Radon: Exposure and Hazards
The World Health
Organization (WHO) and the US Department of Health and Human Services, as well
as EPA, have classified radon as a "Class A" known human carcinogen,
because of the wealth of biological and epidemiological evidence and data
showing the connection between exposure to radon and lung cancer in humans.
Radon is an odorless,
colorless and chemically unreactive inert gas, which
is the densest gas known (nine times denser than air). Radon is not produced as
a commercial product. Radon is a naturally occurring radioactive gas and comes
from the natural breakdown (radioactive decay) of uranium. Most soils contain
varying amounts of uranium. It is usually found in rocks and soil, but in some
cases, well water may also be a source of radon.
Radon is also fairly
soluble in water and organic solvents. Although reaction with other compounds
is comparatively rare, it is not completely inert and forms stable molecules
with highly electronegative materials. Radon is considered a noble gas that
occurs in several isotopic forms. Only two are found in significant
concentrations in the human environment: radon-222, and radon-220. It is
radon-222 that most readily occurs in the environment. Atmospheric releases of
radon-222 results in the formation of decay products that are radioisotopes of
heavy metals (polonium, lead, bismuth) and rapidly attach to other airborne
materials such as dust and other materials facilitating inhalation.
The radon gas and its
highly radioactive metallic products emit alpha and beta particles and gamma
rays. Because it is a single atom gas (unlike oxygen, which is comprised of two
atoms) it easily penetrates many common materials like paper, leather, low
density plastic (like plastic bags, etc.) most paints, and building materials
like gypsum board (sheetrock), concrete block, mortar, sheathing paper
(tarpaper), wood paneling, and most insulation.
Depending on the geographic location, climate and seasonal changes Radon gas
may be a problem in some homes. When the air containing radon is inhaled, it is
possible to increase the risk of getting lung cancer. In fact, radon is the
second leading cause of lung cancer in the
Radon is a national
environmental health problem. Elevated radon levels have been discovered in
virtually every state. The EPA estimates that as many as 8 million homes
throughout the country have elevated levels of radon. State surveys to date
show that 1 out of 5 homes has elevated radon levels.
Radon seeps into homes from the surrounding soil through cracks and other
openings in the foundation. Indoor radon has been judged to be the most serious
environmental carcinogen to which the general public is exposed. Based on
current exposure and risk estimates, radon exposure in single-family houses may
be a causal factor in as many as 20,000 of the total lung cancer fatalities
which occur each year. Radon decay products (polonium) can attach to the
surface of aerosols, dusts, and smoke particles which may be inhaled, and
become deeply lodged or trapped in the lungs. Once lodged, they can radiate and
penetrate the cells of mucous membranes, bronchi, and other pulmonary tissues.
A family whose home has
radon levels of 4 pCi/l is
exposed to approximately 35 times as much radiation as the NRC allows if they
were standing next to the fence of a radioactive waste site. (25 mrem limit, 800 mrem exposure)
An elementary school
student that spends 8 hours per day and 180 days per year in a classroom with 4
pCi/l of radon will receive nearly 10 times as much
radiation as the NRC allows at the edge of a nuclear power plant.(25 mrem limit, 200 mrem exposure)
Most United States
Environmental Protection Agency (EPA) lifetime safety standards for carcinogens
are established based on a 1 in 100,000 risk of death. Most scientists agree
that the risk of death for radon at 4 pCi/l
is approximately 1 in 100.
Routes of Entry:
The primary routes of
potential human exposure to radon are inhalation and ingestion. Radon in the
ground, groundwater, or building materials enters working and living spaces and
disintegrates into its decay products. In comparison with levels in outdoor
air, the concentrations of radon and its decay products to which humans are
exposed in confined air spaces, particularly in underground work areas such as
mines and buildings, are elevated. Although high concentrations of radon in
groundwater may contribute to human exposure through ingestion, the radiation
dose to the body due to inhalation of radon released from water is usually more
important.
Exposure:
Concentrations of radon
decay products measured in the air of underground mines throughout the world
vary by several orders of magnitude. The average radon concentrations in houses
are generally much lower than the average radon concentrations in underground
ore mines. Workers are exposed to radon in several occupations. Underground
uranium miners are exposed to the highest levels of radon and its decay
products. Other underground workers and certain mineral processing workers may
also be exposed to significant levels. Exhalation of radon from ordinary rock
and soils and from radon- rich water can cause significant radon concentrations
in tunnels, power stations, caves, public baths, and spas.
Some scientific studies of
radon exposure indicate that children may be more sensitive to radon. This may
be due to their higher respiration rate and their rapidly dividing cells, which
may be more vulnerable to radiation damage.
Radioactivity
:
The rate of disintegration of a radioactive substance is commonly designated by
its half-life, which is the time required for one half of a given quantity of
the substance to decay.
For example, if you had a
two liter bottle (think of the large soda bottle in the fridge) that was filled
with radon gas and then tighly sealed, at the end of
one half-life (approximately 92 hours or almost 4 days) there would only be one
liter left in the bottle.
Another issue to consider
is the *unusual* property of the radioactive decay chain of
uranium/radium/radon. What makes this seem unusual is that a gas is produced
from a radioactive solid element (a rock) and then the radioactive gas changes
back into radioactive heavy metallic particles. This process and their atomic
size (extremely small) makes possible the transport of
radioactive atoms through a relatively static environment. In other words,
radon's extended half-life (it takes about a month for a specific amount of it
to decay to almost nothing) provides enough time for the gas to migrate through
cracks and crevices in building foundations, then into the internal air volume
where it changes into the more harmful radioactive heavy metals.
This gas and the resulting
very small metallic particles (so small that they will float in air) move
quickly through a building or home, contaminating the air. An analogy that
makes this easier to understand is to think how easily some can detect the
presence of a smoker in another part of the building or the cooking of coffee
or bacon in the kitchen on Sunday morning. In other words, almost nothing will
stop this gas from moving from the basement to other parts of a house if it
makes its way into the basement in the first place.
Acceptable Level of
Radon:
The US EPA has established
the "action level" for deciding when you need to "do
something" about the radon in your home, school, or work place is 4 pCi/l. pCi/l= picocuries
per liter, the most popular method of reporting radon levels. For those
interested in the numbers, a picoCurie
is 0.000,000,000,001 (one-trillionth) of a Curie, an international measurement
unit of radioactivity. One pCi/l means that in one
liter of air there will be 2.2 radioactive disintegrations each minute. For
example, at 4 pCi/l there
will be approximately 12,672 radioactive disintegrations in one liter of air,
during a 24-hour period.
Most Commonly Used
Radon Testing Method:
There are two main methods
used to test for radon gas and radon daughter products. The most popular
involves the use of a "passive" device such as an activated charcoal
test kit which collects radon gas atoms for counting later in a laboratory or
an alpha track device that has a small strip of special plastic that is
"marked" when hit by radon's alpha particles (also counted later in a
laboratory). Another passive device called an electret
has a plastic disc with a static charge. This type is only used by professional
radon inspectors because of the expertise required and the expensive equipment
needed for analysis.
The other main method involves the use of an "active" device called a
CRM (continuous radon monitor). These are mostly used by professional radon
inspectors for short-term (two days or 48 hours) radon testing during a real
estate transaction. There are many different models of CRMs
but they all require some formal training in order to be used properly.
Radon Resistant New Construction (RRNC):
According to the most
recent survey of home builders conducted by the National Association of Home
Builders (NAHB)
- 18.8% of approximately 1,126,000 single-family
detached homes built during 1998 incorporated radon-reducing features,
which translates to nearly 212,000 homes. This brings the cumulative total
of single-family detached homes built with radon-reducing features since
1990 to over 1.6 million homes, based on the results of previous surveys
conducted by the
- 43.3% of approximately 306,000 single-family detached
homes built in Zone 1 (homes with high radon potential) during 1998
incorporated radon-reducing features, which translates to about 132,500
homes. This brings the cumulative total of single-family detached homes
built in Zone 1 with radon-reducing features since 1990 to just over 1
million.
Radon Resistant Construction
Techniques:
The techniques may vary
for different foundations and site requirements, but the basic elements are:
A. Gas Permeable Layer
This layer is placed beneath the slab or flooring
system to allow the soil gas to move freely underneath the house. In many
cases, the material used is a 4-inch layer of clean gravel.
B. Plastic Sheeting
Plastic sheeting is placed on top of the gas permeable layer and under the slab
to help prevent the soil gas from entering the home. In crawlspaces, the
sheeting is placed over the crawlspace floor.
C. Sealing and Caulking
All openings in the concrete foundation floor are
sealed to reduce soil gas entry into the home.
D. Vent Pipe
A 3- or 4-inch gas-tight or PVC pipe (commonly used
for plumbing) runs from the gas permeable layer through the house to the roof
to safely vent radon and other soil gases above the house.
E. Junction Box
An electrical junction box is installed in case an electric venting fan is
needed later.
Figure 1. Radon Resistant Construction
Techniques
LINKS:
Radon Awareness for Public Service Professionals
Unit One (31 frames) Introduction to Radon and Radioactivity
Unit Two (23 frames) Radon Entry and Behavior
Unit Three (27 frames)Radon Mitigation System Design and Installation
Model Building Standards and Techniques (EPA Site)
Building A New Home: Have You Considered Radon?
EPA Map of Radon Zones (
EPA Map of Radon Zones (New
York State)
Last Update: January 9, 2007
By: Serdar Z. Elgun