²²²Rn and ²²⁶Ra concentrations of less than a few to several thousands of Bq L⁻¹ have been observed in several underground bodies of water around the world. Although regulations for these concentrations in water have been implemented internationally, there are currently no regulations in place in Japan. However, concentrations that exceed these internationally recognized regulatory values have also been observed in Japan. In this study, concentrations in spring water in the northern part of Japan were measured and the effective dose from intake of the water was evaluated. ²²²Rn concentrations were measured using a liquid scintillation counter, and ²²⁶Ra concentrations were measured using a high purity germanium detector after chemical preparation. The measured ²²²Rn concentrations (=12.7 ± 6.1 Bq L⁻¹) and ²²⁶Ra concentrations (<0.019–0.022 Bq L⁻¹) did not exceed the reference values set by international and European/American organizations. A conservative estimate of the annual effective ingestion dose of 8 μSv for ²²²Rn and ²²⁶Ra obtained in this study is much smaller than the estimated overall annual effective dose of 2.2 mSv from natural radiation to the Japanese population. However, this dosage accounts for 8% of the WHO individual dosing criteria of 0.1 mSv/year for drinking water.

As part of a nationwide survey of thoron (²²⁰Rn) in Japan, the indoor ²²⁰Rn gas concentrations in 940 dwellings were measured throughout one year, from 1993 to 1996, using a passive type ²²²Rn-²²⁰Rn discriminative monitor. The monitor was placed in a bedroom or a living room in each house for four successive three-month periods. The mean annual indoor ²²⁰Rn concentration was estimated from the four measurements in each house. The arithmetic mean, the median and the geometric mean for indoor ²²⁰Rn concentrations in 899 dwellings were 20.1, 9.6 and 10.0 Bq m⁻³, respectively. The ²²⁰Rn concentrations exhibited a log-normal distribution. It was found that the ²²⁰Rn concentrations were dependent on the nature of the materials used for wall construction and also on the distance of measurement from the wall. Significant seasonal variations in the ²²⁰Rn concentration were not observed. It would seem that the nature of the wall material contributed to the increased indoor ²²⁰Rn concentrations.