Uranium and radon are naturally occurring radioactive elements. Radon is a heavy gas that is an intermediate product of uranium decay. Exposure to uranium and radon can have health consequences, with radon being a more serious hazard. Radon levels in homes depend on various factors, and a level of 4 pCi/L or higher is considered a health risk.
Uranium and radon are naturally occurring radioactive elements. Radon is a heavy gas that is an intermediate product of uranium decay. The uranium present in rocks undergoes a sequence of alpha and beta decay through a series of unstable isotopes of various elements, finally ending with a stable isotope of lead. The rock contains small but variable amounts of uranium, granite with relatively high levels. For this reason, uranium and radon levels tend to be high in areas with granite bedrock; however, other factors affect the actual levels of radon gas reaching the surface.
Radon production results from the decay of radium-226, a heavier isotope in the sequence starting with uranium-238, by the emission of an alpha particle. The isotope created by this process is radon-222, which is itself unstable and emits an alpha particle, forming polonium-218, the next isotope in the sequence. Radon-222 has a half-life of 3.8 days, which means that after this period half of a radon sample will have decayed.
Exposure to uranium and radon can have health consequences. In contrast to the “enriched” uranium used in nuclear reactors, natural uranium-238 is only slightly radioactive. Under normal circumstances it poses no threat, although it is chemically toxic if ingested. Radon is a more serious hazard as it is highly radioactive and, as a gas, can easily leach out of bedrock and build up in homes, where it can be inhaled.
Both uranium and radon produce alpha particles. Although these particles cannot penetrate the skin, if they come into contact with living tissue they can cause damage. When DNA is damaged it can lead to cancer. If radon is inhaled, it is likely to come into contact with lung tissue, and long-term inhalation of radon in homes can lead to chronic lung disease and lung cancer. Most lung cancers are caused by smoking, but radon is the second leading cause.
Radon levels in homes depend on a number of factors besides the uranium content of the bedrock. While the gas will tend to rise to the surface due to pressure differences, it can do so more rapidly if there are fractures in its source rock and if the soil and subsoil above it are highly permeable. Moisture, however, slows its progress as it can dissolve in water. Once on the surface, it can be dragged into houses, especially if the foundations are not well sealed, due to the rise of heated internal air, and accumulate due to limited circulation.
The unit of natural radioactivity is usually the picocurie (pCi), which is roughly equivalent to the decay of two atoms per minute. Radon levels in air are usually expressed as picocuries per liter of air (pCi/L). A typical value for outdoor air is around 0.4 pCi/L, but indoor levels tend to be higher, with 1.3 pCi/L considered average. A level of 4 pCi/L or higher is considered a health risk.
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