|Fig. 1: A geologic fault with embedded granite seams. Faults are a common source of elevated radon levels. (Source: Wikimedia Commons)|
Radon represents an invisible and thus potentially dangerous source of radiation in domestic settings. The gas can occur in surprising places, and its prediction and detection either requires specialized equipment or access to geological surveys. Of particular concern is the tendency of radon to accumulate in the basements of homes that are built near naturally-occuring granite seams, because mitigating this risk requires active preventive measures on the homeowner's part to prevent continuous radon buildup during the life of the home. 
It has been estimated that at least 1 in every 15 U.S. homes has potentially toxic levels of radon in at lease one part of the dwelling.  This is a high enough number that radon alone may be responsible for a detectable fraction of the total number of lung cancer incidents in the U.S, since radon's toxicity primarily targets respiratory tissues. In fact, radon may be responsible for up to half of the yearly background-radiaton exposure experienced by an average U.S. citizen. 
Unlike most other commonly-known radioactive substances, terrestrial radon occurs primarily in the form of a noble gas, and so it is able to seep out of subterranean radioisotope deposits and reach the earth's surface, presenting a significant health hazard. Because it is a noble gas, it is chemically inert and thus unlikely for form compounds that would fixate it as a solid in the terrestrial environment. Instead, it is most likely to decay into further toxic decay products, such as lead and polonium, which tend to form solids under ambient conditions.  This last step makes radon particularly dangerous, as the microparticles of lead and other decay products resulting from the decay of gaseous radon can create microdeposits in living tissues that cause further health effects beyond the initial damage caused by fission.
As the above mechanism suggests, the primary risk factor associated with radon exposure is the development of lung cancer. Gas exchange within the respiratory system gives the gas a particularly long residence time within the body, which increases the amount of time in which it could potentially decay into a toxic byproduct.
A simple calculation can be used to determine whether the radon release rate in a given geological region is sufficient to cause toxic levels of the isotope to develop in a the basement of a household. This calculation assumes that emitted radon does not freely dissipate upon entering a dwelling, resulting in a gradual buildup timescale that is determined by the rate at which air from the interior of the dwelling exchanges with that outside the dwelling.
The EPA-recommended threshold for safe radon levels is 4 pCi per liter of air inside the home.  The "Ci," or Curie, is simply an SI-derived unit for the number of radioactive decays occuring per second---each decay event is associated with at least one fast-moving decay product that can damage tissue, and so areas with a larger number of Ci/L are more likely to be hazardous.
In a moderately active geological site, the flux of radon emanating from a given flat, porous surface is roughly 1 pCi m-2 sec-1.  An average home has a basement that is mostly separated from the rest of the home, leading to low air circulation and thus reduced dissipation of accumulated radon. Assuming that air exchanges with the outside world over a timescale of 2-3 hours, radon would exchange at a slower rate proportional to the square root of its mass relative to oxygen. This leads to a timescale for radon escape from the basement of approximately ten hours. Suppose that the basement of a house is unpaved and has an an average size of 10 m × 10 m = 100 m2 and a height of 3 m. We have
Radon Level = Radon Density/(Total Air Volume) = 1 pCi m-2 sec-1) × 100 m2 × (10 hr × 60 min/hr × 60 sec/min
100 m2 × 3 m × 1000 L m-3
= 12 pCi/L
Thus a well-sealed, large basement that borders a radon-rich geological site (such as a granite seam) is likely to accumulate low, but toxic levels of radon unless proper mitigation methods are used.
As the above calculation suggests, homes in regions with high radon density should take proper precautions to prevent the accumulation of radon in toxic amounts. One option would be to invest in active dissipation methods, such as ventilation systems, that prevent the buildup of radon particles. In the above calculation, if the system can increase circulation by a factor of 3, it can bring radon levels back down to safe concentrations.
Alternatively, the effective surface area through which radon effuses from the earth into the home can be decreased by treating the walls and floor of the basement with a coating that is impermeable to radon. Simple concrete paving will reduce the permeability of the floor, although specialized products exist that are specifically designed to seal structures from radon. 
Individuals who live in areas with particularly high geological radon concentrations, or who live near anthropogenic radioactivity sources like the remnants of the Chernobyl nuclear plant or atomic testing sites, can consider investing in a radon detector. The device resembles a smoke detector, and it can monitor and report background radon levels, as well as alert occupants when the background level spikes unexpectedly (such as due to sudden release from subterranean geological resettlement). 
© William Gilpin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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