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We know radon gas is harmful, but how does it get into our
homes? Knowing this may help keep it
out. That's why it's important to
recognize where radon comes from.
Radon is estimated to cause many thousands of lung cancer
deaths each year. In fact, the Surgeon General has warned that radon is the
second leading cause of lung cancer in the United States. Radon is a naturally
occurring radioactive gas produced by the breakdown of uranium in soil, rock,
and water. Whenever air enters a building from underneath, radon will most
likely come in as well if radon is present. In the United States, radon gas in soil is
the principal source of elevated radon levels in homes.
The Enemy From Below
Normally, radon travels first through the soil before entering
a structure. Radon moves from its higher concentration at its source to areas
of lower concentration in the soil to a point where it will be drawn in by
pressure-driven transport. This process is called diffusion.
Pathways in soil are created from high soil porosity, void
space, fissures in underlying geology and/or preconstruction excavation. All
make soil more permeable. Radon will also follow loose fill in plumbing
trenches. And, although wet clays are low in permeability and can retard radon
entry; dry clays crack and allow soil gas to channel up.
Frost or saturated upper soil horizon can cap the soil so
the negative pressure of a building is exerted on larger areas. This is why
rain can elevate levels of radon within a home.
Your house acts like a vacuum. Soil pressures, referred to
as air pressure differentials, cause air from soil to be pulled through the
smallest of openings in the foundation openings. Soil gas can enter any
structure in contact with the ground and radon in the soil enters with it.
Negative pressure causes most radon entry. Because buildings
are typically at a lower pressure than the surrounding air and soil, and
because it's a gas, radon is drawn into the house air. Environmental factors
like rain, wind and falling barometric pressure also play a role. These
cyclical changes are one of the reasons why levels of radon will vary with
time.
There are several reasons why air pressure differentials
occur. One cause is the effect that exhaust fans have when removing air from a
building. When air is exhausted by mechanical systems, outside air enter the
building to replace it. Home exhaust systems include bathroom fans, clothes
dryers, central vacuums and combustion appliances. Wood fireplaces, open and air-tight wood
stoves also can contribute to air pressure differentials. Related causes are the
effects of unbalanced forced air HVAC Systems due to leaky return air ductwork.
Much of this replacement air comes in from the underlying soil. Another reason
is the stack effect from heating.
Temperature differentials are another contributing factor. When
interior temperatures are higher than outside temperatures, thermal effects
occur inside of the building. As indoor air is heated, warm air rises then vacates
a building and is displaced by cooler outside air. Some of this outside air
comes from the soil.
There are many pathways that will allow radon to enter
including cold joints and expansion joints in basements, hollow blocks, plumbing
penetrations, electrical service penetrations and openings under tubs and
showers. Even the smallest crack in foundations and concrete slab-on-grade
floors can be problematic. Radon can pass through porous drainage beds of "French Drains" towards homes and frequently are routed to interior sump pits.
Some foundation types are bigger culprits than others.
Because of their large soil surface, crawlspaces rank high for potential
problems. Crawlspace vents are of little help, especially in winter. The use of plastic vapor barriers in
crawlspaces, while important in controlling moisture from migrating out of the
soil below a house and into the house environment, is not a radon barrier.
Other Causes
In a small number of homes, the building material (e.g.,
granite and certain concrete products) can also result in elevated levels in
homes by giving off radon. The rate of emanation or release depends on radium
content and surface area. The radon gas is typically dissipated by normal
ventilation.
Radon may also be present in well water and can be released
into the air, in homes when water is used for showering and other household
uses. This process is known as outgassing. In most cases, radon entering the
home through water is a small risk compared with radon entering your home from
the soil.
Radon Entry and Indoor
Concentrations Are Variable
Amounts of uranium and radium beneath a structure, diverse
soil conditions, rapidly changing air pressure and temperature differentials, varying
mechanical systems, different foundation types, and various potential pathways
can make concentrations of radon gas vary from home to home within the same
neighborhood. These are compelling
reasons why the Surgeon General recommends that all homes be tested.
For more information on Radon visit the EPA Radon Website www.epa.gov/radon
Our homeowner tips are only general guidelines. Since each situation is different, please
consult with a specialist regarding your questions or specific issue. More home safety and maintenance information
is available on this website.
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