Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008

RADON, THORON AND THEIR PROGENY
IN LANCASTER PA HOMES
Henry Stewart
Penn Manor High School, Millersville, PA
720 Rohrer Road, Lancaster, PA 17603
Daniel J. Steck1
Physics Department, St. John’s University
Collegeville, MN 56321
Two radon isotopes are commonly found in US homes. Conventional wisdom holds that
220
Rn (thoron) concentrations are lower than 222Rn (radon) concentrations and pose a
smaller health risk in most living spaces. An earlier survey of radon and thoron gas in 30
Lancaster, PA homes found some with elevated concentrations of both isotopes. That
survey found some living spaces with higher short-term concentrations of thoron than
radon. To improve the estimate of the long-term risk from exposure to these gases, we
took both short and long-term measurements in 20 homes. In addition to grab samples of
radon and thoron progeny in two different seasons, we measured the long-term gas
concentrations and surface deposited activity to estimate contributions to the potential
airborne dose from radon and thoron progeny. The following averages were found: longterm average radon concentration 395 Bq m-3 (10.7 pCi/L) with an estimated radon
progeny equilibrium ratio 0.24; long-term thoron concentration 120 Bq m-3 (3.2 pCi/L)
and progeny equilibrium ratio 0.02. Estimated average effective doses calculated from
long-term gas measurements and short-term progeny measurements are: radon progeny
7.3 mSv yr-1 and thoron progeny 0.6 mSv yr-1.

INTRODUCTION
Residential radiation measurements in the US taken during the 1980’s occasionally
included both 222Rn and 220Rn (Schery 1985, 1990). To distinguish these two isotopes we
will follow the common practice of using the nicknames radon for the former and thoron
for the latter. Thoron, like radon, can pose a health risk because its progeny can deliver
radiation dose to lung and other tissues. However, the short half-life of thoron limits its
transport from sources like soil and building materials to indoor living spaces. Thus, it
was generally believed that thoron concentrations indoors would be low and have large
spatial variation making thoron exposures difficult to assess (Nero 1988).
Significant potential alpha energy concentrations (PAEC) can accumulate indoors even at
low thoron concentrations because one of its decay products, 212Pb, has a long half life.
Early surveys concentrated on thoron progeny measurements rather than thoron
measurements to better quantify the potential environmental radiation doses. They found
that thoron can generate a significant fraction of the total PAEC in homes and can be an
1

This work was supported, in part, by a St. John’s University Faculty Development
grant.
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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008

important fraction of the dose delivered to the lungs (Schery 1985, 1990, Dudney et al.
1990, Martz et al.1990, Tu et al. 1992). However, since few measurements of thoron or
its progeny have been made in homes since the early 1990s, its spatial and temporal
distribution indoors is largely unknown.
Interest in thoron has been revived with the discovery of elevated thoron indoors in
China. An earlier study of the radon and thoron in Lancaster suggested that there might
be high thoron homes there (Stewart 2007). These cases raise the question of the effects
of thoron on the potential dose to individuals and on “radon” measurement technologies
used in risk assessment. (Shang et al. 1997, Wiegand et al. 2000, Tokonami et al. 2004,
Yamada et al. 2005, Steck 2006).
While the dose is primarily delivered by the progeny for both radon isotopes,
measurements of radon gas are most common in residential risk assessment since they are
easier to make, especially over extended periods of time. While gas measurements in
homes typically average over days to months, typical progeny measurements are grab
samples that collect the radionuclides over brief periods of time (5 to 10 minutes). This
practice introduces substantial uncertainty in exposure assessment because both gas and
progeny have strong temporal variation on diurnal to seasonal scales. Integrating passive
progeny measurement technologies show promise for improving radiation risk
assessment in homes (Steck et al. 2007).
The primary goals of this study were to measure the long term average concentrations of
radon and thoron gas, to take grab samples of their progeny in two seasons in order to
estimate the doses available in select Lancaster PA homes, and to test the field
performance of passive progeny dosimeters.

METHODS
To estimate the long-term airborne radon and thoron progeny in the living spaces, we
placed track-etch gas and progeny detectors for 90 day exposures. Grab samples were
taken of the gas and airborne progeny upon placement of these detectors, and then again
upon retrieval. Estimates of the long-term equilibrium ratios (FR, FT) were based on the
averages of measurements from these grab samples. Finally, long-term radon progeny
PAEC and dose were estimated from the long-term gas concentrations and the average
grab sampled F’s.
Twenty homes, located in Lancaster County, PA, were measured for radon, thoron and
their progenies. The locations were selected from a prior study (Stewart 2007) to ensure
measurable results. The sites in the houses were selected based on the lowest lived-in
level. An equal number of basements and first floors were sampled. The initial grab
samples were taken and the long-term tests were placed between 8/18/2007 and
11/26/2007. The final grab samples were taken and the long term tests were collected
between 12/19/2007 and 2/25/2008.

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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008

Two types of grab samples were collected for this study, radon and thoron progeny, and
radon and thoron gas. The progeny grab samples were measured using the Ludlum
Measurements Inc. Model 2000 counter and Model 43-9 alpha scintillation detector, and
data were collected to computer storage using DEPOMON interfacing software. The
progeny grab sampling equipment was calibrated in the laboratory with a SARAD EQF
whose calibration is traceable to PTB. After taking a background count and checking the
calibration using a 230Th sealed source, air was pumped through a glass fiber filter for 5
minutes for radon progeny and 10 minutes for thoron progeny respectively. Then counts
from the filter were taken every minute for 50 minutes or 325 minutes, for radon progeny
and thoron progeny respectively. The progeny activities were calculated using the
Tsivoglou method for radon progeny and the Khan-Busigin-Phillips for thoron progeny
(Khan et al. 1982)
The gas grab samples were taken using the Durridge Co. Inc. RAD7. After lowering the
internal relative humidity to 9%, three consecutive 20-minute tests were conducted and
the results averaged.
Long-term average concentrations were measured with track-etch detectors. Radon and
thoron gas detectors, conventional and encapsulated ATD’s, were placed more than one
meter from the external walls, and usually about 1.6 meters from the floor, but not less
than one meter. The surface alpha activity of radon and thoron progeny (specifically
214
Po and 212Po respectively) were measured using open track registration detectors fitted
with several energy absorbing filters. In all but two cases, the surface deposition detectors
were placed on a window pane, as close to the center as achievable. In the exceptions,
they were placed on a structural steel support beam and a plastic wall fixture. These gas
detectors (Steck 2006) and surface progeny detectors have been described elsewhere
(Steck 2007 et al.).

RESULTS
Almost all of the distributions were more lognormal than normal, so geometric mean,
geometric standard deviation and average statistics are given for the major results. The
grab sample distribution statistics for the progeny measurements are given in Table 1.
The spatial and temporal variations are shown in Figure 1 and 2.
Table 1: Progeny Grab Sample Measurement Statistics in Lowest Lived-in Sites

Radon progeny (All 20 sites)
Thoron progeny (All 20 sites)

Median(GSD)
mWL

Average
mWL

38 [2.1]
3.8 [1.9]

48
4.7

Equilibrium
ratio
Median[GSD]
0.22 [1.4]
0.018 [3.4]

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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008

Figure 1: Radon progeny PAEC from the initial and final grab samples

Figure 2: Thoron progeny PAEC from the initial and final grab samples

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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008

The results from the long-term track registration detectors are shown in Tables 2 and 3.
Table 2: Long-term gas measurement statistics
Radon (All sites)
1st Floors
Basements
Thoron (All sites)
1st Floors
Basements

Number
20
10
10
20
10
10

Median [GSD]
Bq m-3
325 [1.9]
108 [3.5]

Average
Bq m-3 (pCi/L)
395(10.7)
289 (7.8)
501(13.5)
121(3.3)
56 (1.5)
185 (5.0)

Figure 3: Site-to-site-variation of the radon and thoron gas concentrations
A statistical summary of the surface activities of 214 Po and 212Po are given in Table 3.
Table 3: Long-term surface activity measurement statistics
Median [GSD]
Number
Bq m-2
214
Radon progeny( Po)
20
28 [2.0]
Thoron progeny (212Po)
20
2.6 [2.3]

Average
Bq m-2
37
3.5

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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008

DISCUSSION
Although measurement of radon gas in homes is common today, simultaneous
measurements of radon and its progeny are rare. Those kinds of measurements were
made occasionally during the early research period of the 80’s and 90’s (Nero 1988). One
reason for the rarity is the cost and difficulty of a progeny sampling protocol that would
average over the temporal variation to give a representative measure of the available
dose. The present work, taking place in select homes in a single locality, should
contribute to the data on the relationship between radon and radon progeny, but no claims
of representativeness are made for other houses. The radon is considerably elevated in
these homes compared to national averages. The average equilibrium ratio (FR), that is
the fraction of the airborne Potential Alpha Energy Concentration (PAEC) compared to
the maximum possible, is lower than the “rule-of-thumb” value; 0.24 vs. 0.4.
Nevertheless, when one calculates the dose rate available from radon progeny using the
UNSCEAR approach of Equilibrium Equivalent Concentration (EEC), the effective dose
rates average to 7.3 mSv/yr and vary from 1 to 18 mSv/yr (UNSCEAR 2000).
Thoron measurements are rare in the US. Few studies report simultaneous radon, radon
progeny, thoron and thoron progeny measurements. Schery’s work in the mid 80’s shows
a reasonable correlation between radon and thoron gas concentrations; R2~0.5 (Schery
1985, 1990 Li and Schery 1992). Our results (shown in Figure 4(a)) show general
agreement with Schery’s results as well as a recent study in the Upper Midwest (Steck
2006). Poorer correlations were found in Gansu Province between the two isotopes in
homes where thoron dominated due to building materials (Tokonami et al. 2004, Shang et
al. 2005, Yamada et al. 2006).
Correlations between thoron, radon and their progenies are not well documented. Figure
4(a) shows the relationship between the gas concentrations alone while Figure 4(b) shows
correlations between the EEC values which include gas and progeny. The R2 in both
cases is approximately 0.5.

(a)

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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008

(b)
Figure 4 Correlation between (a) radon and thoron gas concentrations and (b) equilibrium
equivalent radon and thoron gas concentrations.
Simultaneous measurements of both thoron and its progeny are difficult. Even in
locations where thoron gas is higher than normal, thoron progeny concentrations are low.
Grab samples taken with modest air flow rates and reasonable times yield progeny
concentrations with substantial uncertainties as Figure 2 illustrates. No surveys of
correlations between thoron and its progeny in the US have been published. An abstract
of recent work suggests that a central estimate of the equilibrium ratio between thoron
and its progeny in US homes may be 0.02 (Harley and Chittaporn 2006). This value is
similar to one reported for homes in China (Tokonami et al. 2004, Yamada et al. 2006)
but much less than the value given in UNSCEAR 2000. We found a median equilibrium
ratio for thoron progeny of 0.018 but a variation of a factor of 3 around that median.
Some of the variation undoubtedly arises from measurement variation and counting
statistics. Nonetheless using the UNSCEAR EEC approach, we estimate that the
available effective dose rate from thoron progeny to average 0.6 mSv/yr in this sample of
houses.
Passive progeny dosimeters based on surface deposition hold the promise to improve risk
assessment through integrating the measurement of the progeny. Passive detectors can be
left in place for months tracking the average activity of surface-deposited progeny in an
efficient, effective manner. New technologies have shown reasonably good results in
laboratory and limited field tests (Steck et al. 2007). We deployed surface alpha tracketch detectors in this study. Figure 5 shows that the correlation between the estimated
airborne progeny (EEC) and the average surface deposited activity for both the radon and
thoron is fairly good (R2 ~0.5). Part of the variability is likely due to the use of grab
samples for the F values and partly due to the variation in surface deposition conditions
which were not taken into account in this analysis.

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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008

(a)

(b)
Figure 5 Correlation between estimated airborne progeny and gas concentration for (a)
radon and (b) thoron.

CONCLUSIONS
Elevated radon, thoron and their progenies were found in most of these homes. There was
reasonable correlation between radon and thoron gas concentrations as well as the
corresponding fractions of airborne progeny. Long-term measurements of activities of
surface-deposited progeny show promise for estimating average airborne radon and
thoron progeny dose rates in residential settings.

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Proceedings of the American Association of Radon Scientists and Technologists 2008 International
Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008

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