The coastal belt of Karunagappally, Kerala, India, is known for high background radiation (HBR) from thorium-containing monazite sand. In coastal panchayats, median outdoor radiation levels are more than 4 mGy y-1 and, in certain locations on the coast, it is as high as 70 mGy y-1. Although HBR has been repeatedly shown to increase the frequency of chromosome aberrations in the circulating lymphocytes of exposed persons, its carcinogenic effect is still unproven. A cohort of all 385,103 residents in Karunagappally was established in the 1990's to evaluate health effects of HBR. Based on radiation level measurements, a radiation subcohort consisting of 173,067 residents was chosen. Cancer incidence in this subcohort aged 30-84 y (N = 69,958) was analyzed. Cumulative radiation dose for each individual was estimated based on outdoor and indoor dosimetry of each household, taking into account sex- and age-specific house occupancy factors. Following 69,958 residents for 10.5 years on average, 736,586 person-years of observation were accumulated and 1,379 cancer cases including 30 cases of leukemia were identified by the end of 2005. Poisson regression analysis of cohort data, stratified by sex, attained age, follow-up interval, socio-demographic factors and bidi smoking, showed no excess cancer risk from exposure to terrestrial gamma radiation. The excess relative risk of cancer excluding leukemia was estimated to be -0.13 Gy-1 (95% CI: -0.58, 0.46). In site-specific analysis, no cancer site was significantly related to cumulative radiation dose. Leukemia was not significantly related to HBR, either. Although the statistical power of the study might not be adequate due to the low dose, our cancer incidence study, together with previously reported cancer mortality studies in the HBR area of Yangjiang, China, suggests it is unlikely that estimates of risk at low doses are substantially greater than currently believed.

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As a part of the China-Japan cooperative research on natural radiation epidemiology, we have carried out a dose-assessment study to evaluate the external exposure to natural radiation in the high background radiation areas (HBRA) of Yangjiang in Guangdong province and in the control areas (CA) of Enping prefecture since 1991. Because of the difficulties in measuring the individual doses of all inhabitants directly by personal dosimeters, an indirect method was applied in which the exposed individual doses were estimated from the environmental radiation doses measured by survey meters and the occupancy factors of each hamlet. We analyzed the dose in the hamlets and the variation in the occupancy factors to obtain the parameters of dose estimation on the inhabitants in selected hamlets; Madi and several hamlets of different dose levels in HBRA and Hampizai hamlet in CA. With these parameters, we estimated individual dose rates and compared them with those obtained from direct measurement using dosimeters carried by selected individuals. The results obtained are as follows. 1) The environmental radiation doses are influenced by the natural radioactive nuclide concentrations in building materials, the age of the building and the arrangement of the houses in a hamlet. There existed a fairly large and heterogeneous distribution of indoor and outdoor environmental radiations. The indoor radiation doses were due to exposure from the natural radioactive nuclides in the building materials and were about two times as large as the outdoor radiation doses. The difference between indoor and outdoor doses was not observed in CA. 2) The occupancy factor was influenced by the age of individuals and by the season of the year. The occupancy factor was higher for infants and aged individuals than for other age groups. This lead to higher dose rates of exposure to those age groups. 3) A good correlation was observed between the dose assessed indirectly and that measured directly and the correlation factor was 0.97.

Specific activities of 152Eu:Eu in stone samples exposed to the Hiroshima atomic bomb were determined for 70 samples up to a 1,500-m slant range from the epicenter. The specific activities of 60Co:Co were also determined for six samples near the Hiroshima hypocenter. First, the 152Eu data were investigated to find out the directional dependence of neutron activation. Directional anisotropy was not definite; however, there was an indication that the activation in the west-southwest was lower than in other directions. Second, measured 152Eu and 60Co radioactivity data were compared with activation calculations based on DS86 neutrons. It is clearly shown that the measured data are lower than the calculation near the hypocenter and vice versa at long distances beyond 1,000 m. The calculated-to-measured ratios of 152Eu are 1.6 at the hypocenter, 1.0 at approximately 900 m, and 0.05 at a 1,500-m slant range. Present results indicate that systematic errors exist in the DS86 neutrons concerning the source-term spectrum, neutron transport calculations in air, and/or activation measurements.

To study the effect of low-dose (rate) radiation on human health, we analyzed chromosomes of peripheral lymphocytes of residents in a high background radiation area (HBRA) and compared the results with those obtained from residents in a control area (CA) in Guangdong Province, China. Unstable types of chromosome aberrations (dicentrics and rings) were studied in 22 members of eight families in HBRA and 17 members of five families in CA. Each family consists of three generations. On average 2,600 cells per subject were analyzed. 27 adults and six children in HBRA and 25 adults and eight children in CA were studied with respect to translocations. On average 4,741 cells per subject were examined. We found an increase of the frequency of dicentrics and rings in HBRA, where the natural radiation level is three to five times higher than in the control area. But the increase of translocations in HBRA was within the range of individual variation in the controls.