A poisonous gas that is produced as a product of
radioactive decay product of the uranium decay series. Radon
is a heavy gas, and it presents a serious indoor hazard in
every part of the country. It tends to accumulate in poorly
ventilated basements and well-insulated homes that are built
on specific types of soil or bedrock rich in uranium minerals.
Radon is known to cause lung cancer, and since it is an odorless,
colorless gas, it can go unnoticed in homes for years.
However, the hazard of radon is easily mitigated, and homes
can be made safe once the hazard is identified.
Uranium is a radioactive mineral that spontaneously
decays to lighter “daughter” elements by losing high-energy
particles at a predictable rate, known as a half-life. The halflife
specifically measures how long it takes for half of the
original or parent element to decay to the daughter element.
Uranium decays to radium through a long series of steps with
a cumulative half-life of 4.4 billion years. During these steps,
intermediate daughter products are produced, and high-energy
particles including alpha particles, consisting of two protons
and two neutrons, are released, which produces heat.
The daughter mineral radium is itself radioactive, and it
decays with a half-life of 1,620 years by losing an alpha particle,
forming the heavy gas radon. Since radon is a gas, it
escapes out of the minerals and ground and makes its way to
the atmosphere where it is dispersed, unless it gets trapped in
people’s homes. If it gets trapped, it can be inhaled and do
damage. Radon is a radioactive gas, and it decays with a halflife
of 3.8 days, producing daughter products of polonium,
bismuth, and lead. If this decay occurs while the gas is in
someone’s lungs, then the solid daughter products become
lodged in their lungs, which is how the damage from radon is
initiated. Most of the health risks from radon are associated
with the daughter product polonium, which is easily lodged
in lung tissue. Polonium is radioactive, and its decay and
emission of high-energy particles in the lungs can damage
lung tissue, eventually causing lung cancer.
There is a huge variation in the concentration of radon
between geographic regions, and in specific places in those
regions. There is also a great variation in the concentration of
the gas at different levels in the soil, home, and atmosphere.
This variation is related to the concentration and type of
radioactive elements present at a location. Radioactivity is
measured in a unit known as a picocurie (pCi), which is
approximately equal to the amount of radiation produced by
the decay of two atoms per minute.
Soils have gases trapped between the individual grains
that make up the soil, and these soil gases have typical radon
levels of 20 pCi per liter, to 100,000 pCi per liter, with most
soils in the United States falling in the range of 200–2,000
pCi/L. Radon can also be dissolved in groundwater, with typical
levels falling between 100–2 million pCi/Liter. Outdoor air
typically has 0.1–20 pCi/Liter, and radon inside people’s homes
ranges 1–3,000 pCi/Liter, with 0.2 pCi/Liter being typical.
There is a large natural variation in radon levels in different
parts of the country and world. One of the main variables
controlling the concentration of radon at any site is
the initial concentration of the parent element uranium in
the underlying bedrock and soil. If the underlying materials
have high concentrations of uranium, it is more likely that
homes built in the area may have high concentrations of
radon. Most natural geologic materials contain a small
amount of uranium, typically about one to three parts per
million (ppm). The concentration of uranium is typically
about the same in soils derived from a rock as in the original
source rock. However, some rock (and soil) types have
much higher initial concentrations of uranium, ranging up
to and above 100 ppm. Some of the rocks that have the
highest uranium contents include some granites, some types
of volcanic rocks (especially rhyolites), phosphate-bearing
sedimentary rocks, and the metamorphosed equivalents of
all of these rocks.
As the uranium in the soil gradually decays, it leaves its
daughter product, radium, in concentrations proportional to
the initial concentration of uranium. The radium then decays
by forcefully ejecting an alpha particle from its nucleus. This
ejection is an important step in the formation of radon, since
every action has a reaction. In this case the reaction is the
recoil of the nucleus of the newly formed radon. Most radon
remains trapped in minerals once it forms. However, if the
decay of radium happens near the surface of a mineral, and if
the recoil of the new nucleus of radon is away from the center
of the grain, the radon gas may escape the bondage of the
mineral. It will then be free to move in the intergranular
space between minerals, soil, or cracks in the bedrock, or
become absorbed in groundwater between the mineral grains.
Less than half (10–50 percent) of the radon produced by
decay of radium actually escapes the host mineral. The rest is
trapped inside, where it eventually decays, leaving the solid
daughter products behind as impurities in the mineral.
Once the radon is free in the open or water-filled pore
spaces of the soil or bedrock it may move rather quickly. The
exact rate of movement is critical to whether or not the radon
enters homes, because radon does not stay around for very
long with a half-life of only 3.8 days. The rates at which
radon moves through a typical soil depend on how much
pore space there is in the soil (or rock), how connected these
pore spaces are, and the exact geometry and size of the openings.
Radon moves quickly through very porous and permeable
soils such as sand and gravel, but moves very slowly
through less permeable materials such as clay. Radon moves
very quickly through fractured material, whether it is
bedrock, clay, or concrete.
The large variation in the concentration of radon from
place to place is partly the result of how the rates of radon
movement are influenced by the geometry of pore spaces in a
soil or bedrock underlying a home, and also how the initial
concentration of uranium in the bedrock determines the
amount of radon available to move. Homes built on dry permeable
soils can accumulate radon quickly because it can
migrate through the soil quickly. Conversely, homes built on
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