Terrestrial radiation is a natural, ever-present source of ionizing radiation originating from the Earth itself. This energy radiates from naturally occurring radioactive materials (NORMs) embedded within the planet’s soil, rocks, and water. Terrestrial radiation is a primary contributor to the background radiation dose experienced by every person on Earth. The concentration of these materials determines the baseline radiation exposure for any given location.
The Radioactive Elements in Earth’s Crust
The sources of terrestrial radiation are three primordial radionuclides: Uranium-238, Thorium-232, and Potassium-40. These elements have existed since the Earth formed and possess extremely long half-lives. Uranium-238 and Thorium-232 initiate long decay chains, producing a series of shorter-lived radioactive elements as they break down. Potassium-40 is a standalone radionuclide that decays directly.
The concentration of these elements varies significantly based on local geological composition. Areas with granitic rock or phosphate deposits often exhibit higher levels of natural radioactivity. Conversely, regions with sedimentary rocks like limestone generally show lower concentrations. This geological variability means the natural background radiation dose differs substantially across regions.
Types of Ionizing Radiation Emitted
The decay of terrestrial elements releases energy in three principal forms: alpha particles, beta particles, and gamma rays.
Alpha Particles
Alpha particles are relatively heavy and interact strongly with matter. They have the lowest penetration power, stopping completely after traveling only a few centimeters in air or being blocked by a simple sheet of paper.
Beta Particles
Beta particles are high-energy electrons that are more penetrating than alpha particles, able to travel several meters in air. A thin layer of material, such as aluminum foil, is typically sufficient to stop beta radiation.
Gamma Rays
Gamma rays are high-energy electromagnetic waves. Their lack of electrical charge allows them to penetrate deeply through matter, making them the most penetrating type of terrestrial radiation. Significant shielding, such as thick concrete or lead, is required to substantially reduce exposure.
Radon Gas: The Primary Pathway of Exposure
Radon gas is the largest contributor to the terrestrial radiation dose received by the general public because of its ability to accumulate indoors. This odorless, colorless gas is a product in the decay chain of Uranium-238, forming from Radium-226 found in soil and rock. The gas is released from the ground and moves through porous soil.
Radon rises from the soil beneath a structure and enters the building through openings in the foundation. Entry points include cracks in concrete slabs and gaps around utility pipes. The main driving force is the difference in air pressure, where the lower pressure inside a home creates a vacuum effect that actively draws the gas inward.
Once trapped indoors, the radon concentration can build up. While the gas itself is largely exhaled, its short-lived decay products, known as radon progeny, are solid particles that attach to dust. These radioactive particles are inhaled and deposit in the lungs, where they emit radiation that damages tissue.
Quantifying and Monitoring Terrestrial Dose
The biological effect of radiation exposure is measured using the Sievert (Sv). The smaller unit, the millisievert (mSv), equal to one-thousandth of a Sievert, is commonly used for typical doses. This measurement, known as the effective dose, accounts for the energy absorbed by the body and the biological harm caused by different radiation types. The average annual background dose from all natural sources is estimated to be a few millisieverts.
Monitoring terrestrial exposure focuses on testing for indoor radon accumulation, which is the most variable and controllable part of the dose. Inexpensive testing kits measure the average radon concentration, providing an estimate of indoor exposure. If testing reveals elevated levels, mitigation strategies are implemented to reduce the inhalation dose.
Effective mitigation often involves installing a sub-slab depressurization system, which uses a fan and piping to draw radon gas from beneath the foundation and vent it safely outside. Sealing major entry points, such as foundation cracks, also helps block the gas from entering the living space.