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Radon is a naturally occurring, chemically
inert, radioactive gas. Because radon is chemically unreactive
with most materials, it is free to travel as a gas. It can
move easily through very small spaces such as those between
particles of soil and rock. Radon is odorless, invisible,
and without taste; thus, it cannot be detected with the
human senses. Radon is also moderately soluble in water
and, therefore, can be absorbed by water flowing through
rock or sand. Its solubility depends on the water temperature;
the colder the water, the greater radon's solubility.
The two natural sources of radon, thorium
and uranium, are common, naturally occurring elements that
are found in low concentrations in rock and soil. Through
radioactive decay, both are constant sources of radon. Radon
is produced from the radioactive decay of the element radium,
which is itself a decay product of either uranium or thorium.
Radioactive decay is a process in which an unstable atomic
nucleus undergoes spontaneous transformation, by emission
of particles or electromagnetic radiation, to form a new
nucleus (decay product), which may or may not be radioactive.
The level of radioactivity is measured in curies, where
1 curie equals 37 billion disintegrations per second. The
time required for a given specific activity of an isotope
to be reduced by a factor of two is called its half-life.
A Picocuries (pCi) is equal to one-trillionth of a curie.
Specific activity concentrations are typically measured
in Picocuries per gram (in a solid) or Picocuries per liter
(in a gas, such as air).
Uranium-238 decays in several steps to radium-226,
which decays into radon-222. Radon-222 has a half-life of
3.8 days and, therefore, has enough time to diffuse through
dry, porous soils or to be transported in water for a considerable
distance before it decays. Similarly, thorium-232 decays
into radon-220 (a different radon isotope, also called thoron),
which has a half-life of only 55 seconds. Because of its
short half-life and limited ability to migrate into residences,
radon-220 is usually a less important source of radon exposure
to humans. The United Nations Scientific Committee on the
Effects of Atomic Radiation has estimated the average exposure
from indoor radon-220 decay products to be about 25 percent
of that from radon-222. Only radon-222 is addressed specifically
in the Citizen's Guide because it is the radon isotope of
most concern to the public. Although radon-220, or thoron,
has not been measured separately in most homes, radon mitigation
will also reduce exposure to thoron.
Radon-222 is preceded in the uranium-238
decay series by radium-226, which has a half-life of 1,600
years. Radon-222 decays in several steps to form radioactive
isotopes with short half-lives: Polonium-218, lead-214,
bismuth-214, and polonium-214. These isotope particles are
commonly referred to as radon decay products, daughters,
or progeny. Radon decay products are chemically reactive
and can attach themselves to walls, floors, or airborne
particles that are inhaled into the lungs. Unattached radon
decay products also can be inhaled and, subsequently, can
become deposited on lung tissue.
The four radon-222 decay products just mentioned
all have half-lives of less than 30 minutes. This short
half-life is significant since, once deposited on lung tissue,
the radon decay products can undergo considerable decay
before the action of mucus in the bronchial tubes can clear
these radioactive particles. Two of the short-lived decay
products, polonium-218 and polonium-214, emit alpha particles
during the decay process. An alpha particle is a subatomic
particle that has two protons and two neutrons and has a
double positive electrical charge. It is identical to a
helium nucleus.
Radon-222 is found virtually everywhere
in at least small amounts because its predecessor, radium-226
(or, more distantly, uranium-238), is found in all rock
and soil. In outdoor air, radon concentrations are usually
less than one Picocuries per liter (pCi/L). Higher concentrates
of radon outdoors may be observed during brief periods,
such as during a temperature inversion, when a warm air
mass traps a colder one beneath it. The highest individual
concentration, in contrast, can vary from around 0.5 pCi/L
to over 2,000 pCi/L, with results from EPA's National Residential
Radon Survey indicating that over 6 percent of all homes
nationwide have average annual indoor radon levels above
4 pCi/L. Most indoor radon comes from rocks and soil around
a home, although other, usually less significant, sources
of indoor radon are water and some construction materials.
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