*3.4. Dependency of 220Rn Concentration on Wall Structure and Distance from Wall*

The present survey on 220Rn concentrations considered four categories of material which were used for wall construction in the houses. The mean annual 220Rn concentrations obtained by passive measurement for the different wall materials in the houses are presented in Figure 3. Inspection of the results (Figure 3) reveals that high 220Rn concentrations occurred for houses with mud walls, and the values decreased gradually with distance from the surface of the wall as shown in Figure 4. Yonehara et al. reported similar behavior for 220Rn concentrations at locations near the wall surfaces in Japanese dwellings [26].

**Figure 3.** 220Rn concentrations for various wall materials.

**Figure 4.** 220Rn concentrations at different distances from the wall surface for all wall materials.
