*3.5. 220Rn and 222Rn Correlation*

The correlation between the indoor 220Rn and 222Rn concentrations was investigated. The relationship between the 220Rn and the 222Rn concentrations is illustrated in Figure 5. The concentration distributions for both radioisotopes follow approximately a log-normal distribution. Consequently, both datasets were calculated after taking the logarithms of the respective data. The linear regression analysis shows a weak positive correlation (R = 0.25). The ratio for the concentrations of 220Rn/222Rn ranged from 0.007 to 40.3 and reveal a log-normal plot. The arithmetic mean for 220Rn/222Rn was 1.64 and geometric mean was 0.78.

**Figure 5.** Correlation between the mean annual concentrations of 220Rn and 222Rn.

#### **4. Conclusions**

The mean annual indoor 220Rn concentrations were measured in 899 houses using a passive 222Rn–220Rn discriminative monitor. The arithmetic mean, the median and the geometric mean were 20.1, 9.6 and 10.0 Bq m−3, respectively. The 220Rn concentration plot exhibited a log-normal distribution. The maximum 220Rn concentration found in the present survey was 383 Bq m−<sup>3</sup> for a wooden house with mud walls. The survey data for the indoor 220Rn concentrations in Japan did not exhibit a significant seasonal variation. There was a marked difference in the 220Rn concentration depending on the nature of the house structure. Relatively higher concentrations of 220Rn were found in wooden and concrete block houses compared to other housing types. In general, the 220Rn concentrations in traditional wooden houses with mud walls tended to be higher than those for houses with different wall types. Further, it was demonstrated that the 220Rn concentrations decreased with distance of measurement from the wall.

**Funding:** This research was carried out in collaboration with 47 prefectural institutes under the auspices of the former Science and Technology Agency of Japan.

**Institutional Review Board Statement:** Not applicable for studies not involving humans or animals.

**Informed Consent Statement:** Not applicable for studies not involving humans.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **References**

