Radioactivity occurs when an unstable atomic nucleus releases energy as particles or electromagnetic waves to achieve a more stable configuration. This spontaneous emission is known as radioactive decay, and the energy released is radiation. While the term often conjures images of nuclear power or weapons, radiation is a constant, ubiquitous feature of the environment that has existed since the formation of the Earth. Everyone is exposed to naturally occurring radiation every day from the ground, the sky, and even within their own body.
Radioactivity from the Earth (Terrestrial Sources)
The largest source of natural radiation exposure comes from the Earth’s crust, which contains primordial radioactive elements that have existed for billions of years. These terrestrial sources are primarily long-lived isotopes like Uranium-238 and Thorium-232, which are found in varying concentrations within rocks, soil, and building materials. These heavy elements undergo a series of radioactive decays, known as decay chains, until they reach a stable, non-radioactive element like lead.
A significant byproduct of the Uranium-238 decay chain is Radon-222, an odorless, colorless, and chemically inert gas. Radon forms when Radium-226 breaks down, and because it is a gas, it migrates easily through soil and rock fractures. While radon quickly dissipates to harmless levels in the open atmosphere, it can accumulate to higher concentrations in enclosed spaces like basements and houses.
Radon-222 has a relatively short half-life of 3.82 days, but its subsequent decay products are solid and chemically active. These products attach to dust particles that can be inhaled. Once lodged in the lungs, the alpha particles emitted by these decay products deliver a localized dose, making radon the single largest contributor to the public’s natural radiation exposure. The concentration of uranium and thorium in local geology dictates the amount of radon produced, explaining why certain geographical regions have higher background radiation levels.
Radiation from Space and Within the Body (Non-Terrestrial Background)
Beyond the Earth’s crust, another major source of natural exposure originates from space, known as cosmic radiation. This radiation consists of high-energy charged particles, primarily protons and atomic nuclei, that constantly bombard the Earth from the sun and distant galactic sources. When these primary cosmic rays enter the atmosphere, they collide with gas molecules, creating a cascade of secondary particles like neutrons, muons, and photons that reach the surface.
The Earth’s magnetic field and dense atmosphere provide significant shielding, but exposure increases with altitude because less air is overhead to absorb the radiation. Individuals who live in high-elevation cities or frequently fly are exposed to a higher dose of cosmic radiation. Exposure at typical commercial aircraft altitudes is significantly greater than at sea level, making cosmic rays a consideration for airline crew members.
A final natural source of radiation is found within our own tissues, primarily from the isotope Potassium-40 (\(^{40}\)K). Potassium is a biologically essential element necessary for cell function, and approximately 0.012% of all natural potassium is the radioactive \(^{40}\)K isotope. With a half-life of 1.251 billion years, this isotope decays by emitting beta particles and gamma rays, contributing a consistent internal radiation dose to all soft tissues. The body tightly regulates potassium levels through homeostasis, meaning the concentration of \(^{40}\)K remains nearly constant throughout a person’s life, regardless of diet.
Sources Generated by Human Activity (Anthropogenic Uses)
Human activity has introduced additional sources of radiation into the environment, with medical procedures being the largest contributor to the public’s man-made radiation dose. Diagnostic tools like X-rays and Computed Tomography (CT) scans do not use radioactive material but rather generate ionizing radiation by accelerating electrons onto a target inside a vacuum tube. The X-rays produced pass through the body to create images, with CT scans delivering a higher dose because they generate multiple cross-sectional images from a rotating source.
Other medical applications involve nuclear medicine, which administers small amounts of radioactive isotopes directly into the body as radiopharmaceuticals. For diagnostic imaging, the most common isotope is Technetium-99m (\(^{99m}\)Tc). This isotope is chemically bound to tracer molecules that target specific organs or tumors and emit detectable gamma rays.
For therapy, stronger beta- or alpha-emitting isotopes are used. Examples include Iodine-131 (\(^{131}\)I) for treating thyroid conditions or Lutetium-177 (\(^{177}\)Lu) for neuroendocrine tumors, delivering a high, localized dose of radiation.
Beyond the medical field, many consumer and industrial products contain radioactive material. For instance, most household ionization-type smoke detectors contain a small source of Americium-241 (\(^{241}\)Am), a synthetic element created from the decay of plutonium. This isotope emits alpha particles that ionize the air between two electrodes. When smoke enters the chamber, it disrupts this current, triggering the alarm.
Residual radiation from historical events, such as global fallout from atmospheric nuclear weapons testing and nuclear power accidents, has distributed isotopes like Cesium-137 (\(^{137}\)Cs) across the globe. Cesium-137, with a half-life of 30.17 years, is a long-term contaminant that binds to soil and is taken up by plants, though its contribution to the average annual dose is very small.