Since the UNSCEAR Report published in 1982, radiation exposure to the respiratory tract due to radon and its progeny has been regarded as the single largest contributor to the natural radiation exposure of the general public. In Japan, the measurement of radon gas concentrations in many types of buildings have been surveyed by national and private institutes. We also carried out the measurement of radon gas concentrations in different types of residential buildings in Tokyo and its adjoining prefectures from October, 1988 to September 1991, to evaluate the potential radiation risk of the people living there. One or two simplified passive radon monitors were set up in each of the 34 residential buildings located in the above-mentioned area for an exposure pepiod of 3 months each. Comparing the average concentrations in the buildings of different materials and structures, those in the concrete steel buildings were always higher than those in the wooden and the prefabricated mortared buildings. The radon concentrations were proved to become higher in autumn and winter, and lower in spring and summer. Radon concentrations in an underground room of a concrete steel building showed the highest value throughout our investigation, and statistically significant seasonal variation was detected by the X-11 Method developed by the U. S. Bureau of Census. The values measured in a room at the first floor of the same concrete steel building also showed seasonal variation, but the phase of variation was different. Another multivariate analysis suggested that the building material and structure are the most important factors concerning the levels of radon concentration among other factors such as the age of the building and the use of ventilators.
To obtain the basic data for radon, we carried out the measurement of the 222Rn concentration in outdoor air at Wakasa district (21 measurement points) with a cup monitor for 449 days. From the comparisons between 222Rn concentrations and exposure rates or 226Ra radioactivities in soil, the following conclusions were made. 1. The range of 222Rn concentrations averaged for 449 days were from 12.4Bq/m3 to 1.9Bq/m3. The seasonal variation of 222Rn concentration showed low concentrations from spring to summer. 2. 1Bq/m3 of 222Rn concentration was estimated to contibute to about 0.5nGy/h of exposure rate. 3. The clear correlation between 222Rn concentration and 226Ra radioactivity in soil was observed. From the correlation obtained, the remote source component of 222Rn concentration in outdoor air was estimated to be about 2Bq/m3. 100Bq/kg of 226Ra radioactivity in soil was estimated to produce about 7Bq/m3 of 222Rn concentration in outdoor air through various complex paths from soil.
A new cellulose-glass fiber filter paper (HE-40TA) was produced on a trial basis to improve the collection efficiency of Toyo HE-40T filter paper, which is widely used for sampling of radioactive aerosols at nuclear facilities in Japan. HE-40TA filter paper was tested for collection efficiency, pressure drop and mechanical strength. It was observed that the collection efficiency of HE-40TA was more than 87%, under the conditions of this study: the face velocity from 20 to 130cm/s, the size of DOP aerosol from 0.07 to 1.0 μm in diameter. The pressure drop of HE-40TA filter paper was about 1.2 times higher than that of HE-40T. And the mechanical strength of HE-40TA filter paper was sufficient for practical use.
The survey of the parameters of diagnostic X-ray apparatus, that is, tube voltage, tube current and exposure time, was carried out. The nominal parameters chosen were 70kV, 200mA and 0.1s. Forty-seven institutions were visited, using a total of 63 machines. Most devices meet the JIS for the voltage (±7% to the nominal value) and exposure time (±10%). As for the current, 19 (30.2% of all) devices were out of the JIS (±10%). No significant differences were found between the rectifying methods. If the specific dose (mGy/mAs) is estimated by the nominal parameters, the error of the dose would be 20 to 30%. The quality assurance of the device is very important for the optimization of medical exposure.
This report summarizes the analytical results of deposition, soil and total diet in regard to 90Sr and 137Cs concentrations in the area of Fukuoka prefecture during the period of fiscal year 1963-1990 as well as measurements of the γ-ray dose rate in the same area during the period of fiscal year 1970-1990. The annual fallout peaked value in fiscal year 1963 showing 639MBq/km2 for 90Sr and 839MBq/km2 for 137Cs and then decreased on an exponential curve until fiscal year 1985, reflecting the effect of Chinese nuclear tests. 137Cs fallout at a level of 34.4MBq/km2 was detected upon the accident at the Chernobyl nuclear power plant in 1986. However, it began to decrease in the following year and had shown a lower annual level since fiscal year 1989 than those which were detected before the accident. The results of soil analysis showed a sign of accumulation of both 90Sr and 137Cs, indicating a longterm effect. The radioactivity of the two nuclides in total diet showed a sloping down curve similar to that of the deposition. The γ-ray dose rate was in the range 51.3-101nGy/h, averaging 70.7±5.24nGy/h. The dose equivalent due to external exposure to 137Cs in the soil was estimated to be 14.5μSv/year on average and that due to internal exposure to 90Sr and 137Cs was 4.7μSv/year on average. After the survey, it was found that the 90Sr and 137Cs concentration levels in the fallout in Fukuoka prefecture were lower than that of the national average and the mean dose equivalent due to both internal and external exposure to the two nuclides was estimated to be 18.0μSv/year. This value is within the error limit of 0.495±0.0367mSv/year, which represents an annual dose equivalent due to the γ-ray dose rate, and is less than 1% of the dose of 2.4mSv/year, which represents the mean dose due to the total natural radiation in the world. It is also 1.8% of 1.0mSv/year, that represents the dose equivalent limit based on the ICRP recommendation. This level of an additional radioactivity was deemed to have virtually no effect on human health.
Samples of sea water and sea sediments collected during a period of fiscal year 1971-1992 and those of marine products during fiscal year 1975-1992 from the territorial waters of Fukuoka prefecture were analyzed and the results are summarized. As for the concentration of radioactive substances in sea water, the measurements fell in a range from 1.70 to 7.31mBq/l for 90Sr and from 2.80 to 6.29mBq/l for 137Cs. Their values in 1992 came down to one third and one half of their maximum values, respectively. The 137Cs concentration in sea sediments was found in a range from 1.70 to 9.62Bq/kg dried soil. The distribution coefficient of sea sediments vs sea water was as large as 887 on average. The 137Cs concentration in sea bream was found in a range from 0.180 to 0.311Bq/kg wet, representing a higher level than those of other foods such as cereal crops, vegetables and milk. At the time of nuclear explosion tests in China in 1976 and 1980 as well as the accident at the Chernobyl nuclear power plant in 1986, the concentration of 137Cs increased in sea water and sea sediments, while no apparent change was detected in sea bream. As a result of the present survey, it was concluded that the measurements of 90Sr and 137Cs concentration in marine samples have shown no significant difference between the values for Fukuoka prefecture and the national mean values. Furthermore, Fukuoka's level has always stayed at or below what has been representative in the national coastal waters of the Sea of Japan.
A food consumption survey was done in the south-eastern part (Sanpachi-Kamikita area) of Aomori prefecture. The annual averages of the total daily intake were 1, 902, 2, 224 and 1, 912g/p/d for fishery's, farmer's and other worker's families, respectively. These figures are distinctly larger than the reference value of the national daily consumption rate in Japan. A small seasonal variation was observed in the total consumption, being slightly higher in autumn and lower in spring. The highest food consumption was observed in the farmers among the three groups. The shares of agricultural, livestock and marine products were around 60, 20 and 10%, respectively, for the averaged total consumption of the whole families. The contribution of marine products to the total consumption was relatively high compared with that of the national average. The coastal fishery's group showed the highest consumption of marine products. The representative foods in order of high consumption rates were as follows: salmon, squid and wakame (seaweeds) in marine products; rice, radish, tofu (soybean cake), cucumber and apple in agricultural products; and milk, egg and pork in livestock products.