Radiation and Radon from Natural Stone Dr. Llope May 7, 2008 report exerpts
Radiation and Radon from Natural Stone
May 7, 2008
Rice University, Houston, TX 77005
The natural and varied colors of natural stone countertops in home kitchens and other
rooms have long been seen as stylish and impressive. In fact, their natural beauty makes
them the center of attention in rooms that are otherwise painted and tiled in simple
single colors. They are durable and resistent to stains and heat. As a result of their high
heat capacity, which keeps them cool even in warm kitchens, experienced bread-makers
often prepare dough directly on them . Their aesthetic value and durability adds real
monetary value to the homes that have them.
It is thus not surprising that recent market research  implies the residential demand
for natural stone countertops will continue to increase. The demand increased by 5%
per year in the period from 2001 to 2006. The continued consumer preference for larger
kitchens and more bathrooms are expected to continue to promote growth from the residential
remodelling sector, which accounted for 70% of the volume in 2006. The August
2007 edition of Consumer Reports magazine  includes an article on stone countertops
entitled “Countertops, The Hottest Rocks,” and notes that these have “evolved from utilitarian
work surface to stylish focal point.” One might wonder, however, if homeowners
that opt (at considerable additional expense) for natural stone countertops are putting
themselves and their families in danger by doing so. It is certainly not well-known that
natural stone could potentially be dangerous. This key question is the focus of this article.
Natural stone finds its way into homes as part of the bricks, cement, sheetrock, floor
and wall tiles, as well as our beautiful heavy countertops. This stone is mined from the
earth’s crust in quarries, and then made available to home builders, throughout the world.
The earth’s crust contains elements that were originally produced in the (supernova)
explosions of stars in our galaxy over the billions of years that the earth has existed.
Some of these elements are radioactive, and therefore any building materials extracted
from the earth’s crust can potentially be radioactive too. So, perhaps the title of the
Consumer Reports article noted above has a second, more insidious, interpretation.
The potential dangers of radioactive elements in homes are two-fold: one is “radiation”
and the other is “radon.” Radioactive elements, by their very nature, seek stability by
decaying into lighter elements via the emission of radiation. This radiation comes in
the form of energetic light particles - namely, “gamma-rays”, “beta rays”, and “alpha
particles.” These can enter the body from the outside and cause damage to cells and
DNA, potentially causing cancer. Also, naturally occurring Uranium nuclei decay into
an element called Radium, which produces an unstable gas called Radon. Breathing air
that contains radon thus deposits radioactive elements directly into the soft-tissues of the
lungs and can cause lung cancer.
According to articles made available by the industry that are typically the top results
in web searches on the subject, stone countertops pose no danger whatsoever. An oftRadiation
and Radon from Natural Stone, W.J. Llope, May 7, 2008, page 2
cited reference of this kind is an e-mail from Prof. D. Langmuir in 1995 . This opinion
was however based on a model, not experimental data, and there is no delineation of the
assumptions that were made. There is thus no way to know if the opinion is of general
applicability. A more detailed analysis  from the Cold Spring Granite company also
indicates a very low risk. However, the quoted results apply only to the granite from this
one quarry. Contrast this with the fact that websites such as graniteland.com now list
as available 1600 different varieties of granite from 64 different countries! More recent
research published in peer-reviewed journals, in which all assumptions and experimental
methods are carefully and plainly described, calls this general lack of concern into question.
Before we go into the details on this reseach, it is important to point out that there
is a low level of radiation surrounding all of us at all times. We need to understand this
“background” before we can put into context the possible dangers from stone countertops
in our homes above and beyond this background level. Here I need to use two different
units, “milli-rem per hour” (mrem/hr) and “picoCurie per liter” (pCi/L) to give a sense of
scale. The mrem/hr number is the relevant quantity for the absorbed dose from external
radiation, while the pCi/L number is the relevant quantity for the radon in the air we
On average, we receive about 360 mrem per year (0.04 mrem/hr) from natural and
man-made sources. The natural sources include radon emanating from the soil around
our homes and the cosmic rays entering our atmosphere from space. Man-made sources
include dental and medical X-rays, and appliances such as television sets and smoke
detectors. Flying on a commercial airplane results in approximately 1 mrem per 1000
miles travelled, so every round trip from New York to Los Angeles results in about 6 mrem
of additional exposure. Sleeping next to another person for 8 hours per night results in
approximately 2 mrem per year due to the natural levels of radioactive potassium in our
bodies. People that work in Grand Central Station in New York City are exposed to 120
mrem/year from the high Uranium content in the granite walls. Brazil nuts have 1000
times the radioactivity of most foods due to their radium content. So, we live in a world
filled with radiation.
Now, to put the “mrem/hr” and “pCi/L” numbers for an additional exposure above
background into context into terms of cancer risk, one must depend on the results of
very complicated long-term studies on the relationship between the dose received and the
incidence of cancer. This is subtle business. However, there are general guidelines. The
excess risk of cancer from an additional exposure to radiation above that from background
sources is estimated to be 4 per 10000 per rem. Thus, the exposure to a 4 mrem/hour
radioactive source for a total of 250 hours would amount to one rem. If 10,000 people
were all exposed to this one rem of radiation, 4 of these people (0.04%) would develop
cancer as a result of this exposure. For radon, the risk assessment is phrased differently.
If 1000 non-smokers breathed air that contained 4 pCi/L of radon over the course of their
entire lifetime, 7 of these people would be expected to develop lung cancer as a result of
the radon exposure.
The cancer rates are roughly linear in the exposure - doubling the exposure doubles
the cancer risk. The U.S. Department of Energy (DOE)  and the Environmental Protection
Agency (EPA)  have thus set limits on the excess amount of radiation and radon
exposure, respectively, in order to insure the excess cancer risks are very small. The limit
for the acceptable whole-body exposure to radiation to visitors of DOE research facilities
Radiation and Radon from Natural Stone, W.J. Llope, May 7, 2008, page 3
is 100 mrem/year, and these facilities locally enforce much stricter limits. The EPA suggests
remediation for radon concentrations exceeding 2 pCi/L, and strongly recommends
remediation for concentrations exceeding 4 pCi/L.
These administrative limits are our best guide of what levels of exposure are dangerous
and what levels aren’t. However, there is no safe amount of radiation or radon. There is
no threshold for the onset of cancers that result from exposure to radiation or radon. If it
is reasonably achievable to make the exposure zero, then that is by far the best approach.
Given these scales for the cancer risk from exposure to radiation and radon, let’s
turn to the published research. I have found approximately 20 articles published in peerreviewed
journals that describe direct measurements of the direct radiation and radon
emanation from natural stone and other building materials. These results paint a slightly
less rosy picture of the potential risks.
I found 95 data points  of the rate of radon emanation from specific natural stones
used in construction in the literature. These results indicate the potential for both signficant
direct radiation and radon emanation from measured stones. Indeed, a tight correlation
between the radiation and radon emanation is typical, which is not surprising.
Radon Concentration (pCi/L)
0 0.5 1 1.5 2 2.5 3 3.5 4
Number of Samples
Radon Saturation (pCi/L) Frequency
Figure 1: The frequency distribution of the radon concentrationn
(pCi/L) from various granites described in the published literature if
placed in a hypothetical unventilated 20’×10’×10’ room and assuming a
granite surface area of 5 m2.
To give a feel for the radon concentration that would result from using these stones
as countertops, I need to make a few assumptions. I will assume that there is 5 square
meters (54 square feet) of surface area of this countertop and that it is sitting in an
unventilated 20×10×10 foot room. Figure 1 indicates the saturation concentration of
radon that would result in this hypothetical room. The vertical axis of this plot is the
Radiation and Radon from Natural Stone, W.J. Llope, May 7, 2008, page 4
number of samples, and the horizontal axis is the saturation radon concentration in pCi/L.
One can see that the majority of the samples would result in a very low risk from radon
(below 1 pCi/L). The mean value is ∼0.3 pCi/L. However, a handful of samples imply a
saturation radon concentration of several pCi/L. The EPA would consider this dangerous
assuming long-term exposure and would recommend remediation.
Similar results apply for the direct radiation. The majority of the stones are consistent
with, or are only mildly above, the background, and hence do not pose a significant risk.
A few however, emit radiation of several mrem/hr.
The home I visited as part of this story had natural stone countertops that, in general,
emitted radiation at 0.1-0.3 mrem/hr, which is close to the background rate. In five
specific locations on these countertops, however, the rate was 3-4 mrem/hour! This rate
is a factor of some tens above background and is certainly not negligible.
I measured a section of this same countertop with a detector that was able to measure
the energies of the emitted particles. As the decay of a radioactive nucleus produces
particles of very specific and well-defined energies, the measurement of the energies of the
emitted radiation directly and unambiguously identifies the nucleus that is decaying. The
result is shown in Figure 2.
γ Energy (MeV)
0.5 1 1.5 2 2.5 3
Figure 2: The γ energy spectrum measured from a section of granite
countertop from a home in the Houston area. The positions of the peaks
directly identify the radioactive elements in this countertop. For the
details, see the text.
The majority of the peaks in this figure result from the decay of 214Bi that is a progeny
of 238U. The countertop thus contains Uranium ore. Where there is Uranium, there is
Radon. There is also an admixture of 232Th (indicated by the 2.615 MeV peak from the
decay of 208Tl) and 40K (indicated by the 1.46 MeV peak). The observed mixture of
Radiation and Radon from Natural Stone, W.J. Llope, May 7, 2008, page 5
Uranium, Thorium, and Potassium is consistent with all of the published literature.
These results imply that most natural stone countertops in the home are “safe,” but
some fraction are most certainly not safe. While the risk of cancer from these extreme examples
is generally small and requires many years of exposure, “forewarned is forearmed,”
especially if there are pregnant women or young children in these homes.
If you own natural stone countertops, here are some suggestions. Ventilate your home
as often as possible by opening windows - this exhanges potentially radon-laden air from
the inside of your home with essentially radon-free air (at least in the Houston area)
from the outside. The radon concentration in an unventilated volume of air reaches its
saturation density in about 27 days - so open your windows for a half-day once every few
weeks. Home radon test kits can be obtained at most hardware stores. Look for ones
that state “meets EPA requirements,” and follow the instructions on the test kit carefully.
Radon decays with a half-life of about 4 days, so if the test kit needs to be shipped to
a lab for analysis, do this expeditiously. Make sure that you follow the manufacturer’s
recommendations on the regular maintenance of your countertops, which typically call for
regular resealing of the surface (once per year). Measuring the direct radiation requires
at least a Geiger Counter, and these are not inexpensive. Feel free to contact me about
this, or any other questions that you might have, via e-mail at the address listed below.
To summarize, natural stone naturally contains some amount of radioactive elements.
The amount is most likely negligible compared to the other sources of radioactivity that
we are unavoidably exposed to every day. In some cases, however, the amount can be
significant. This fact raises some disturbing questions. Homeowners that have, at considerable
additional expense, already installed natural stone countertops in their homes are
now wondering if their beautiful stone is safe. This requires research on their part that
they should not have been put in the position of having to do after the fact. It would make
much more sense if natural stone countertops were checked for radiation and/or radon
emanation by the EPA and/or the manufacturer, and thus officially qualified as “safe for
homes” with respect to simple well-defined protocols before installation. Also, as the natural
stone countertops installed in U.S. homes are quite commonly quarried abroad, there
are also real Homeland Security concerns here. A number of companies sell detectors for
use by port security personnel to detect radiation potentially indicating the makings of
a “dirty bomb.” These companies recognize  natural stone as a common false positive!
Thus, shipments of large blocks of stone, or other common false positives, are presumably
typically cleared through port security checkpoints without direct inspection . Natural
stone is dense enough that a significant amount of a much purer radioactive material
could be hidden inside and not significantly increase the external radiation signature.
About the Author:
W.J. Llope is a Research Associate Professor of Physics at the T.W. Bonner Nuclear
Laboratory at Rice University in Houston, Texas. His research concentrates on the physics
of relativistic heavy-ion collisions at the RHIC accelerator facility at Brookhaven National
Laboratory on Long Island, New York. He specializes in the detection of the particles
emitted in these collisions via new high-performance and low-cost detector technologies.
This expertise in particle detection applies as nicely to countertops as it does to heavyion
collisions. Prof. Llope wishes to thank Daniel McDonald and Profs. Stan Dodds
and Gordon Mutchler for helpful comments regarding the gamma spectrum measurement
Radiation and Radon from Natural Stone, W.J. Llope, May 7, 2008, page 6
noted in this article, and Judy Krieger for helpful comments on this text. Prof. Llope
can be contacted regarding this article via e-mail at the address
SaxumSubluceo@gmail.com. The comments received, his replies, and the late-breaking
results from his ongoing studies of local stone countertops will be made available at the
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