Radon is a naturally occurring, colorless, odorless, and chemically inert radioactive gas. This gas is a continuous product of the earth’s geology, making it present in varying concentrations across the globe. The accumulation of radon in enclosed spaces is a concern because the subsequent decay of its particles is the second leading cause of lung cancer. Understanding how this gas is generated is key to addressing the risk it presents.
Uranium The Primary Source
Radon is a byproduct of the natural breakdown of heavier, primordial elements. The primary source for the most common radon isotope, Radon-222, is Uranium-238 (U-238). This long-lived radioactive material has a half-life of 4.5 billion years, which ensures a continuous supply of its decay products. U-238 is universally present in trace amounts within nearly all rocks, soils, and granite formations. Concentrations vary widely depending on the local geology, with higher levels often found in igneous rocks like granite, metamorphic rocks, and shale.
The Step by Step Decay Process
The creation of Radon-222 occurs within the long sequence known as the Uranium-238 decay chain. The U-238 atom undergoes several intermediate steps, including the formation of Thorium and Protactinium isotopes, before it reaches the immediate precursor to radon. The immediate precursor in this process is Radium-226, which has a much shorter half-life of about 1,600 years compared to its parent uranium. The production of Radon-222 gas occurs when a Radium-226 atom decays by emitting an alpha particle. This transforms the solid Radium-226 into the gaseous Radon-222, providing the gas with enough recoil energy to potentially escape the solid rock matrix where it originated. The resulting Radon-222 isotope is short-lived, with a half-life of only 3.8 days, making it the most significant gaseous intermediary in the decay chain. This short life span means the gas must quickly move through the soil before it decays into its solid, highly radioactive progeny, such as Polonium-218.
Movement from the Earth to the Atmosphere
Once created, the Radon-222 atom is chemically inert and does not readily form chemical bonds with the surrounding soil or rock material. This inertness allows the gas to move freely through the pores and fractures within the earth. The rate at which the gas can travel is heavily dependent on the permeability of the underlying geological materials.
Highly permeable soils, such as coarse gravel or sand, allow radon gas to migrate quickly and over greater distances. Conversely, dense clay or unfractured bedrock significantly restricts the movement, trapping the gas before it can travel far. The main mechanism for transport is the convective flow of soil gas, where differences in air pressure drive the movement.
This pressure-driven flow is pronounced near buildings, where the “stack effect” often develops, particularly during cold weather. Warm indoor air rises and escapes through the upper levels of a structure, creating a slight vacuum or lower pressure zone at the foundation level. This pressure differential actively draws soil gas, and the radon it contains, from the ground and into the building through cracks, floor drains, and utility penetrations.
Once the gas escapes the ground, it is rapidly diluted in the outdoor atmosphere. However, when it enters an enclosed space like a basement, the lack of ventilation allows the gas to accumulate to elevated levels, concentrating the risk to occupants.