Decrease of radon exposure by continuously adjusted and controlled ventilation.

A new mechanical ventilation system which continuously controlled the indoor-outdoor pressure difference was installed in six houses, where the long-term radon levels ranged from 670 to 3 080 Bq/m3. When the new system had operated for several months, the indoor radon levels decreased to levels from 120 to 600 Bq/m3 , the effective dose reductions being from 40 % to 88 %.