In order to understand the source of atmospheric radon concentration in the Gyokusendo which is the limestone cave on Okinawa Island, the radon concentrations in the drip water from the ceiling were measured using a liquid scintillation counter. The arithmetic mean (± standard deviation) of radon concentration was 8.7 ± 2.0 kBq m-3. This result suggests that the atmospheric radon in the cave was supplied from the drip water. When comparing the monthly precipitations which concentration in the drip water was shown the delay of 60-90 days. The radon concentration in the soil water was estimated to be 47.3 kBq m-3. Furthermore, the radon concentrations in the percolation water at the cave over a period of 9 to 10 days were calculated to be 9.3-7.7 kBq m-3. The estimated radon concentration in the percolation water was almost equivalent with the measurement concentration in the drip water. These results suggest that the radon atoms in the drip water were generated from the soil above the cave. In other words, emanated radon from the soil particles might be the source of radon in the Gyokusendo.

In order to explain the distribution of natural radiation level in the Asia, in situ measurements of dose rate in air due to terrestrial gamma radiation have been conducted in a total of 21 islands that belong to Ryukyu Islands (Ryukyu Archipelago), subtropical rejoin of southwest Japan. Car-borne surveys have also been carried out in Okinawa-jima, the biggest island of the archipelago. Based on the results for these measurements, arithmetic mean, the maximum and the minimum of the dose rates at 1 m in height from the unpaved soil ground in the archipelago were estimated to be 47, 165 and 8 nGy h(-1), respectively. A comparative study of car-borne data obtained prior to and subsequent to the 2011 Fukushima nuclear accident, as for Okinawa-jima, indicated that the nuclear accident has no impact on the environmental radiation at the present time.

Radon and thoron exhalation rates from the ground surface were estimated in three islands of Okinawa Prefecture, a subtropical region of Japan. In situ measurements of the exhalation rates were conducted at a total of 88 points using an accumulation technique with a ZnS(Ag) scintillation detector. The radon and thoron exhalation rates were calculated to be 1-137 (arithmetic mean: 21) mBq m(-2) s(-1) and 32-6244 (1801) mBq m(-2) s(-1), respectively. In the surface soil samples collected at 53 measurement points, (238)U and (232)Th series concentrations were estimated to be 17.9-254.0 (64.0) Bq kg(-1) dry and 17.8-136.1 (58.8) Bq kg(-1) dry, respectively. The maximum rates and concentrations were observed in the dark red soil area. Recent studies strongly suggest that the base material of the soils may be the eolian dust derived from the southeastern part of China, a high background radiation area. The eolian dust is, therefore, considered to be an enhancer for the radon and thoron exhalations in Okinawa.