6.1. Radiation is Natural

Our world is radioactive and has been since its creation. Natural radiation has been with us since the Big Bang and the appearance of the Universe. Radionuclides and radiation are constituents of the Earth and play important roles in natural processes, long-lived radionuclides, for example, maintain enhanced temperatures deep underground thereby preserving the Earth's heat balance. Living systems have adapted to, or even been stimulated by, existing levels of natural radioactivity; the soft radiation of ¹⁴C (49.5 keV beta particles) is suspected of causing suitable conditions for development of life itself.

The Earth's current population receives some level of radiation from a number of natural, as well as man-made sources including cosmic rays from outer space and the Sun's surface, terrestrial radionuclides that occur in the Earth's crust, in building materials and in air, water and foods and in the human body itself. Humans contain ⁴⁰K and ¹⁴C and hence potentially irradiate one another as the decay of ⁴⁰K causes emission of 1.46 MeV photons. Some of the exposure sources are fairly constant and uniform for all individuals everywhere, for example, the dose from ingestion of ⁴⁰K in foods. Other exposures vary widely depending on location; cosmic rays are more intense at higher altitudes. Exposures can also vary as a result of human activities and practices. In particular, the building materials of houses and the design and ventilation systems strongly influence indoor levels of ²²²Rn and its decay products, which...