Abstract
The results of radon $(^{222}Rn)$ concentrations and working levels (WL) for forty rooms in Kwanak Campus, Seoul National University on granite bedrock of Jurassic age showed that radon concentration have mean value of 3.0 pCi/L and 0.011 for working level. A number of rooms where these values exceed the EPA's action level are five (13%). It was also suggested that indoor basement rooms in poor ventilation condition can be classified as extremely high radon risk zone having more than 4 pCi/L and 0.020 WL. It was proved that inflow of soil-gas was a primary factor that governs indoor radon level by comparison of soil-gas radon concentrations with indoor radon concentrations.