What Is Radon?

Radon is a naturally occurring radioactive gas with atomic number 86 and chemical symbol Rn, classified as a noble gas and the heaviest naturally occurring member of that group. It forms as an intermediate product in the uranium-238 decay series, produced when radium-226 undergoes alpha decay. Because it is a gas at room temperature, radon migrates readily through porous soil and rock, making it the primary mechanism by which natural radioactivity from the ground reaches inhabited spaces. Radon-222, the isotope of primary health and regulatory concern, has a half-life of 3.82 days before decaying into a sequence of short-lived radioactive progeny that can be inhaled and deposited in lung tissue.

Radon occupies a central position in radiation protection science because it constitutes the single largest source of natural background radiation exposure for most of the world's population. Its recognition as a public health hazard developed progressively through the twentieth century, first from studies of uranium miners who developed lung cancer at elevated rates, and later from epidemiological investigations in residential settings.

Physical Properties and Decay

Radon is colorless, odorless, and chemically inert under normal conditions, properties that make direct sensory detection impossible. Its decay chain proceeds through polonium-218, lead-214, bismuth-214, and polonium-214 before reaching lead-210, a longer-lived intermediate. The immediate decay products are electrically charged solids that attach to airborne dust particles and can be inhaled and deposited on bronchial epithelium, where the alpha particles they emit cause ionizing damage to DNA. The energy of the alpha particles emitted by radon-222 and polonium-218 ranges from approximately 5.5 to 7.7 MeV. A review of radon health hazard studies published in PMC surveys the epidemiological evidence linking radon progeny to lung cancer at residential exposure concentrations. Radon dissolves in water and can be released from groundwater during showering or other household uses, adding a secondary exposure route beyond soil gas entry.

Indoor Accumulation

Radon enters buildings primarily through pressure-driven transport: the interior air pressure in a building is typically slightly lower than soil gas pressure beneath the foundation, drawing radon upward through cracks, joints, and unsealed penetrations in floors and basement walls. The resulting indoor concentration depends on soil permeability, the uranium and radium content of underlying geology, building construction type, and ventilation rate. The U.S. Environmental Protection Agency has established guidance on radon health risk and recommended mitigation action levels, recommending that homeowners address radon when measurements reach 4 pCi/L (148 Bq/m³) or above. The EPA estimates that radon exposure causes approximately 21,000 lung cancer deaths annually in the United States, making it the second leading cause of lung cancer after smoking. Indoor radon concentrations vary widely, from less than 1 Bq/m³ in well-ventilated structures to hundreds or thousands of Bq/m³ in poorly sealed basements in high-radon geology.

Detection and Mitigation

Radon testing is the only reliable way to determine indoor exposure because the gas is imperceptible to human senses. Short-term charcoal canister tests, deployed for two to seven days, provide an initial screening result; long-term alpha track detectors, placed for 90 days or more, yield a more representative annual average. Both test types are passive, inexpensive, and widely available. The CDC provides guidance on radon testing and remediation through its environmental health programs. Mitigation typically involves sub-slab depressurization: a pipe inserted through the foundation slab connects to a small fan that draws radon-laden soil gas from beneath the structure and exhausts it above the roofline, bypassing interior spaces. This approach routinely reduces indoor concentrations by 50 to 99 percent.

Applications

Knowledge of radon's behavior has applications in a range of technical and scientific areas, including:

• Radiation protection and building codes, where radon-resistant construction techniques are specified for new construction in high-risk regions
• Geoscience and mineral exploration, where elevated soil radon concentrations indicate underlying uranium and thorium deposits
• Seismology research, where anomalous radon flux in groundwater and soil has been studied as a potential precursor to seismic events
• Epidemiology, where residential radon exposure data inform lung cancer risk models used by regulatory agencies worldwide