Radon is a naturally occurring radioactive gas that forms from processes within the Earth’s crust. It is a colorless, odorless, and tasteless noble gas, making it impossible to detect without specialized equipment. Its presence indoors is a product of natural geological phenomena.
Uranium as the Source
Radon originates from the natural radioactive decay of heavier elements found in rocks and soil. The primary parent element in this process is Uranium-238 (U-238). Uranium-238 is present in varying concentrations throughout the Earth’s crust, found in most rocks, soil, and even water. This presence of uranium is a continuous, natural geological occurrence.
It occurs naturally in low concentrations, typically a few parts per million, in soil, rock, and water. The widespread distribution of uranium ensures a continuous, albeit slow, production of radon globally.
The Radioactive Decay Process
Radon-222 (Rn-222), the most stable isotope of radon, is produced through a series of steps in the Uranium-238 decay chain. Radioactive decay is a process where an unstable atomic nucleus loses energy by emitting radiation. This chain begins with Uranium-238, which undergoes several transformations to eventually become Radium-226 (Ra-226).
Radium-226 then decays directly into Radon-222. This specific decay involves the emission of an alpha particle from the Radium-226 nucleus. The release of this alpha particle transforms the radium atom into a radon atom. Radon-222 has a half-life of about 3.8 days, meaning half of a given amount will decay in that time.
The gaseous nature of radon is a critical factor in its mobility. Unlike its parent elements, uranium and radium, which are fixed in solid rock and soil, radon is a gas that can move freely. This allows it to escape from the mineral grains where it is formed and enter pore spaces in the surrounding soil.
Movement from Ground to Buildings
Once formed in the soil and rock, radon gas can move through porous soil, cracks in bedrock, and even water. Its gaseous state enables it to travel greater distances than its solid precursors. Radon can enter buildings through various entry points where the structure contacts the ground.
Common entry points include:
Cracks in concrete slabs
Gaps around utility pipes
Floor-wall joints
Pores and cracks in concrete blocks
Exposed soil in sumps or crawl spaces
Water supplies, particularly well water
A significant factor in radon entry is the pressure difference between the soil and the indoor air. Homes often have slightly lower indoor air pressure compared to the soil beneath, creating a vacuum effect that draws radon gas inward. This pressure differential is influenced by factors like warm air rising within a building, exhaust fans, and wind.
What Influences Levels
Radon levels can vary significantly from one location or building to another due to a combination of geological and building-specific factors. Geological factors include the concentration of uranium and radium in the underlying soil and rock. Areas with higher concentrations of these parent elements generally have a greater potential for elevated radon levels.
Soil permeability also plays a role, as radon moves more easily through permeable soils like coarse sand and gravel than through less permeable clays.
Building characteristics further influence indoor radon concentrations. These include the type of foundation (basement, crawl space, or slab-on-grade), the integrity of the foundation (presence of cracks or gaps), and the effectiveness of ventilation systems. Seasonal changes and weather conditions, such as atmospheric pressure fluctuations, temperature, and precipitation, can also impact indoor radon levels.