Background radiation

Background radiation is the ubiquitous ionizing radiation present in the natural environment that originates from various natural and artificial sources, and to which every person is continuously exposed. It constitutes the baseline level of radiation against which all other radiation exposures are compared.

Sources of background radiation

Background radiation is broadly classified into natural and artificial (anthropogenic) sources. Natural sources account for the majority of human exposure, typically around 80-85% of the total annual effective dose.

Natural sources

Cosmic radiation originates from outer space and the Sun. It consists of high-energy particles (primarily protons and alpha particles) that interact with Earth's atmosphere, producing secondary radiation such as neutrons, muons, and gamma rays. The intensity of cosmic radiation increases with altitude, so people living at high elevations or frequent fliers receive a higher dose. At sea level, cosmic radiation contributes about 0.3 mSv per year, but at 10 km altitude, the dose rate can be 100 times higher.

Terrestrial radiation comes from radioactive elements naturally present in the Earth's crust, soil, rocks, and building materials. The most significant contributors are the primordial radionuclides uranium-238, thorium-232, and potassium-40, along with their decay products such as radium and radon. Radon-222, a radioactive gas produced from the decay of uranium, is a major source of indoor background radiation, particularly in poorly ventilated buildings. Terrestrial radiation dose varies widely depending on local geology; for example, regions with granite or phosphate deposits have higher levels.

Internal radiation arises from radionuclides that are taken into the body through ingestion and inhalation. The most important internal sources are potassium-40 (present in all living tissues), carbon-14 (produced by cosmic rays and incorporated into organic matter), and radon decay products that deposit in the lungs. These contribute a relatively constant dose, typically around 0.3–0.4 mSv per year.

Artificial (anthropogenic) sources

Artificial background radiation originates from human activities, though it is generally a small fraction of total exposure. The primary contributors include:

• Medical exposures: Diagnostic procedures such as X‑rays, CT scans, and nuclear medicine imaging. These are not strictly "background" but are often included in discussions of population exposure. The average annual dose from medical sources has increased significantly in recent decades.
• Nuclear weapons testing fallout: Atmospheric testing in the mid-20th century released radionuclides like cesium-137 and strontium-90 that persist in the environment at low levels.
• Nuclear power and industrial sources: Routine discharges from nuclear facilities, coal-fired power plants (which release uranium and thorium), and industrial uses of radioactive materials (e.g., radiography, smoke detectors) contribute small amounts.
• Consumer products: Items such as luminous watches, ceramic glazes, and some building materials (e.g., granite countertops) may contain trace amounts of radioactive elements.

Measurement and units

Background radiation is measured in units of absorbed dose (gray, Gy) or equivalent/effective dose (sievert, Sv). Because doses from background are small, they are commonly expressed in millisieverts (mSv) or microsieverts (μSv). The global average annual effective dose from all natural sources is about 2.4 mSv, with a typical range of 1–10 mSv depending on location. Artificial sources add roughly 0.6 mSv on average (including medical, occupational, and other exposures). Instruments such as Geiger counters, scintillation detectors, and ionization chambers are used to measure background radiation levels.

Variations in background radiation

Background radiation levels vary significantly around the world due to differences in geology, altitude, and human activities. Some notable examples:

• High background areas: Ramsar (Iran) has some of the highest natural radiation levels on Earth, reaching up to 260 mSv per year due to local hot springs and deposits of radium. Guarapari (Brazil) and Kerala (India) also have elevated levels from thorium-rich monazite sands.
• Low background areas: Underground laboratories, such as the Sudbury Neutrino Observatory in Canada, are built deep underground to shield from cosmic rays and achieve extremely low radiation backgrounds for sensitive experiments.
• Altitude effect: Cosmic radiation at 3000 m altitude is about 4 times higher than at sea level. Air travel adds approximately 0.02 mSv per flight hour.

Health effects and significance

The health effects of chronic exposure to low-level background radiation are a subject of ongoing scientific study. The linear no-threshold (LNT) model, adopted for radiological protection, assumes that any dose above zero carries some risk of cancer or genetic damage. However, epidemiological studies in high-background regions have not consistently demonstrated increased cancer rates, and some suggest a possible hormetic effect (adaptive response) at low doses. Nevertheless, international bodies such as the International Commission on Radiological Protection (ICRP) advise keeping all exposures as low as reasonably achievable (ALARA), while recognising that background radiation cannot be eliminated.

Understanding background radiation is essential for:

• Setting regulatory dose limits and safety standards.
• Interpreting results of environmental monitoring around nuclear facilities.
• Designing experiments in particle physics and dark matter detection that require ultra-low backgrounds.
• Assessing radiological risks to astronauts and frequent flyers.

See also

• Environmental radioactivity
• Radon
• Cosmic ray
• Ionizing radiation