Background radiation refers to the omnipresent ionising radiation originating from natural sources in the environment. It is an unavoidable component of human existence, varying in intensity based on geographical location, altitude, and local geological characteristics. Understanding background radiation is fundamental to the discipline of radiation protection, as it provides a baseline against which occupational and medical exposures are assessed.
The primary sources of natural background radiation can be categorised into several key areas:
This radiation originates from naturally occurring radioactive materials (NORM) present in the Earth's crust, soil, water, and building materials. Key radionuclides include isotopes from the uranium and thorium decay series, and Potassium-40 (). Radon () and thoron (), gaseous decay products of uranium and thorium respectively, are significant contributors to internal exposure, particularly within enclosed spaces. Exposure levels from terrestrial sources vary significantly depending on the local geology; for instance, areas with granite bedrock typically exhibit higher levels due to elevated concentrations of uranium and thorium.
Cosmic radiation originates from outer space, comprising high-energy protons and atomic nuclei from galactic and solar sources. These primary cosmic rays interact with the Earth's atmosphere, producing secondary particles such as neutrons, muons, and electrons, which reach the ground. The intensity of cosmic radiation increases with altitude, as there is less atmospheric shielding, and also varies with latitude due, in part, to the Earth's magnetic field. Aircrew and frequent flyers experience higher doses from cosmic radiation.
The human body naturally contains various radioactive isotopes absorbed through food, water, and air. The most significant contributor is Potassium-40 (), an essential element for bodily functions, which is present in soft tissues. Other internal radionuclides include Carbon-14 () and minute quantities of lead and polonium isotopes from the uranium and thorium decay series. These contribute to a continuous, unavoidable internal dose.
While not strictly considered "natural" background radiation, certain human activities contribute to the overall environmental radiation exposure that can sometimes be indistinguishable from natural sources over large areas. This includes residual fallout from past atmospheric nuclear weapons testing and releases from some industrial processes. However, these contributions are generally minor compared to natural sources in most regions, though they are subject to strict regulatory control.
The effective dose from background radiation is typically measured in milliSieverts per year (). The global average effective dose from natural background radiation is approximately , though this can range from to over in certain high natural background radiation areas. Understanding these baseline levels is crucial for establishing and adhering to radiation dose limits for workers and the public. It allows safety professionals to characterise and differentiate between natural, unavoidable exposure and exposures resulting from controlled or accidental releases, informing effective radiation protection strategies and risk assessments.