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| Radon |
Radon is an odorless, colorless, radioactive, though chemically unreactive gas. It has an atomic number of eighty-six, which corresponds to the number of protons found in the nucleus of any isotope of radon.
There are more than thirty known isotopes of radon, and each one emits some combination of alpha, beta, and gamma radiation when undergoing radioactive transformation, commonly referred to as “decay.”
Radon gas is ubiquitous in the natural environment. This is because the precursors to radon, such as the aforementioned radium isotopes, and others such as radium, thorium, and uranium isotopes, are present in some rock formations. Radon is also found in the man-made environment because many of the materials, consumer products, and foodstuffs of everyday life come from the naturally radioactive environment.
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Radon is one of the few examples in nature of a gaseous element that results from the decay of a solid element and then decays into another solid element. This increases its potentially harmful effect in humans. For example, radon-222, the most common isotope of radon, is a product of the alpha decay of radium-226 atoms, found in rocks. Radon-222 atoms subsequently produce polonium-218 in a similar alpha-decay process, and it is this solid substance that can lodge in human tissue.
Solid-state radionuclides remain where created by decay processes unless they are redistributed by dissolving in groundwater or by becoming airborne. Given the chemically inert nature of radon, there are no known compounds that include this element. Thus isotopes of radon may diffuse away from their place of origin and usually end up dissolved in ground water or mixed with air above the soil and rocks that bear their solid precursors.
People’s exposure to radon primarily occurs when radon seeps out of air spaces above soil or rocks and into surrounding indoor or outdoor air, such as the basements of houses built over radium-bearing rocks.
It is not exposure to radon gas that actually may lead to harm, but exposure to the decay products of radon, specifically the ones with short half-lives that emit alpha radiation. Radon-222 offspring, like polonium-218 and polonium-214, become attached to dust particles that may be breathed in by people exposed to the gas and become lodged in the respiratory tract.
Decay of the radon progeny while in the lungs is the means by which the radiation dose is delivered to the lungs. This dose, which is the energy of alpha particles absorbed by cells that line the lungs, is what gives rise to the potential for lung cancer associated with exposure to radon.
Radon has been labeled by the Environmental Protection Agency as the second-leading cause of lung cancer in humans (after tobacco smoke), based on mathematical risk estimates derived from many published studies of exposure of subsurface uranium miners to highly elevated levels of the gas, primarily radon-222.
Many radiation health scientists have challenged such findings because of the vast difference in exposure levels between homes and buildings on the one hand, and subsurface mines on the other. However, a variety of action levels and exposure limits for radon gas exposure have been recommended or set into law for the protection of the public.
The Surgeon General and the EPA recommend that radon levels of four picocuries or more inside homes be reduced. The EPA states that radon levels less than four picocuries still pose a risk, especially for smokers.
Methods to both detect and mitigate indoor radon exposure have been devised as well. Detection and measurement methods usually make use of a device to collect radon gas atoms or the offspring particles.
The simplest real-time method would be a “grab sample,” in which air is drawn into an evacuated flask that is then taken back to a laboratory for analysis. The most popular short-term measurement device is the activated charcoal canister, a small container of steam-treated charcoal that is opened and left at the sampling location for a prescribed time.
Radon is adsorbed by the charcoal, and the decay products of the radon are analyzed after the canister has been resealed and retrieved. The simplest mitigation methods include sealing cracks and penetrations through foundations, as well as diverting the radon away from the slab or out of the ground, with vacuum or ventilation systems.
