Radon () is a naturally occurring radioactive gas produced from the decay of uranium in soil and rocks. While radon itself is an alpha emitter with a relatively long half-life ( days), the primary health hazard arises from its short-lived solid decay products, often referred to as radon progeny or radon daughters. These progeny are isotopes of polonium, lead, and bismuth, which readily attach to airborne dust particles and aerosols, becoming easily inhalable.
Radon undergoes alpha decay to Polonium-218, initiating a series of short-lived decay products:
These short half-lives mean that once inhaled, these particles decay rapidly within the lungs, depositing significant alpha radiation dose to lung tissue, which is the primary cause of radon-induced lung cancer.
In an enclosed space, the concentration of radon decay products builds up over time, eventually reaching a state of secular equilibrium with the radon gas itself. However, due to air movement, ventilation, and plate-out (deposition on surfaces), this equilibrium is rarely complete. The 'equilibrium factor' () quantifies the degree of disequilibrium, defined as the ratio of the equilibrium equivalent radon concentration () to the actual radon concentration ():
Typically, values in indoor environments range from to , indicating that the activity of progeny is usually less than that of the parent radon.
When is formed from radon decay, it recoils as a positively charged free ion, typically referred to as the 'unattached' fraction. These free ions are highly mobile and have a very small physical diameter (nanometre scale). Within seconds, these unattached ions tend to attach to airborne aerosol particles (e.g., dust, smoke, water droplets), forming the 'attached' fraction, which has a larger diameter (micrometre scale).
The distinction between attached and unattached fractions is critical for radiological protection and dose assessment:
Effective radiation protection strategies for radon focus on reducing overall radon concentrations through improved ventilation, sealing entry points, and filtration to minimise both fractions, particularly the more hazardous unattached component.