Radon (Rn) is a naturally occurring element with the atomic number 86, distinguished as a radioactive noble gas. It is the only noble gas chemically generated through radioactive decay, specifically as a product of the uranium decay chain found in soils and rocks. Radon is a colorless, odorless, and tasteless gas, making detection challenging. Because it is radioactive and can accumulate in indoor environments, understanding its properties and origin is important for public health.
Radon’s Position on the Periodic Table
Radon’s designation as element 86 places it in Group 18 and Period 6 of the periodic table. Group 18 is known as the noble gases, which also includes elements like helium, neon, and argon. This group is characterized by a full outer electron shell, which gives the elements an extremely low level of chemical reactivity. Its position in Period 6 indicates that a radon atom has six electron shells, accounting for its relatively large size and high standard atomic weight of approximately 222. This placement allows radon to escape the solid rock and soil lattice where its parent elements are chemically bound.
Fundamental Physical and Chemical Properties
Radon is the heaviest known gas under standard conditions, with a density about 7.5 times greater than that of air. This high density contributes to its tendency to accumulate in lower, enclosed areas like basements and crawl spaces.
The most significant property of radon is its inherent radioactivity. The most stable and common isotope, Radon-222, has a relatively short half-life of 3.82 days. Radon atoms decay by emitting an alpha particle, which transforms the radon atom into a series of short-lived radioactive decay products, often called radon progeny.
The Origin of Radon in the Environment
Radon is continuously created through a specific natural process called a decay chain. The entire process begins with Uranium-238, an element naturally present in trace amounts in the Earth’s crust. Uranium-238 undergoes a long series of radioactive transformations, eventually decaying into Radium-226. Radon-222 is the immediate decay product of Radium-226, which is found in common materials like granite, shale, and phosphate rocks. Once formed, its noble gas nature allows it to escape the solid matrix of the soil and rock. The gas then migrates through air pockets and cracks in the soil, eventually being released into the atmosphere. If a structure is built on top of this ground, the gas can seep through foundation cracks, utility openings, and porous concrete, leading to accumulation in indoor air.
Health Risks and Detection Methods
Health Risks
The primary health concern associated with radon exposure is an increased risk of lung cancer. When inhaled, radon gas rapidly decays into solid, radioactive particles known as radon progeny, which attach to dust and aerosols. These decay products become lodged in the lining of the lungs, where they release alpha radiation that damages the DNA of lung cells. This process can lead to the uncontrolled cell growth characteristic of cancer. Radon is considered the second leading cause of lung cancer globally, and the leading cause among non-smokers.
Detection and Mitigation
The only way to determine the concentration in a home or building is through testing. Residential testing typically involves either short-term kits, measuring levels over a few days, or long-term kits, used for 90 days or more to provide a more accurate annual average. If testing reveals elevated levels—the U.S. Environmental Protection Agency (EPA) recommends fixing levels of 4 picocuries per liter (pCi/L) or higher—mitigation is necessary. The most common and effective mitigation technique is sub-slab depressurization, where a system of pipes and a fan draws radon gas from beneath the foundation and vents it outside. Sealing major cracks in the foundation is also a component of the overall mitigation strategy.