The health effects of low-dose radiation have generated considerable concern after the accident at the Fukushima Daiichi Nuclear Plant. Although the risk of acute direct exposure to high-dose radiation could be avoided, the risk remains for low-level continuous exposure to radiation by long-lived environmental contaminants, such as cesium-137 that is released from the nuclear plant. Scientists have engaged in a contentious debate regarding the actual risk of low-dose radiation. To understand the actual risk of radiation scientifically, the Open Symposium of Japanese Environmental Mutagen Society (JEMS) was held on May 26, 2012 at Keio University in Tokyo. Eight scientists and a special guest from Fukushima were invited to participate in this symposium. We understand that it is difficult to draw a proper conclusion scientifically concerning the actual (absolute) risk of low-dose and low-dose rate radiation from the available data. The risk of radiation exposure can only be estimated in a relative manner if we compare the risk to other confounding risk factors, such as smoking. Being unafraid and controlling risk factors in our lifestyle are important in helping us to cope with the inevitable exposure to low-dose radiation that was caused by the Fukushima accident. It is critical to communicate and to advise people in the nearby environment regarding their risk of radiation exposure and the need to make a rational decision to avoid undue exposure and excess risk concerning radiation emerging from the accident site.

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When we consider the risk of radiation caused by the Fukushima Daiichi Nuclear Plant accident, we may feel the situation to be much like the formation of rain spots on a car. The dirty spots are difficult to tolerate by the owner of a brand-new car but can be accepted by a used car owner who does not clean his or her car frequently. In the course of collecting information to prepare a webpage concerning radiation risk on the Japanese Environmental Mutagen Society (JEMS) homepage following the Fukushima accident, I have learned that we have already unconsciously been exposed to an unexpected level of radiation. Therefore, our body is not like that brand-new car affected by rain spots or, in this case, radioactive contamination. We are internally exposed to ⁴⁰K radiation through the foods we eat on a daily basis, and we have already been exposed to the 1,000-10,000 times higher background of the nuclear fallout that occurred during the 1960s because of world-wide nuclear bomb experiments. It is important to know these facts to consider the excess risk derived from the Fukushima accident and thereby learn to be more cautious. Obtaining a proper answer scientifically about the health effects of low-level radiation exposure is very difficult when using available data on radiation biology. Increasing risk awareness and communication is also important together with proving the real risk of low-level radiation. Radiation risk should be considered in a relative manner by comparing it with other confounding factors, which together can be treated as a total risk. The increased risk posed by radiation exposure can be traded-off by reducing other risk factors affecting our lifestyle. The most important task for us is to transfer available scientific knowledge to the public such that the information is more understandable to help people make their own decisions on how to face radiation risk.

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Radiation hygiene survey has been conducted in the East Japan earthquake area, about Fukushima Dai-Ichi nuclear power station disaster caused by tsunami on March 11, 2011. Our surveys revealed that a public annual dose was 10 mSv following the disaster and health hazards should not be concluded by the methods of in situ dose evaluation. This study has been focused on internal dosimetries of iodine-131 in thyroid and of cesium-134, 137 in whole body. Further, we continuously have been studying radiation hygiene on cattle livestock in Namie town located within 20 km zone around the Fukushima Daiichi nuclear power station, and found no problem for the recovery.

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The March 11 earthquake and tsunami crippled the Fukushima Daiichi Nuclear Power Plant of Tokyo Electric Power Company. Consequently, wide areas were contaminated with radioactive substances. The evacuation zone was set as the area within a 20 km radius from the power plant. People who lived in that zone were forced to evacuate immediately. Fukushima Prefectural Disaster Headquarters (FPDH) and the Japanese Governmental Disaster Headquarters (JGDH) planned to let them return to their homes temporarily and asked for help from related organizations, one of which is the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The ministry asked all Japanese universities and colleges to take part in screening radioactivity of the temporal returnees from the areas, with aids of the temporal returning program. We applied to the screening activities and measured radioactivity of the returnees at Bajikoen Off-site Center on July 14-17, 2011, and at Kawauchi Gymnasium or Hirono Central Gymnasium Off-site Center on July 23-25, 2011, in Fukushima Prefecture. During these periods, 4009 returnees in all were screened at these three Off-site Centers. None was found to be contaminated with radioactivity over 13,000 cpm. Before the returnee screening program, emergency screening had been conducted soon after the disaster. In all, 33,598 people out of 78,000 evacuees (43%) were screened during March 12-December 7, 2011, during which 3,686 volunteers were recruited by MEXT for the screening program. This is a tiny fraction of the temporal returning program. FPDH reported that 102 people were contaminated with more than 100,000 cpm out of 259,108 people generally screened from March 13, 2011-October 10, 2012. The 102 were detected only in the screening in March in 2011. No hazardous effect, however, was detected among them, so far. Screening is still underway. The present article was compiled to record aspects of the large scale radioactivity monitoring program planned by FPDH and JGDH.

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Whether chronic exposure has an cancer risk per dose different from that of acute exposure is a topic of debate. This paper discusses the effect of dose rate on the strength of relationship between cancer risk and external exposure to low-LET (Linear Energy Transfer) ionizing radiation (such as X-rays and gamma rays), by reviewing important epidemiological studies. The study of atomic-bomb survivors, who had acute exposure mainly to low-LET ionizing radiation, has shown that the excess relative risk per gray (ERR/Gy) of leukemia increases in a linear-quadratic manner with an increase of radiation dose. The estimate of ERR/Gy for medium-high dose ranges was shown to be approximately two-fold larger than that for a low-dose range in the atomic-bomb survivor study. The estimate of ERR/Gy associated with acute exposure appears to be larger than those obtained from the studies of low-dose-rate exposure. On the other hand, the risk of solid cancer (or all cancers excluding leukemia) showed a linear dose-response relationship. Regarding the risk modification by dose rates, lower dose-rate exposure to high LET radiation is suspected to be associated with a larger risk. In the case of low-LET exposure, however, the cancer incidence study of residents in high natural background radiation areas of Karunagappally Taluk in Kerala State, India suggests that the ERR per dose for solid cancer after chronic radiation exposure is significantly lower than that associated with acute exposure such as that experienced by atomic-bomb survivors.

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Human epidemiological studies have revealed significant increases in the cancer incidence rates by exposure to 100 mSv or higher doses of ionizing radiation. However, the relationship between lower doses of ionizing radiation and cancer incidence is still unclear. In general, oxidative DNA damage is closely related to cancer generation. We studied the oxidative DNA damage elicited by low dose ionizing irradiation. The γ-ray irradiation of a deoxyguanosine solution caused a linear increase in the 8-hydroxydeoxyguanosine (8-OHdG) levels, in the range of 20-300 mGy. Thus, 8-OHdG seems to be a good marker of the oxidative DNA damage caused by ionizing radiation. In contrast, in the case of the whole body X-ray irradiation of mice, the 8-OHdG levels in liver DNA and urine increased from about 500 and 200 mGy, respectively. These results indicate that living organisms have a defense mechanism against the oxidative damage caused by ionizing radiation. Considering the 8-OHdG levels as an ionizing radiation effect marker for living organisms, a threshold level of irradiation seems to exist for oxidative damage and tumorigenesis. Furthermore, diet imbalances increased radiation damage. Lifestyle may affect the radiation hazard.

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