The low dose considered here is about 2R in 40 years which corresponds to the difference in the highest and the lowest background radiations in several prefectures in Japan. Examples of risk are cancer of adults (40-79 years old) and of childhood (0-9 years old) and childhood vital statistics.
The quantity of risk is to be expressed in 10^-5 Y^-1 for cancers and in 10^-3 Y^-1 for childhood vital statistics with standard deviation (SD) which gives uncertainties of estimates. Risk factors of the ICRP¹⁾ are given in terms of 10^-2 ?? 10^-5 Sv^-1. The risk factors provide convenient measures for point assessment of lifetime detriment after the relevant population is expired. But the risk factor is not satisfactory for public health purposes because it does not give annual rate nor statistical variation of the total detriment. The purpose of this report is to supplement some insufficiency of risk estimation.

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The degree of inhibition of DNA replication and the level of DNA repair synthesis induced by thermal neutron irradiation was determined in human lymphoblastoid cell lines. Dose response experiments revealed that, when compared with equivalent doses of ⁶⁰Co-γ-irradiation, thermal neutrons gave rise to similar levels of inhibition of DNA replication but induced relatively low levels of DNA repair synthesis. The amount of repair synthesis after exposure of cells to thermal neutrons, was about ten-fold lower than that observed after γ-radiation. Reasons for this difference are discussed in terms of the type and incidence of damage induced by thermal neutrons.

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An Escherichia coli K-12 ultraviolet (UV)-sensitive mutant KMBL92 (dar-4) was identified as an amber uvrC mutant. In a suppressor-free genetic background it showed high sensitivity to UV, no reactivation of UV-irradiated λ phage and positive Weigle reactivation, whereas upon introduction of a suppressor mutation it exhibited moderate resistance to UV, intermediate capacity for host-cell reactivation and no Weigle reactivation. Other properties, like DNA degradation during post-UV incubation and frequencies of UV-induced mutations in bacteria and bacteriophage λ, were similar to those of a uvrC mutant.

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Somatic chromosomes derived from lymphocytes of 23 Hiroshima A-bomb survivors were analyzed to determine the type and frequency of radiation-induced chromosome aberrations, using both the ordinary staining method (0-method) and the trypsin G-banding method (G-method). Of 896 cells examined, 342 had aberrations, including 31 unidentifiable cells even by the G-method. The number of aberrations detected was 376 in 311 cells. The majority of them were intra or inter-chromosomal symmetric exchanges, while only 24 were found to be asymmetric exchanges (dicentrics, rings and interstitial deletions). Further, 28 aberrations included acentric fragments and terminal deletions, and the remaining 36 were complex intraand inter-chromosomal exchanges showing insertions and double translocations.
An analysis of the same metaphases examined by sequential 0 and G-methods was carried out independently on 361 aberrations. It was found that 78 were detectable only by the Gmethod; among these were 14 paracentric inversions, 48 reciprocal interchanges of chromosome segments with either equal or unequal length, 14 minor deletions and 2 complex rearrangements, all of which were judged as the normal variation by the O-method. In contrast, 25 aberrations detected by the 0-method were found to show normal banding patterns by the G-method.

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The radiosensitivity of mouse zygotes fertilized in vitro (BC3F₁ ovum x ICR sperm) to X-rays has been measured as a function of time from fertilization to first cleavage. The dose of X-rays (LD₅₀) required to prevent development of 50% of the zygotes to the blastocyst stage in vitro varied markedly depending on the time of irradiation from about 40 to 400 R. Sensitivity is relatively low shortly after sperm entry and becomes extremely high (LD₅₀, 40 R) at the stage just before pronuclear formation (4 to 6 hr after insemination). In the later pronuclear stage (12 hr of fertilization) the sensitivity becomes low (LD₅₀, about 400 R), and it increases again just before first cleavage.

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