Risks of fatal cancer from inhalation of 239,240plutonium by humans: a combined four-method approach with uncertainty evaluation.

The risk per unit dose to the four primary cancer sites for plutonium inhalation exposure (lung, liver, bone, bone marrow) is estimated by combining the risk estimates that are derived from four independent approaches. Each approach represents a fundamentally different source of data from which plutonium risk estimates can be derived. These are: (1) epidemiologic studies of workers exposed to plutonium; (2) epidemiologic studies of persons exposed to low-LET radiation combined with a factor for the relative biological effectiveness (RBE) of plutonium alpha particles appropriate for each cancer site of concern; (3) epidemiologic studies of persons exposed to alpha-emitting radionuclides other than plutonium; and (4) controlled studies of animals exposed to plutonium and other alpha-emitting radionuclides extrapolated to humans. This procedure yielded the following organ-specific estimates of the distribution of mortality risk per unit dose from exposure to plutonium expressed as the median estimate with the 5th to 95th percentiles of the distribution in parentheses: lung 0.13 Gy(-1) (0.022-0.53 Gy(-1)); liver 0.057 Gy(-1) (0.011-0.47 Gy(-1)); bone 0.0013 Gy(-1) (0.000060-0.025 Gy(-1)); bone marrow (leukemia), 0.013 Gy(-1) (0.00061-0.05 Gy(-1)). Because the different tissues do not receive the same dose following an inhalation exposure, the mortality risk per unit intake of activity via inhalation of a 1-microm AMAD plutonium aerosol also was determined. To do this, inhalation dose coefficients based on the most recent ICRP models and accounting for input parameter uncertainties were combined with the risk coefficients described above. The following estimates of the distribution of mortality risk per unit intake were determined for a 1-microm AMAD plutonium aerosol with a geometric standard deviation of 2.5: lung 5.3 x 10(-7) Bq(-1) (0.65-35 x 10(-7) Bq(-1)), liver 1.2 x 10(-7) Bq(-1) (0.091-20 x 10(-7) Bq(-1)), bone 0.11 x 10(-7) Bq(-1) (0.0030-4.3 x 10(-7) Bq(-1)), bone marrow (leukemia) 0.049 x 10(-7) Bq(-1) (0.0017-0.59 x 10(-7) Bq(-1)). The cancer mortality risk for all sites was estimated to be 10 x 10(-7) Bq(-1) (2.1-55 x 10(-7) Bq(-1))--a result that agrees very well with other recent estimates. The large uncertainties in the risks per unit intake of activity reflect the combined uncertainty in the dose and risk coefficients.

Dose limits for astronauts.

Radiation exposures to individuals in space can greatly exceed natural radiation exposure on Earth and possibly normal occupational radiation exposures as well. Consequently, procedures limiting exposures would be necessary. Limitations were proposed by the Radiobiological Advisory Panel of the National Academy of Sciences/National Research Council in 1970. This panel recommended short-term limits to avoid deterministic effects and a single career limit (of 4 Sv) based on a doubling of the cancer risk in men aged 35 to 55. Later, when risk estimates for cancer had increased and were recognized to be age and sex dependent, the NCRP, in Report No. 98 in 1989, recommended a range of career limits based on age and sex from 1 to 4 Sv. NCRP is again in the process of revising recommendations for astronaut exposure, partly because risk estimates have increased further and partly to recognize trends in limiting radiation exposure occupationally on the ground. The result of these considerations is likely to be similar short-term limits for deterministic effects but modified career limits.

The linear no-threshold response: why not linearity?

ICRP and NCRP recommend risk coefficients for use in radiation protection that are based on a linear quadratic response in the low dose region. This is a derivative of the linear no threshold (LNT) hypothesis with allowance for low dose and dose rate effects. The risk coefficients are derived from the Lifespan Study of the A-bomb survivors but are supported by many other epidemiological studies some, such as occupational, at low doses. Nevertheless, the risk coefficients are uncertain and range (90% confidence intervals) over a factor of 2-3 above and below the nominal values. Various possible dose responses in the low dose region are considered including those that may result from adaptive responses. Laboratory studies show linearity in some systems to doses as low as 2.5 mGy. Epidemiological studies include several with significant excess risks at 100 mGy or less with at least one at 10 mGy. The linear quadratic response seems, therefore, the most likely response in the very low dose region. Adopting the linear quadratic response in the low dose region does not prevent common sense judgements about dismissing small radiation risks. NCRP defined first a negligible individual risk (1987) and then an individual dose (1993) to encourage common sense judgements in the low dose region. More consideration might be given to dismissing minor risks in common sense applications in radiation protection.

The present system of quantities and units for radiation protection.

The International Commission on Radiation Units and Measurements has over the last decade developed operational quantities, the ambient, directional and personal dose equivalent, suitable for the measurement of radiation fields in a variety of circumstances. Experience with the use of these quantities to represent the dose limitation quantities defined by the International Commission on Radiological Protection in 1977 has been an important part of recent radiation protection metrology. The definition by International Commission on Radiological Protection in 1991 of new limitation quantities, the equivalent dose and the effective dose has necessitated a redirection of this work. The metrology field has made good progress, however. It has found that for photons, at least above 50 keV, the effective dose can be measured by the ambient dose equivalent about as well as the former effective dose equivalent. Unfortunately, for neutrons the existing and already quite severe complications have been made somewhat worse by the new quantities although not any worse in the important region between 0.1 and 1 MeV. Neutron measurements over a broad energy range are the subject of extensive evaluation and some new suggestions as the metrology field wrestles with these problems. Values of wR constitute an important part of the International Commission on Radiological Protection recommendations. A brief history of the development of higher relative biological effectiveness values for fission neutrons and alpha particles leading to the selection of 20 for wR in each case, is provided.

Radiation protection recommendations on dose limits: the role of the NCRP and the ICRP and future developments.

The purpose of this paper is to review the role of the National Council on Radiation Protection and Measurements (NCRP) and the International Commission on Radiological Protection (ICRP) in making recommendations on dose limits for ionizing radiation exposure for workers and for the public. The text describes the new limits for workers and public recommended by ICRP in 1991 and NCRP in 1993 and the composition of the radiation health detriment on which they are based. The main component of this detriment is the risk of radiation induced cancer which is now estimated to be about three times greater than a decade or so earlier. Uncertainties in these risk estimates are discussed. Some special radiation protection problems, such as those for the embryo or fetus are described. The article also addresses future progress in radiation protection particularly with regard to future improvements in the scientific basis for radiation protection recommendations.

Radiation protection issues in galactic cosmic ray risk assessment.

Radiation protection involves the limitation of exposure to below threshold doses for direct (or deterministic) effects and a knowledge of the risk of stochastic effects after low doses. The principal stochastic risk associated with low dose rate galactic cosmic rays is the increased risk of cancer. Estimates of this risk depend on two factors (a) estimates of cancer risk for low-LET radiation and (b) values of the appropriate radiation weighting factors, WR, for the high-LET radiations of galactic cosmic rays. Both factors are subject to considerable uncertainty. The low-LET cancer risk derived from the late effects of the atomic bombs is vulnerable to a number of uncertainties including especially that from projection in time, and from extrapolation from high to low dose rate. Nevertheless, recent low dose studies of workers and others tend to confirm these estimates. WR, relies on biological effects studied mainly in non-human systems. Additional laboratory studies could reduce the uncertainties in WR and thus produce a more confident estimate of the overall risk of galactic cosmic rays.

Recent estimates of cancer risk from low-LET ionizing radiation and radiation protection limits.

Estimates of the risk of cancer induction, formerly about 1%/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. Thus the effective dose equivalent may require modification. These changes are modifying ICRP and NCRP thinking about recommendations on protection limits, especially for radiation workers.

The 1986 and 1988 UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) reports: findings and implications.

The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has published a substantive series of reports concerning sources, effects, and risks of ionizing radiation. This article summarizes the highlights and conclusions from the most recent 1986 and 1988 reports. The present annual per person effective dose equivalent for the world's population is about 3 mSv. The majority of this (2.4 mSv) comes from natural background, and 0.4 to 1 mSv is from medical exposures. Other sources contribute less than 0.02 mSv annually. The worldwide collective effective dose equivalent annually is between 13 and 16 million person-Sv. The Committee assessed the collective effective dose equivalent to the population of the northern hemisphere from the reactor accident at Chernobyl and concluded that this is about 600,000 person-Sv. The Committee also reviewed risk estimates for radiation carcinogenesis which included the new Japanese dosimetry at Hiroshima and Nagasaki. These data indicate that risk coefficient estimates for high doses and high dose rate low-LET radiation in the Japanese population are approximately 3-10% Sv-1, depending on the projection model utilized. The Committee also indicated that, in calculation of such risks at low doses and low dose rates, a risk-reduction factor in the range of 2-10 may be considered.

Trends in radiation protection--a view from the National Council on Radiation Protection and Measurements (NCRP).

The present status of ionizing radiation protection in our society, with the exception of extraordinary events such as the Chernobyl accident, can be considered reasonably satisfactory. Occupationally, average exposures have risks no greater than accident rates in "safe" industries and show a downward trend in concert with results of safety practices in other occupations; higher exposures are being addressed specifically, and a new NCRP guideline may prove useful. An important concern relating to the quality factor for neutrons is at least partially accounted for by recent International Commission on Radiological Protection (ICRP) and NCRP recommendations. Among public exposures, the most important by far is exposure to indoor Rn. However, this problem is being addressed on all fronts, and its magnitude and the means to deal with it will soon be better known. For the near future, we should see a stabilizing of risk estimates, albeit at levels very probably higher than formerly. There may also be an increasing tendency to use incidence rather than mortality for calculating these estimates. These changes may require some adjustment in our perspective on limits. As the difference in risk between the sexes becomes more definite, we may wish to adopt a policy of equal risk rather than one of equal dose. Age data also emphasize, more and more, the decline of risk with age; consequently, using older workers when feasible in radiation-exposure circumstances becomes more desirable. For the longer-term future, various developments can be expected, including, possibly, a more suitable climate for a risk system, a more appropriate way to express differences in radiation quality, further knowledge of the role probabilities of causation may play in radiation control, the effect of mitigating and enhancing factors, and progress in fundamental oncology. All of these are exciting possibilities which may provide a variety of options for the most effective radiation protection in the future.