Radon is a naturally occurring, invisible, odorless, and radioactive gas that originates deep within the Earth’s crust. It is a direct byproduct of the natural radioactive decay of uranium, an element found almost universally in rocks and soil. The continuous breakdown of uranium-238 is the primary source that generates the radon found globally. This decay process is responsible for the presence of radon in the ground.
The Mechanism: Uranium’s Path to Radon Gas
The creation of radon gas is initiated by the radioactive decay of Uranium-238, which begins a long series of atomic transformations. Uranium-238, with a half-life measured in billions of years, slowly breaks down into a chain of shorter-lived radioactive elements. The key element in this lengthy sequence is Radium-226, the immediate parent material to Radon-222.
Radium-226 is a solid, radioactive element with a half-life of about 1,600 years, ensuring a persistent source of its gaseous offspring. The transformation from solid radium to gaseous radon occurs through alpha decay. During this decay, the Radium-226 atom ejects an alpha particle, forming a new Radon-222 atom.
Radon is a noble gas, making it chemically inert; it does not bond with surrounding minerals. The recoil energy from the alpha decay physically propels the gaseous atom out of the mineral grain and into the pore spaces within the soil or rock. This process, known as emanation, allows the mobile radon atom to travel through the ground.
Geological Factors Influencing Radon Presence
The concentration of uranium and radium determines the potential for radon production in a specific area. These parent materials are naturally concentrated in certain types of bedrock, such as granite, shales, and phosphate rock. High levels of uranium are found in granitic areas, which often exhibit deep weathering profiles that further facilitate radon release.
The amount of radon that reaches the surface is not solely dependent on the uranium concentration in the ground. The permeability of the soil and rock is an equally important factor, as it dictates how easily the gas can migrate. Highly permeable materials, such as coarse sands, gravels, or fractured bedrock, allow the radon gas to move freely through the subsurface.
Conversely, dense, low-permeability materials like clay or uncracked shale may trap the gas, limiting its escape to the atmosphere. Therefore, an area with moderate uranium content but highly permeable soil may pose a greater risk than an area with high uranium content in dense, impermeable rock. Localized geological features, including natural faults and fractures, also act as pathways that channel radon toward the surface.
How Radon Enters and Accumulates in Structures
Radon that has emanated from the soil and rock moves into buildings due to pressure differences between the indoor air and the soil gas. Homes typically operate at a slight negative pressure relative to the surrounding soil, especially at the foundation level. This pressure differential acts like a vacuum, actively drawing soil gas, including radon, into the structure.
A phenomenon known as the “stack effect” often intensifies this negative pressure, particularly during colder months. As warm air inside the building rises and escapes through upper-level openings, it draws replacement air from the lowest portions of the structure. This suction pulls the radon-laden soil gas into the home through any opening in the foundation.
Radon gas enters the living space through various pathways connecting the house to the underlying soil. Common entry points include:
- Cracks in concrete slabs and foundation walls.
- Construction joints.
- Gaps around utility penetrations like water pipes and electrical conduits.
- Sump pits and hollow block walls.
In homes that use private well water drawn from fractured bedrock, the water itself can also contribute to the indoor radon concentration.
Why Radon Exposure is a Health Concern
Radon gas is a recognized human carcinogen and is considered the leading cause of lung cancer among non-smokers, ranking second overall only to cigarette smoking. When inhaled, the gas itself is largely exhaled due to its inert nature. The danger arises from the short-lived radioactive decay products, often called “radon progeny,” that form when radon atoms break down.
These progeny are solid, electrically charged particles that readily attach to dust, aerosol droplets, and other airborne particles. When a person breathes in these contaminated particles, they become lodged in the lining of the lungs and bronchial tubes. Once deposited, the progeny continue their rapid decay, emitting high-energy alpha radiation directly into the sensitive lung tissue.
Alpha particles cause intense ionization over a very short range, damaging the double-stranded DNA within the lung cells. This cellular damage can lead to genetic mutations that may result in the development of malignant tumors. To mitigate this risk, public health organizations recommend reducing indoor radon levels when concentrations exceed specific guidelines.