Occupational exposures to ionising radiation mainly occur at low-dose rates and may accumulate effective doses of up to several hundred milligray.The objective of the present study is to evaluate the evidence of cancer risks from such low-dose-rate, moderate-dose (LDRMD) exposures.Our literature search for primary epidemiological studies on cancer incidence and mortality risks from LDRMD exposures included publications from 2002 to 2007, and an update of the UK National Registry for Radiation Workers study. For each (LDRMD) study we calculated the risk for the same types of cancer among the atomic bomb survivors with the same gender proportion and matched quantities for dose, mean age attained and mean age at exposure. A combined estimator of the ratio of the excess relative risk per dose from the LDRMD study to the corresponding value for the atomic bomb survivors was 1.21 (90% CI 0.51 to 1.90).The present analysis does not confirm that the cancer risk per dose for LDRMD exposures is lower than for the atomic bomb survivors. This result challenges the cancer risk values currently assumed for occupational exposures.
The interpretation of a best estimate for a value of the DREF depends on the appropriateness of including the Mayak study. This study indicates a range of uncertainty in the value of DREF between 1 and about 2 after protracted radiation exposure. The LDR data provide direct evidence regarding risk from exposures at low dose rates as an important complement to the LSS risk estimates used for radiation protection purposes.
Soil samples collected from three forest sites within the
30-km zone around the Chernobyl reactor were analyzed for
239Pu and 240Pu by ICP-MS. The average 240Pu/239Pu atom
ratio in contaminated surface soil samples, values of which
are scarce in the literature, was 0.408. There were
almost no differences in the 240Pu/239Pu ratios between
the individual samples analyzed, although the 239+240Pu levels
varied very widely (i.e. from 6.3 to 1430 Bq kg-1 dry
weight) depending on the distance from the reactor and
on the soil layers investigated. This result corresponded to
area-related activities for 239+240Pu between 1.1 kBq m-2
and 13.3 kBq m-2. It was estimated that about half of the Pu
migrated from the organic layers to the underlying
mineral layers. The 240Pu/239Pu ratio observed in the
Chernobyl area was much higher than that attributed to
weapons fallout (ca. 0.18). The high ratio was related to the
high burn-up grade of the reactor fuel. The 240Pu/239Pu
ratio observed might be used as a “fingerprint” in identifying
the distribution of Chernobyl-derived Pu in the environment
and in distinguishing it from other sources, e.g. global
fallout. Relationships between the concentrations of Pu
and those of 137Cs, 60Co, and 125Sb were also discussed.
The biological effects on humans of low-dose and low-dose-rate exposures to ionizing radiation have always been of major interest. The most recent concept as suggested by the International Commission on Radiological Protection (ICRP) is to extrapolate existing epidemiological data at high doses and dose rates down to low doses and low dose rates relevant to radiological protection, using the so-called dose and dose-rate effectiveness factor (DDREF). The present paper summarizes what was presented and discussed by experts from ICRP and Japan at a dedicated workshop on this topic held in May 2015 in Kyoto, Japan. This paper describes the historical development of the DDREF concept in light of emerging scientific evidence on dose and dose-rate effects, summarizes the conclusions recently drawn by a number of international organizations (e.g., BEIR VII, ICRP, SSK, UNSCEAR, and WHO), mentions current scientific efforts to obtain more data on low-dose and low-dose-rate effects at molecular, cellular, animal and human levels, and discusses future options that could be useful to improve and optimize the DDREF concept for the purpose of radiological protection.
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