保健物理

The frame work of chemical risk assessment consists of the four key steps: hazard identification, dose-response assessment, exposure assessment, and risk characterization. After the hazard identification step, generally, a threshold approach or a non-threshold approach is selected as a procedure of dose-response assessment, in accordance of genotoxic characterization. Traditionally, default uncertainty factors had been used for TDI(tolerable daily intake) derivation in the threshold approach, and specific mathematical models had been used for low-dose extrapolation, such as VSD (virtually safe dose) derivation, in the non-threshold approach. In order to derive more scientifically rational TDIs or to increase transparency, use of chemical specific adjusting factors (CSAFs) and/or a benchmark dose approach method have been recommended recently by WHO/IPCS for the guidance value derivation. The procedures and current trends of guidance value setting, such as TDIs or VSDs, for human health effects by chemical exposure was summarized in this article.

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Large-scale injection of huge amounts of any pollutant into the atmosphere has health and environmental impacts, but also provides an opportunity for understanding associated bio-geochemical cycles. Radiologically important, long-lived radionuclides emitted by atmospheric nuclear tests, such as ⁹⁰Sr, ¹³⁷Cs, and plutonium isotopes, are good examples. These radionuclides remain in the biosphere, although dilution and radioactive decay have mitigated their radiation impact, which is at present negligible compared with natural radiation. Nevertheless, the study of their redistribution over the Earth's surface can enhance understanding of various environmental processes. In this review, available up-to-date information on the source, transport, and deposition of these radionuclides in the atmosphere is collected and these aspects of radionuclides in various global regions are reviewed. In regions adjacent to arid or semi-arid lands, aeolian dust is likely the dominant source of these radionuclides, whereas in northern regions adjacent to contaminated boreal forests, forest fires are a more important source, especially of ¹³⁷Cs in air. Understanding of the atmospheric processes responsible for the redistribution of these radionuclides can improve forecast models in various environmental research fields. Future tasks include continued long-term monitoring and data integration with global data consistency and data analysis with modern chemical transport models.

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The scanning method with GM survey meter is useful for detection of surface radiological density. The detection ability is varied with several parameters; those are the distance between the sample and the window of GM tube, the scanning speed and the background dose rates. We made an experimental system. In that system, a radiation source can be moved at any constant velocity with a linear scanning device. The obtained data shows that the change of count rate has some patterns due to each scanning velocity. The count rate increased with the source coming closer, and decreased with the source moving away. The maximum count rate under the scanning condition, those are 5 mm distance and 50 mm/s scanning velocity, point source on a stainless steel plate was 23% of the count rate on the fixed source. The detection ability in various scanning conditions was evaluated from the measured data. As a result, it was confirmed to be able surely to detect the lower level of the surface radiological density limit by means of the scanning condition applied to the routine work, 5 mm distance and 50 mm/s scanning velocity.

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