Radon gas is the result of the radioactive decay of radium-226, an element that can be found in varying concentrations throughout many soils and bedrock. Figure 31.1 shows the series of elements that begins with uranium-238, and, after undergoing a series of radioactive decays, leads eventually to lead-210. At the time radium decays to become radon gas, energy is released.9 Of all the elements
and isotopes illustrated in Figure 31.1, radon is the only one that behaves like a gas and can easily slip through the small spaces between bits of soil. While many of the isotopes in the uranium-238 decay series exist for a long time before they decay, radon does not remain radon for very long. It has a half-life of 3.8 days. If 1 lb of radon were put in a jar, 3.8 days later, only 1/2 lb of radon would be left; the other 1/2 lb would have decayed into short-lived decay products, namely, polonium,
bismuth, and lead. After another 3.8 days, only 1/4 lb of radon would be left in the jar. The radon decay products shown inside the building have even shorter half-lives than radon, and decay within a few hours to the relatively stable isotope lead-2l0. It is this rapid release of energy that causes radon and radon decay products to pose such a significant health risk.
If radon and radon decay products are present in the air, they will be inhaled. Because the decay products are not gases, they will stick to lung tissue or larger airborne particles that later lodge in the lung. The energy given off as these isotopes decay can strike the cells in the lung, damage tissue, and may eventually develop into lung cancer. The amount of risk depends on how long a person is exposed to how high a concentration of radon and radon decay products. Estimates of the number of lung cancer deaths in the United States attributable to radon and radon decay products range from 5000 to 20,000 deaths per year.9 In all, 1.2 million new homes have been built with radon-resistant features since 1990. U.S. EPA continues to focus its risk reduction on mitigating existing homes and building new radon-resistant homes. As a result of these actions through 2003, U.S. EPA estimates that as many as 650 future lung cancer deaths are prevented (lives saved) each year.10
31.2.2 Radon Entry into Buildings A house will contain radon if the following four conditions exist:
1. A source of radium exists to produce radon.
2. A pathway exists from the radium to the building.
3. A driving force exists to move the radon to the building.
4. An opening in the house exists to permit radon to enter.
If one of these conditions does not exist, then the building will not have a radon problem. An estimated 10-20% of the existing homes in the United States have annual average radon concentrations above 4pCi/L. This may seem like a small percentage of problem homes until one considers that, of the million or so U.S. houses built each year, 100,000-200,000 homes will likely have radon concentrations higher than 4pCi/L. Similar radon problems do exist in commercial and industrial buildings with basements.
The most common way radon enters a building is when lower indoor air pressure draws air from the soil, bedrock, or drainage system into the house. If there is radon in the soil gas, it will also be drawn in. Just as gravity will make water flow from a high elevation to a lower elevation, pressure differences will make radon-laden air move from an area of higher pressure to an area of lower pressure. For a variety of reasons, most buildings tend to maintain an indoor air pressure lower than outdoor air pressure. If cracks and holes in the foundation are open to the soil, radon will be drawn indoors. Radon movement by pressure differences is called pressure-driven transport.
Radon can also enter buildings when there are no pressure differences. Place a drop of food coloring in a glass of water; eventually, the coloring will spread out (diffuse) and color the water— even without stirring. Radon will do the same thing—spread from an area of higher concentration to an area of lower concentration until the concentrations are equal. Radon movement in this way is called diffusion-driven transport.
A less common entry mechanism is the outgassing of radon from well water. A well supplied by groundwater that is in contact with a radium-bearing formation can transport the dissolved radon into the home. It is estimated that the health risks associated with breathing radon gas released from the water are 10 times higher than the risks associated with ingesting water containing radon.9
Radon can also emanate from the building materials themselves. The extent of the use of radium-contaminated building materials is unknown but is generally believed to be small.
Figure 31.2 illustrates the percentages of contribution by each type of radon entry made to a specific group of study houses in the Pacific NW.11 Any one house can vary significantly from these figures. However, on a national basis, this is an indication of the relative importance of each of the contributors.
Figures 31.3 through 31.5 illustrate typical radon entry routes found in basement, crawlspace, and slab-on-grade construction, respectively.
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