Based on the low-dose risk assessment method established in ICRP Pub.26, the log-normal function of annual worker dose was adopted in paragraph 100 of the Recommendations to set dose limits that meet the typical average risk level of safety industries. However, in planned exposure situations where the annual dose approaches the dose limit moderately, it is necessary to add a normal function to the lognormal function to account for dose reduction due to the dose limit. As such a probability distribution, the hybrid log-normal (HLN) distribution was developed in the former JAERI (1980). The genesis mechanism of the HLN distribution has the characteristics of "log + linear increasing/ decreasing" (Kumazawa and Ohashi 1986). On the other hand, cell survival curves with shoulders have "log + linear decrease" characteristic due to the sublethal damage repair effect, and the radiation response per surviving cell have "log + linear increase" characteristic of dose via its logarithmic transformation due to expressing the product of power function and exponential function for dose. As a result, the generalized dose-response relationship, corrected for cell surviving fraction, can be quantitatively modeled based on the HLN genesis characteristics. Thus, the qualitative model of radiation effects and protective risk management can be integrated on the base of the HLN genesis characteristics. In addition, the HLN-generating characteristics are also found in the enzymatic reaction of Michaelis-Menten and the suppression of neutron excess due to increased number of protons in the nucleus, plant growth regulation and reactor power control, and the effectiveness of these management effects becomes apparent in the pure hybridization region bridging between logarithmic and linear scale regions (Kumazawa 2019). Similarly, it is important to adjust the control of "low dose risk increase" associated with "beneficial activity increase" so that the effective result of risk management appears in the pure hybridization region. The HLN genesis characteristics are also found in the temporal and spatial attenuation of air dose rates after the 1FNPP accident (Kumazawa, Toyota, and Kato, 2019) and in the estimated individual dose distributions of evacuated residents (Kumazawa 2020), also applicable to environmental radiation protection. We would like to propose for the future of radiation protection that the concept applying the “log + linear increase” (HLN-generating property) of the gas pedal and brake be included in the system of radiological protection.
Keywords: ICRP Publication 27; individual dose distribution; hybrid log-normal (HLN) distribution; log + linear increase/decrease; quantitative low dose risk sense