Natural background radiation refers to the ionising radiation that is ubiquitous in our environment and is not a result of human activities. It is an inherent part of life on Earth, originating from various natural phenomena, and is the primary contributor to the annual effective dose received by the general population globally. Understanding its sources and levels is fundamental to the principles of radiation protection.
Natural background radiation can be broadly categorised into three main sources: cosmic radiation, terrestrial radiation, and internal radiation from radionuclides incorporated within the body, with radon gas being a significant component.
Cosmic radiation originates from outer space, primarily from the Sun and other celestial bodies in our galaxy. These high-energy particles (protons, alpha particles, and heavier nuclei) interact with the Earth's atmosphere and geomagnetic field, producing secondary radiation, including neutrons, muons, and electrons. The dose from cosmic radiation varies significantly with altitude and latitude. Higher altitudes, such as those experienced during air travel or in mountainous regions, result in increased exposure due to less atmospheric shielding. For instance, aircrew often receive higher doses compared to individuals at sea level.
Terrestrial radiation arises from naturally occurring radioactive materials (NORM) present in the Earth's crust, soil, rocks, and building materials. The primary radionuclides responsible for this source are uranium-238 (U), thorium-232 (Th), and potassium-40 (K), along with their decay products. The concentration of these radionuclides varies geographically, leading to differing background radiation levels across regions. Areas with high granite content, for example, typically exhibit higher terrestrial background radiation.
Humans are also exposed to radiation from within their own bodies due to the ingestion and inhalation of natural radionuclides. Key contributors include potassium-40 (K), which is a vital element in human physiology and is uniformly distributed throughout soft tissues, and carbon-14 (C), an isotope present in all organic matter. Additionally, short-lived decay products of radon gas, inhaled from the air, contribute significantly to internal exposure, particularly to the lung tissue.
Radon (Rn) is a naturally occurring radioactive gas produced from the decay of uranium present in soil and rocks. Being a gas, it can migrate through the ground and accumulate in enclosed spaces such as homes, offices, and mines. Once inhaled, its short-lived decay products can attach to lung tissue, delivering an internal dose. Radon is globally recognised as the largest single contributor to the average effective dose from natural background radiation for the general public, and its levels can vary widely based on local geology and building characteristics.
The average annual effective dose from natural background radiation for an individual globally is approximately . However, this figure can vary substantially, ranging from about to over in certain high background areas, influenced by local geology, altitude, and housing construction materials. Understanding these variations is crucial for assessing potential health effects and for establishing appropriate dose limits for artificial radiation sources.
The pervasive nature of natural background radiation provides a baseline against which all other radiation exposures are measured. Its study is vital for establishing reference levels in environmental monitoring and for characterising the risks associated with both natural and artificial sources. While the health risks from typical natural background radiation levels are generally considered low, exceptionally high local levels, particularly of radon, may warrant specific protective measures and further assessment.