Radon
What is Radon?
• Radon is a gas produced by the radioactive decay of the element radium.
• Radon itself is radioactive, losing an alpha particle and forming the element
polonium.
• Each of the three naturally-occurring series-decaying radionuclides releases
radon (only the uranium-238 series contributes significantly to the hazard).
• Uranium is everywhere (soils, building materials, etc.)
• Uranium decays eventually to radium.
• Radium, the parent of radon is very soluble.
• Therefore radon is also everywhere.
•
The major source of radon is the soil, but ground water, natural gas, and
building materials also contribute.
What is the Hazard?
• The half-life of radon is only 3.8 days.
• It is the daughter products of radon decay that are more of a health risk.
• Polonium, bismuth, and lead (among others) are produced by radon decay in
the air and in the lung.
• Alpha particles are released in the decay of the radon daughters.
•
Radon
The biological effects of very low doses of alpha particles are uncertain
(extrapolated from high doses to miners).
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Radon History:
• As early as the 1500s an unusually high mortality from respiratory disease
(“black lung disease”) was documented among underground metal miners in the
Erz Mountains of Eastern Europe. In the 1800s pulmonary malignancy was
documented in miners from that area, and in the early 20th century shown to be
primary carcinoma of the lung.
• Rutherford and Owens studying the “emanations from thorium” in 1899 wrote
“the radiation from thorium was not constant but varied in a most capricious
manner”. Rutherford made the key observations that “the emanation…acts like
an ordinary gas” and that “… the intensity of the radiation has fallen to one half
of its value after an interval of about one minute.” Rutherford and Owens had
discovered radon-220 (thoron), half life 51 sec.
• Pierre and Marie Curie, studying the emanations from radium in 1899,
concluded that “..it stayed radioactive for several days.” The Curies had
detected radon-222, half life 3.8 days, but they still had doubts as to whether the
activity was really in the form of a gas.
• Later work by Rutherford (1901) and also by Ernst Dorn (1900) confirmed that
the radiations emitted by radium were a radioactive gas.
• In the 1920s high levels of radon were detected in mines in Czechoslovakia,
where miners also had high lung cancer rates. It was proposed that the radon
was the cause, but the hypothesis was not generally accepted until the reports of
epidemiological studies on other groups of radon-exposed miners in the 1950s
and 1960s.
•
The mining industry begins to monitor mine air in the 1950s and 1960s and
limit exposure.
•
Radon itself was initially assumed to be the direct cause of the lung cancers in
miners. In the 1950s, Harly, in a doctoral thesis at Rensselear Polytechnical
Institute, proposed that it was the decay products of radon that delivered the
pertinent dose to the lung cells.
Radon
Page 2 of 22
Image removed due to copyright restrictions.
Table B-4 in [BEIR VI]. National Research Council. Health Effects of Exposure to
Radon: BEIR VI. Washington DC: National Academies Press, 1999.
See http://books.nap.edu/books/0309056454/html/194.html#page_top.
[WL, working level, is a measure of radon daughter concentration in air.]
• Radon was found to be a constituent of indoor air, but the potential health
implications received little attention until the late 1970s. High indoor radon
levels were found in Scandinavia and traced to building materials containing a
particular type of shale. Sparse data from the United States reported similar
levels.
• Only since the 1970s that radon has been recognized as a major source of
radiation exposure to the general public.
• Home construction practices (in response to the energy crisis) in the 1970s led
to better insulated and more tightly sealed homes.
• In 1984, a nuclear power plant worker in Pennsylvania set off a radiation
monitor on his way IN to work. His home was found to be contaminated with
radon at levels higher that the limits for underground mines.
• Other homes were later found to contain similarly high radon levels. As the
database grew, the problem was recognized as widespread.
Radon
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Radon
Uranium: The source
• All rocks contain some uranium: average values between 1 and 3 parts per
million (ppm).
• In general, the uranium content of a soil will be about the same as the
uranium content of the rock from which the soil was derived.
• Some types of rocks have higher than average uranium contents (lightcolored volcanic rocks, granites, dark shales, sedimentary rocks that contain
phosphate, and metamorphic rocks derived from these rocks).
• These rocks and their soils may contain as much as 100 ppm uranium.
• Layers of these rocks underlie various parts of the United States.
• The amount of radon in a house is greatly affected by other factors in
addition to the presence of uranium in the underlying soil.
Radon
Page 4 of 22
Source: U.S. Geological Survey. “Geologic Radon Potential of the U.S.” [updated 13
October 1995, cited 29 March 2004]. http://energy.cr.usgs.gov/radon/rnus.html.
Courtesy of the U.S. Geological Survey.
Radon
Page 5 of 22
Uranium → Radium → Radon
• Alpha recoil is the most important factor affecting the release of radon from
mineral grains.
• Some radon escapes the grain into the pore space.
•
The presence of water in the pore space can slow the radon and increase the
likelihood that it will stay in the pore space.
Source: U.S. Geological Survey. “The Geology of Radon” [updated 13 October 1995, cited
29 March 2004]. http://energy.cr.usgs.gov/radon/georadon/3.htmll.
• For most soils, only 10 to 50 percent of the radon produced actually escapes
from the mineral grains and enters the pores.
• Most soils in the United States contain between 0.33 and 1 pCi of radium per
gram of mineral matter and between 200 and 2,000 pCi of radon per liter of
soil air.
Radon
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Soil conditions affect radon movement
Radon can more easily leave the rocks and soils by escaping into fractures and
openings in rocks and into the pore spaces between grains of soil.
Factors affecting radon movement:
• the amount of water present in the pore space (the soil moisture content)
• the percentage of pore space in the soil (the porosity)
•
the soil permeability, the "interconnectedness" of the pore spaces that
determines the soil's ability to transmit water and air.
Source: U.S. Geological Survey. “The Geology of Radon” [updated 13 October 1995, cited
29 March 2004]. http://energy.cr.usgs.gov/radon/georadon/3.htmll.
Courtesy of the U.S. Geological Survey.
• Radon moves more rapidly through permeable soils, such as coarse sand and
gravel, than through impermeable soils, such as clays.
• Fractures in any soil or rock allow radon to move more quickly.
•
Radon
The distance that radon moves before most of it decays is less than 1 inch in
water-saturated rocks or soils, but it can be more than 6 feet, and sometimes
tens of feet, through dry rocks or soils.
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Radon entry into buildings
Source: U.S. Geological Survey. “The Geology of Radon: Preface” [updated 13 October
1995, cited 29 March 2004]. http://energy.cr.usgs.gov/radon/georadon/1.html.
Courtesy of the U.S. Geological Survey.
Characteristics of homes with elevated radon levels
• Houses with basements rather than crawlspaces.
• Houses with walls below grade.
• Houses with exposed earth in basements/sumps.
• Houses on ridges/slopes rather than in valleys. The soil on slopes is usually
better drained and more permeable.
• Houses with hollow concrete block foundation walls rather than solid poured
concrete walls.
• Tightly sealed rather than drafty houses.
Radon usually escapes into the atmosphere.
Where a house is present, however, soil air often flows toward its foundation for
three reasons:
• differences in air pressure between the soil and the house,
• the presence of openings in the house's foundation
• increases in permeability around the basement (if one is present).
Radon
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Radon concentrations
Radon levels in outdoor air, indoor air, soil air, and ground water can be very
different.
Radon in outdoor air
ranges from less than 0.1 pCi/L to about 30 pCi/L,
average 0.2 pCi/L.
Radon in indoor air:
ranges from less that 1 pCi/l to about 3,000 pCi/L,
averages between 1 and 2 pCi/L.
Radon in soil air (the air that occupies the pores in soil)
ranges from 20 or 30 pCi/L to more than 100,000 pCi/L;
most soils in the United States contain between 200 and 2,000 pCi of radon
per liter of soil air.
Radon dissolved in ground water ranges from about 100 to nearly 3 million
pCi/L.
Approximate contributions to indoor radon concentration in a single family
home.
• Soil
1.5 pCi/L
• Public water supplies
0.01
• Private wells
0.4
• Building materials
0.05
• Outdoor air
0.2
Radon
Page 9 of 22
Geometric Mean = 0.96 pCi/l
Geometric Standard Deviation = 2.84
Arithmetic Mean = 1.66 pCi/l
20
Percent of Houses
GM
15
10
AM
5
>8
0
0
2
222Rn
4
6
8
Concentration (pCi/l)
Figure by MIT OCW.
Distribution of indoor radon concentrations in 552 U.S. homes in data from 19
different studies (above) and in similar data from Sweden (below). (A
concentration of 4 pCi/L is equal to 160 Bq/m3 ).
Radon Decay
Image removed due to copyright restrictions.
Fig. 16.2 in Alpen, E. L. Radiation Biophysics, 2nd ed. San
Diego, CA: Academic Press, 1998.
Radon itself, due to its fairly short half-life (222Rn) is not a major concern.
Radon is also an inert gas and is typically exhaled after breathing it in (although
some will dissolve in the blood).
The concern is over the daughter products of radon that are particulate
(attached to aerosol particles), α-emitting, and decay within hours to 210Pb (T1/2
= 22 years).
Radon
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The Working Level
By convention, the concentration of radon daughters is measured in working levels
(WL), and cumulative exposures over time are measured in working-level months
(WLM).
The working level (WL) is a single value used to describe the radon daughter
concentrations in a way that also reflects their biological hazard.
The hazard is most directly related to the total amount of energy that will be
deposited in the lung tissue by the alpha decays of 218Po and 214Po.
The working level is that combination of radon daughters in one liter of air which
will result in the emission of 1.3 x 105 MeV of alpha particle energy.
1 WL = 100 pCi/L of radon daughters at equilibrium (each = 100 pCi/L)
1 WL = 1.3 x 105 MeV/L
Typical working level concentrations of radon daughters:
Outdoors
Indoors, Basement
Indoors, First Floor
0.0016 WL
0.008 WL
0.004 WL
The Working Level Month is a cumulative exposure to radon daughters. It
results from an exposure to a radon daughter concentration of 1 WL for one
working month (170 hours).
Exposure (WLM) =
Radon
Concentration (WL) x exposure time (hrs)
170
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Equilibrium
Image removed due to copyright restrictions.
Fig. B-1 in [BEIR VI].
See http://books.nap.edu/books/0309056454/html/178.html#p200063c49960178001.
Given enough time, a closed system initially containing only 222Rn will come
into equilibrium with the daughter products.
Most atmospheres show an equilibrium factor of about 0.5
[daughters ]
= 0.5
[radon]
All daughters at 100 pCi/L = 1 WL.
Or, radon at 200 pCi/L , with equilibrium factor of 0.5 = 1 WL.
Radon
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Potential Alpha Energy Concentration (PAEC)
The convention for description of radon progeny concentration is moving towards
more standardized SI units expressed in joules/m3.
PAEC refers to radon daughters, not radon, which is inert and not retained in the
lungs.
Cp(J/m3) = (5.79 C1 + 28.6C2 + 21.0C3) x 10-10
Where:
Cp = PAEC in J/m3
C1 = activity concentration of 218Po (RaA) in Bq/m3
C2 = activity concentration of 214Pb (RaB) in Bq/m3
C3 = activity concentration of 214Bi (RaC) in Bq/m3
Potential Alpha Energy
(13.69 MeV)
(7.69 MeV)
(7.69 MeV)
Exposure is expressed as Jh/m3
1 WLM = 3.5 x 10-3Jh/m3
Image removed due to copyright restrictions.
Fig. 4.7 in Turner J. E. Atoms, Radiation, and Radiation
Protection, 2nd ed. New York: Wiley-Interscience, 1995.
Radon
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Attachment to aerosols
When radon decays the daughter product (218Po) is usually stripped of several
electrons and left with a net positive charge.
The 218Po most likely combines with oxygen or becomes hydrated as water
condenses around it. This “unattached” 218Po may be anywhere from 0.001 –
0.02 µm in size.
The unattached 218Po suffers one of three possible fates:
• it remains unattached until decay,
• it attaches to ambient aerosols
• or it plates out on some available surface.
Image removed due to copyright restrictions.
Estimates place the unattached fraction between 1 and 10%; commonly quoted
average values are 4% in mines and 7% in homes.
The higher the concentration of ambient aerosols, the larger the attached
fraction.
Radon
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Lung Physiology
Source: U.S. National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program.
“Respiratory System: Conducting Passages.” [cited 29 March 2004.]
Courtesy of the U.S. National Cancer Institute.
Source: U.S. National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program.
“Respiratory Tract.” [cited 29 March 2004.]
http://training.seer.cancer.gov/ss_module03_lung/unit02_sec02_resp_tract.html Used with permission.
Radon
Page 15 of 22
Aerosol particle size affects lung deposition pattern.
Image removed due to copyright restrictions.
Fig. B-19 in [BEIR VI].
See http://books.nap.edu/books/0309056454/html/207.html#page_bottom
Image removed due to copyright restrictions.
Fig. B-8 in [BEIR VI].
See http://books.nap.edu/books/0309056454/html/185.html#page_middle
• Smallest particles (unattached fraction) are retained in the nose and throat (long
considered a major dose contributor).
• ~5 nm: deposit in bronchial region
• 3-10 nm: dose to lung increases greatly
• >10 nm: deep lung deposition
• ~100 nm: deposition effectiveness decreases due to less effective diffusion of
larger particle
• >500 nm: impaction helps deposit in respiratory tract
• >2500 nm: deposition in nose and throat
Radon
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Dose to the lung: the dosimetric approach
There are several ways to compute the dose from radioactive dust deposited in
the lung.
The simplest is to assume that the energy is deposited uniformly throughout the
entire lung mass (taken to be 1000 grams in the adult male).
The dose to the bronchial epithelium is orders of magnitude higher than that
calculated on the assumption that the energy is absorbed in the whole lung.
Most lung cancers arise in the bronchial epithelial tissue.
Factors that influence the dose to target cells in the respiratory tract from radon
exposure.
• The unattached fraction
• Aerosol characteristics
• Breathing pattern: Tidal volume; Nose or mouth breathing
• Characteristics of the lung: mucus thickness; morphology of bronchi
The particle size distribution can affect the dose distribution.
For radon the energy is assumed to be absorbed to a depth of 40-70 µm, which is
the range of the most energetic alpha particle emitted in the radon decay scheme.
• Dose estimation is still experimental.
• Active research in this area continues.
• Not used yet for risk estimation by radiation protection agencies.
Radon
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The special nature of the biologic damage induced by alpha
particles.
Image removed due to copyright restrictions.
Fig. 2-1 in [BEIR VI].
See http://books.nap.edu/books/0309056454/html/51.html#page_bottom.
• For any particular cell the probability of being hit is VERY low.
• Target cells for lung cancer induction are still not known.
• What is the effect of exactly one alpha particle traversal?
Image removed due to copyright restrictions.
Table 2-1 in [BEIR VI].
See http://books.nap.edu/books/0309056454/html/52.html#page_top
Documented increased risk of lung cancer in miner cohorts.
The problem is how to extrapolate this risk down to the exposure levels seen by the
general public.
Typical exposure to the general public:
~ 0.2 WLM/year
~ 10-17 WLM/lifetime.
An average lifetime exposure to 200 Bqm-3 (5.4 pCi/L), which is about 4 times the
average indoor exposure, would result in, on average, about 1 alpha particle
traversal per bronchial epithelial cell nucleus (0.6 for the bronchial basal cell
nucleus, and 2-4 for the secretory cell nucleus).
Radon
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Lung Cancer and Estimation of Risk: The Epidemiological Approach
Risk factors for lung cancer (from BEIR VI Table 1-1)
•
•
•
•
•
•
•
•
Active cigarette-smoking
Passive cigarette-smoking
Radon
Occupational carcinogens
Arsenic
Asbestos
Chromates
Chloromethyl ethers
Nickel
Polycyclic aromatic hydrocarbons
Family history
Fibrotic lung disorders
Ambient air pollution
Image removed due to copyright restrictions.
Table 3-2 in [BEIR VI].
See http://books.nap.edu/books/0309056454/html/77.html#page_top.
Image removed due to copyright restrictions.
Figure 3-6 in [BEIR VI].
See http://books.nap.edu/books/0309056454/html/113.html#page_bottom
ERR: excess relative risk—A model that describes the risk from exposures as a multiplicative
increment to the excess disease risk above the background level.
Image removed due to copyright restrictions.
Figure G-1 in [BEIR VI].
See http://books.nap.edu/books/0309056454/html/377.html#page_middle
Radon
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The EPA Action Level
In the U.S and Europe the indoor radon concentration varies from 20-60 Bq/m3
(0.5 to 1.6 pCi/L)
Finland, Norway and Sweden average about 100 Bq/m3 (2.7 pCi/L)
Conversion of radon concentration to a dose has many uncertainties. 20 Bq/m3
(0.5 pCi/L) will give a dose to the bronchial epithelial cells of 1 mSv/year (100
mrem/year).
The U.S. EPA has set a recommended action level of 160 Bq/m3 (4 pCi/L).
•
This is ~ 4 x the average radon concentration in U.S. homes (1 in 12
homes exceed this level).
•
This is the most stringent action level in the world.
• Germany and the U.K. are 2x higher. Finland, Sweden and Norway are
5x higher.
The following are estimated doses based on radon exposures:
1 WLM = 14.2 rem, (0.71 rads) adult male
1 WLM = 12.6 rem, adult female
1 WLM = 25 rem, 10 year old child
Typical cumulative exposures:
0.2
WLM/year
~3 rem/year to the bronchial endothelium
10 – 17
WLM/lifetime
BEIR VI estimate: reducing all homes to < 4 pCi/L will reduce lung cancer
incidence by 3-4%.
Radon
Page 20 of 22
Risk Assessment
How to predict the risk from low levels of exposure?
Image removed due to copyright restrictions.
Figure 1-3 in [BEIR VI].
See http://books.nap.edu/books/0309056454/html/26.html#page_bottom
Dosimetric approach: focuses on computing the doses deposited in lung target
cells by alpha particles. The approach relies on applying risk estimates from
populations exposed to other radiations, such as the Japanese atomic bomb
survivors, to the doses estimated in lung cells.
Biological approach: would use data from molecular, cellular and animal studies
to generate parameters which could be used to refine models of carcinogenesis.
The miner data could be used in conjunction with the models to test reliability.
Empirical approach: Uses statistical methodology to analyze data from miner
and residential studies.
Problems:
1. Use of gamma-ray-exposed groups requires assumptions about extrapolation
from acute low-LET exposure to prolonged high-LET localized exposures.
2. Residential radon exposure data limited in statistical power. Few studies,
small groups.
3. Dosimetric approach has many uncertainties in the extrapolation of effects
of acute whole body exposure to gamma rays to localized exposure to highLET alphas.
4. Biological approach: limited data on mechanisms of radiation induced
carcinogenesis.
BEIR VI summary: choose the empirical approach.
Radon
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Reference:
National Research Council. Health Effects of Exposure to Radon: BEIR VI.
Washington DC: National Academies Press, 1999. Viewable online at
http://www.nap.edu/catalog/5499.html
Radon
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