
W.C. Roesch.
12.
W.K. Sinclair and D.L. Preston. Revisions in the Dosimetry of the A—Bomb Survivors at Hiroshima and Nagasaki and the Consequences. In: Radiation Research. Proceedings of the 8th International Congress of Radiation Research, eds. E.M. Fielden, J.F. Fouler, J.H. Hendry and 0. Scott, Edinburgh, July 1987. Taylor & Francis, London (1987).
13.
D.L. Preston and D.A. Pierce. The Effect of Changes in Dosimetry on cancer Mortality Risk Estimates in the Atomic Bomb Survivors. Radiat. Rae. 114, 437—466, (1988).
14.
Y. Shimizu, H. Kato and W.J. Schull. Life Span Study Report 11. Part 2. Cancer Mortality in the Years 1950—85 Based on the Recently Revised Doses (DS86). Technical Report RERF TR—5—88. Radiation Effects Research Fou~dation, Hiroshima, Japan, (1988).
15.
Y. Shimizu, H. Kato and W.J. Schull. Studies of the Mortality of A—Bomb Survivors 9. Mortality, 1950—85: Part 2. Cancer Mortality Based on the Recently Revised Doses (DS86). Radiat. Res. 121, 120—141, (1990).
16.
NCRP. Influence of Dose and its Distribution in Time on Dose—Response Relationships for Low—LET Radiation. 64, NCRP, Bethesda, Maryland, (1980).
17.
NAS—NRC. Health Effects of Exposure to Low Levels of Ionizing Radiation. National Academy Press, Washington, D.C., (1990).
18.
UNSCEAR. Sources and Effects of Ionizing Radiation, 1977 Report, United Nations Publication No. E.77.IX.1. United Nations, New York, New York, (1977).
19.
NIH. Report of the National Institutes of Health Ad Hoc Working Group to Develop Radioepidemiological Tables. Publication 85—2748. U.S. Government Printing Office, Washington, D.C., (1985).
20.
ICR?.
21.
NCR?. Recommendations on Limits for Exposure to Ionizing Radiation. Bethesda, Maryland, (1987).
22.
NCR?. Exposures from the Uranium Series with Emphasis on Radon and Its Daughters. NCR?, Bethesda, Maryland, (1984).
Hiroshima and
U.S.—Japan Joint Reassessment of Atomic Bomb Radiation dosimetry in Hiroshima and Nagasaki. Final Report. Radiation Effects Research Foundation, Hiroshima, Japan, Vol. I & II, (1987).
Recommendations of the Commission, 1990.
Draft G/O1.
ICR?,
BEIR V,
(1990). 91.
NCR?,
77,