The IAEA Safety Fundamentals publication: Fundamental Safety Principles [1] establishes the safety objective and principles for the protection of people and the environment from harmful effects of ionizing radiation. Safety Principle 10 states: ―Protective actions to reduce existing or unregulated radiation risks must be justified and optimized‖. One type of situation covered by this principle concerns exposure due to natural sources of radiation, including exposure due to radon in dwellings and workplaces, and external gamma exposure due to radionuclides of natural origin in building materials.