Correlation between natural background radiation exposure rates and cancer mortality rates in 46 Japanese prefectures was statistically studied over the period between 1968 and 1978. With regard to the correlation between natural background radiation exposure rates and cancer mortality rates, statistically significant positive correlation coefficients were observed in most cases of cancers with only a few exceptions.

Porcelain teeth, some of which contain uranium compounds for aesthetic purpose, have been widely used in dental clinics. Hazardous effects due to uranium radiation have been suggested in recent publications.In the present study, the authors carried out the determination of uranium concentrations in four major brands of porcelain teeth marketed in Japan using the fission track method, and the absorbed doses to oral tissues were calculated by a formula introduced by the authors for calculation of alpha radiation.Average uranium concentrations of the brands studied were determined to be 3.6 ppm (0.33-10 ppm, Japan), 18 ppm (0.69-81 ppm, Japan), 9.4 ppm (2.5-14 ppm, Japan) and 82 ppm (1 1-205 ppm, US.), respectively. The corresponding dose equivalents at the surface of oral mucosal membrane were 2.9 rem . yr-', 14 rem . yr-', 7.6 rem . yr-' and 66 rem . yr-'.

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Whole body counting of potassium-40 and anthropometric measurements were done on 88 urban and rural Japanese boys 12 to 14 years of age. The log-normal distribution provided a good fit to the observed data of potassium content in both groups. The urban and rural boys did not differ significantly in age, weight, body and sitting height, chest and abdomen circumferences, antero-posterior diameter of chest and abdomen, and grip strength. On the other hand, the mean potassium content, K/body weight, K/fat-free mass and estimated upper-arm muscle circumference were significantly greater in rural boys. Skinfold thickness and percentage fat were significantly greater in urban boys. In a regression analysis estimating whole body potassium from anthropometric variables, body weight and skinfold thickness were the most significant variables, accounting for 85.2% of the total variation. Estimation errors were smaller in the rural than in the urban group. Factor analysis was used to identify the factors which could explain the items measuring body composition and strength. Factor 1, in which potassium content and grip strength are the most important items, was designated as the 'Muscular Factor', and Factor 2, in which skinfold thickness and upper-arm circumference are the most significant items, was designated as the 'Fatness Factor'. The factor scores of rural subjects were scattered in a considerably narrower range than those of urban subjects.

In the previous studies, using Pearson's product moment correlation coefficient, we found that in most cases of cancers, statistically significant positive correlations were observed between natural background radiation exposure rate and crude cancer mortality rate over the period . Furthermore, we found that the statistical significance of correlation between natural background radiation exposure rate and the age-adjusted cancer mortality rate in the same period mostly disappeared. We studied the cause of this apparent correlation and found that the prefecture with a higher natural background radiation exposure rate had a greater component ratio of older people. In Japan, a number of prefectures with a higher natural background exposure rate are located in relatively thinly populated districts which have been experiencing an outflow of the younger generation to more highly industrialized and urbanized areas. Therefore, statistically significant positive correlations were observed for almost all cancers between natural background radiation exposure rate and crude cancer mortality rate. In the present investigation, we statistically tested the frequency distributions of natural background radiation exposure rate and ageadjusted cancer mortality rate, and calculated Spearman's rank correlation coefficient between natural background radiation exposure rate and the age-adjusted cancer mortality rate. The frequency distribution of the natural background radiation exposure rate and that of the age-adjusted mortality rate appeared normal in most cases of cancer, and the statistical significance of correlation between natural background exposure rate and the age-adjusted cancer mortality rate did not differ much on the whole, even though we used Spearman's rank correlation coefficient between them.