Radon is a natural, radioactive gas that migrates from the ground into buildings. Prolonged exposure to high levels of this gas can cause lung cancer. Radon is, thus, a serious health threat in workplaces, schools, and especially homes. It is important that Ohio's citizens be well informed about this hazard.
Radon is a naturally occurring, gaseous element that
is a by-product of the radioactive decay of another element, uranium. When
an atom of uranium decays to radon it does so by transforming itself into
a series of different radioactive elements, each a decay by-product of
the preceding one. The series passes ultimately through radium directly
to radon, which, in turn, decays to other elements. Of all the elements
produced in the uranium decay series, only radon is a gas.
Where does radon come from?
At least some uranium is present in all earth materials. On continental surfaces the rocks, sediments and soils typically contain between 1 and 3 parts per million (abbreviated ppm) of this element. In other words, a million pounds of rock (500 tons) will have 1 to 3 pounds of uranium scattered through it on average. Some earth materials have uranium contents significantly above this amount, and as a consequence, may be a cause of locally high indoor radon levels. Such radon sources are found throughout much of Ohio.
Because radon is a gas, it easily drifts upward through
the ground to the Earth's surface. How much of it reaches the surface depends
on the uranium content of the underlying earth materials together with
their depth and permeability (that is, the presence of fractures and interconnected
pore spaces that act as conduits for radon). Radon will enter the lowest
level of a building using whatever pathways are available. For structures
with basements or slab-on-grade foundations, the entry points include (1)
cracks and pores in floor slabs, walls, and floor-wall joints; and (2)
openings around sump pumps, floor drains, and pipes penetrating floors
and walls. Structures with a crawl space between the ground and lowest
floor level may be less vulnerable to radon, which tends to escape to the
outside air when appropriate vents are installed, but can still admit some
of the gas through cracks in the flooring.
It has been observed in numerous studies that radon levels in the living areas of the houses are about 1.6 times higher in the winter than in summer, and are in between the winter and summer values during the fall and spring. For the same houses, the annual average radon level in basements is up to 2.5 times higher than it is on the first floor. It was also found that well-weatherized (tight) houses have average radon levels about 1.4 times higher than poorly-weatherized (drafty) houses. The reasons for these differences are easy to understand. There should be more radon in basements because that is where it enters a house, and radon levels should be higher during the cold months when the stack effect is greater and indoor/outdoor exchange is very low (especially in tight houses).
In nearly all cases, indoor radon is derived only from
the earth materials underlying a building. However, it can also come from
the construction materials if uranium-enriched rock is used for fireplaces,
field-stone walls or concrete aggregate, or from private well water if
it is drawn from an uraniferous aquifer. In rare instances, radon can also
be released from the natural gas burned in furnaces and household appliances.
Radon can also enter into homes through the water system.
This is mainly true for houses in which ground water is used as the
main water supply. Small public water works and private domestic wells
often have closed systems and short transit times that do not allow radon
to decay to harmless by-products before entering a home. Once inside, radon
escapes from the water to the indoor air as people take showers, wash clothes
or dishes, or otherwise use water. The areas most likely to have problems
with radon in ground water are those with have high levels of uranium in
the underlying rocks.
Water in rivers and reservoirs usually contains very little radon, because it escapes into the air. Thus homes that rely on surface water usually do not have a radon problem from their water. In big cities, water processing in large municipal systems aerates the water, which allows radon to escape, and also delays the use of water until most of the remaining radon has decayed.
Although it has been suggested that drinking water containing radon may cause stomach cancer, this health effect has not been conclusively demonstrated. In Ohio it is, in any case, a very minor risk in comparison to that of radon-induced lung cancer from release of water-borne radon to indoor air. Radon is readily soluble in water and enters it from the ground through which the water flows. Public drinking water from wells or surface water sources is normally treated in ways that reduce radon at or near the water source before it is distributed to homes.It takes about 10,000 pCi/l of radon in water to raise the radon in indoor air by 1 pCi/L. Water samples from residential wells in Ohio only rarely have radon concentrations above 1,000 pCi/L, and municipal water has much lower levels of radon.
When a radioactive element decays it does so by emitting one or more of the following types of radiation:
(1) an alpha particle, which is a fragment of the atom's nucleus consisting of two protons and two neutrons
(2) a beta particle, which is an electron and
(3) gamma rays, which are not subatomic particles but rather a form of energy similar to x-rays.
Radon, for example, emits an alpha particle and gamma rays when it decays, and in the process is transformed into a new element called "polonium." Polonium then decays, in succession, into lead, bismuth, another form of polonium (that is, a different isotope), and finally into a stable (non-radioactive) form of lead. The alpha and gamma radiation of these decay by-products are used to determine the concentration of radon in indoor air.
The concentration of radon is most commonly expressed in terms of the number of alpha particles it generates. The units of concentration are "picocuries per liter of air" (abbreviated pCi/l). One pCi/l of radon represents an average of 2.22 alpha particles produced in each liter of air every minute. An average size room measuring 15 by 15 feet with an 8 foot ceiling would have a volume of 1800 cubic feet or 50,970 liters of air, and if the radon concentration is, say, 4 pCi/l there would be a total of 452,614 alpha particles produced in the room every minute.
Another, seldom used, measure
is expressed in units of "working levels" (abbreviated WL). Instead of
representing the radon concentration directly, working levels reflect the
amount of decay by-product atoms and, hence, are an indirect measure of
radon concentration. Radon decay by-products are more dangerous than radon
itself is. There is no exact equivalence between the two concentration
measures but for the typical house, 4 pCi/l of radon corresponds to about
0.02 WL of decay by-product atoms.