THE EFFECTIVENESS OF MITIGATION FOR REDUCING RADON RISK IN
SINGLE-FAMILY MINNESOTA HOMES
Daniel J. Steck
AbstractVIncreased lung cancer incidence has been linked with
long-term exposure to elevated residential radon. Experimental
studies have shown that soil ventilation can be effective in re-
mitigation systems in the U.S. are installed by private contrac-
tors. The long-term effectiveness of these systems is not well
known, since few state radon programs regulate or indepen-
dently confirm post-mitigation radon concentrations. The effec-
tiveness of soil ventilation systems in Minnesota was measured
for 140 randomly selected clients of six professional mitigators.
Homeowners reported pre-mitigation radon screening concen-
trations that averaged 380 Bq mj3(10.3 pCi Lj1). Long term
post-mitigation radon measurements on the two lowest floors
show that, even years after mitigation, 97% of these homes have
concentrations below the 150 Bq mj3U.S. Environmental Pro-
tection Agency action level. The average post-mitigation radon in
the houses was 30 Bq mj3, an average observed reduction of
990%. If that reduction was maintained over the lifetime of the
1.2 million Minnesotans who currently reside in single-family
homes with living space radon above the EPA action level, ap-
proximately 50,000livescouldbe extended for nearly two decades
by preventing radon-related lung cancers.
Health Phys. 103(3):241Y248; 2012
222Rn; radiation protection; radiation risk; risk
RADON (222RN) HAS been identified as the most important
source of ionizing radiation exposure from natural sources
(NCRP 2009; UNSCEAR 2009). Radon is ubiquitous;
outdoor concentrations are low, on the order of 10 Bq mj3,
but indoor radon concentrations can be more than an order
of magnitude higher and vary substantially from house
to house and from time to time. The effective dose rate
available in residences can exceed 10 mSv yj1(UNSCEAR
2009). The U.S. has large regionswhere many houses have
annual average indoor radon concentrations that exceed
recommended radiation protection action levels (USEPA
2010, 2011). Many public health organizations recom-
mend that people measure radon in their living spaces and
take protective actions when radon concentrations are el-
evated (USDHHS 2005; WHO 2009; USEPA 2010). In
the U.S., the Environmental Protection Agency (EPA) rec-
ommends a screening protocol that starts with a radon
measurement that lasts for a minimum of 2 d in the lowest
lived-in level of a closed-up home. The EPA recommends
that the house be mitigated when radon concentrations
exceed 150 Bq mj3(4 pCi Lj1).
and Boice 1997; Field et al. 2000; Darby et al. 2005;
Krewski et al. 2005). This risk can be reduced by lowering
depressurization of the soil underneath a slab or a crawl-
space membrane, is the most common technique currently
used to reduce radon concentrations in single-family homes
in North America (Rahman and Tracy 2009). This tech-
nique uses a fan to create a pressure difference between the
soil gas underneath the house and the atmosphere (Scott
1988). The radon-laden soil gas is exhausted to the atmo-
sphere at a location where soil gas reentry into the house is
unlikely. This technique has been found to be effective for
many dwellings in different climates and countries (WHO
2009). Radon reduction rates ranging from 65% to 99%
using soil ventilation techniques were reported for early
Leovic et al. 1988; Henschel 1994). After the development
of standard practices and technologies, most of the radon
sector contractors. The only systematic survey of the actual
radon reduction created by private sector-installed systems
has been routinely used as the de facto rule of thumb for
mitigation’s attainablegoal in advice to the public (USEPA
Physics Department, St. John’s University, Collegeville, MN 56321.
The author declares no conflict of interest.
For correspondence contact: Daniel J. Steck, Physics Department,
St. John’s University, Collegeville, MN 56321, or email at dsteck@
(Manuscript accepted 16 February 2012)
Copyright * 2012 Health Physics Society
et al. 1999).
Significant risk reduction requires that mitigation sys-
tems maintain low radon concentrations in the living spaces
for years. Current practice calls for the system’s performance
to be confirmed by a post-mitigation test conducted by the
contractor, the homeowner, or an independent measure-
ment technician between 1 and 30 d after the installation.
This screening test is usually done under closed-house
level of the building. Most mitigators will guarantee that
this test result will be below the action level. Long-term
radon concentrations in living spaces are not usually mea-
sured after the system is installed. Radon can increase over
time due to fan failures, pipe blockages, leaks, and house or
HVAC modifications. Although follow-up radon measure-
ceased its radon proficiency program, which had provided
some oversight and guidance to radon measurement and
mitigation professionals. Since then only a few states have
instituted radon programs that regulate or independently
confirm post-mitigation radon concentrations. Therefore,
the long-term radon reduction created by and maintained
by mitigation systems is not well known.
Long-term radon concentrations were measured in 18
of homeowners who had received a free radon detector
from a health department (Steck 2005). Five of these mit-
igated houses had radon concentrations in the living spaces
thatexceededthe actionlevel. One of the 12 professionally-
mitigated houses had a failing system, while four of the
sample raised questions about the long-term effectiveness
of mitigation systems.
The present work examines radon reduction effec-
tiveness of mitigation systems installed by private-sector
contractors who work in a radon-prone state that exercises
minimal governmental oversight. Long-term radon mea-
surements were made in the living spaces of an unbiased
sample of single-family homes whose mitigation systems
were more than 6 mo old. In addition, screening tests were
conducted to investigate their performance as diagnostic
tests for long-term radon exposure. The potential effects of
radon reduction on lung cancer mortality in Minnesota
were estimated from the population-weighted distribution of
post-radon mitigation concentrations, pre-mitigation radon,
risk coefficients, and population in single-family homes.
MATERIALS AND METHODS
Ten mitigation contractors listed on the Minnesota
Department of Health website in 2007 were asked to coop-
eratein this research project. These mitigators were chosen
in an attempt toget a representativeyet unbiased sample of
homesmitigatedbyprofessionals. Thecontractors provided
a list of clients whose homes had been mitigated for 6 mo
or longer. Confidentiality was assured for both mitigators
and their clients. Six mitigators agreed to participate. They
had been in the business from 2 to 20 y. Their client lists
ranged from 5 Y 250 single-family homeowners. Invitations
offering freepost-mitigation radon measurementswere sent
to 300clients. These clientswere selected togivea balance
of new and old systems, rural and urban locations, expe-
rienced and inexperienced mitigators. The invitation in-
cluded an enrollment postcard with questions about the
status of their mitigation system and radon measurement
practices. Sixty-seven invitations were returned as unde-
liverable. One hundred sixty-six homeowners returned
questionnaires. The locations of the participating homes
(triangles) are shown in Fig. 1 superimposed on a map of
Minnesota that is shaded to show county median radon
concentrations measured in earlier radon surveys of un-
mitigated homes (Steck 2005).
Between 1 February and 15 March 2008, each home-
owner received an activated charcoal detector (AC) (Air
Chek, Inc., 1936 Butler Bridge Rd., Mills River, NC
28759 USA) for a closed-house screening measurement.
Homeowners were instructed to perform a 4-d long radon
measurement with the AC detector at the location where
they had made their earlier pre-mitigation measurement.
This location is referred to as the primary site. All these
Fig.1. Mitigated homelocations (triangles) superimposed on a map
shaded to reflect countyVmedian radon concentrations.
242 Health Physics September 2012, Volume 103, Number 3
short-term measurements were completed during the win-
ter season. Two alpha track detectors (ATD) (RADTRAK,
Landauer Inc., 2 Science Rd., Glenwood, IL 60425 USA)
were sent for long-term measurements in the home during
an extensive period of normal house operation. One ATD
was to be placed at the primary site. The second ATD was
to be placed in a frequently occupied room on another
level if possible, preferably a bedroom. This location is
referred to as the secondary site. The ATDs were returned
after 90 or more days of exposure between mid-June and
the end of September 2008. Previous research had shown
that long-term measurements taken across heating and
non-heating seasons would produce results within 25% of
the true annual average radon concentration in Minnesota
homes (Steck 1992, 2005).
Both the AC and ATD detectors met the goals of a
quality assurance program,which included 10% duplicates,
8% spikes, and 5% blanks. Duplicate detectors showed a
reproducibility of 15% or less for radon concentrations
near the action level. With the exception of one ATD, all
spikes were within 25% of the exposure target value. No
unexposed detector reported a concentration above its
lower level of detection (LLD). The LLD for the AC de-
was 1,100 Bq d mj3(30 pCi d Lj1), which corresponded
to an exposure for 150 to 90 d at radon concentrations of
7 to 12 Bq mj3(0.2 to 0.3 pCi Lj1).
All the mitigation systems used activesoil ventilation.
Theaverage system agewas 2.3 yin 2008 and rangedfrom
0.5 to 7 y. Homeowners who had documentation of radon
screening results prior to 2008 reported pre-mitigation
concentrations that ranged from 100Y2,400 Bq mj3in
128 houses. Ninety-one percent of the homes had a living
space in the basement, which was the primary location for
these measurements. The pre-mitigation concentration was
lognormally distributed with an average of 380 Bq mj3
(10.3 pCi Lj1), geometric mean of 295 Bq mj3, and a
factor of 1.8 variations (geometric standard deviation).
Seventy-six percent of the respondents had a post-mitigation
measurement result taken shortly after the installation.
These post-mitigation concentrations averaged 45 Bq mj3
(1.2 pCi Lj1). However, the distribution shown in Fig. 2
has a distortion from the expected shape at the LLD of AC
detectors. Based on these homeowner reported radon re-
sults, the mitigation systems reduced radon an average of
85% (median 89%). For the 76 systems that were more
than 2 y old, 55% had been retested as recommended by
the EPA. The average number of years since the last
‘‘check-up’’ radon measurement was 2.6 y (median 3 y).
To assess possible bias in results between this sur-
vey’s participants and other clients, pre- and post-mitigation
radon results were analyzed from 200 randomly selected
job sheets from one of the experienced mitigators. The
short-term radon concentration distributions of the 20 par-
ticipants and the larger sample were statistically identical.
Unfortunately, no prior long-term measurements were avail-
able to test for bias.
Long-term and screening measurementswere made in
the 2008 survey. Only 9% of the primary measurements
were on the first floor; the rest were in the basement. Sec-
ondary site measurements were taken on the first floor with
only 6% on the second floor or higher. Complete post-
mitigation measurements are available for 127 homes; 140
long-term measurements are available at the primary site,
132 at the secondary site, and 129 at both sites. Short-tem
screening results are available at the primary site for 137
homes. The results of the 2008 post-mitigation measure-
results in Table 1.
reported by homeowners from measurements before (black circles)
and after (grey squares) mitigation. A plot of the expected value of
the cumulative probability against the log of the variable would be
a straight line for a pure lognormal distribution.
Table 1. Pre- and post-mitigation radon concentration distributions.
Results reported by homeownera
Post-mitigation measured in 2008
Short-term: primary site
Long-term: secondary site
128 380 300100Y2400
aMeasurements made prior to 2008.
bLLD for ATD detector exposed for 150 d.
cLLD for AC detector.
243 Reducing radon risk in Minnesota homescD. J. STECK
Post-mitigation screening radon distributions are sim-
ilar for the homeowner-reported measurements and those
measured in 2008, except at the low and high extremes as
illustrated in Fig. 3. Since both distributions depart from
log normality at the LLD of AC detectors, nonparametric
to test if there is a statistically significant difference be-
tween these distributions. Both tests indicate that the dis-
tributions are statistically different.
Post-mitigation long-term radon concentration distri-
butionsshowninFig. 4 arevirtually identical atthe primary
(basement) and secondary sites (first floor). Nonparametric
statistical significance tests support this observation. When
the data are restricted to houses with results from both sites,
the first-floor distribution has a nonsignificant higher me-
dian than the basement. In contrast, unmitigated homes in
the basement than on the first floor (Steck 1992, 2009).
post-mitigation radon concentration in the home’s living
spaces. The house-average radon was above the current ac-
tion level in only 3% of the houses. Six percent of the
compared to other possible reference levels, only 6% had
radon concentrations above 110 Bq mj3(3 pCi Lj1), 9%
system’s effectiveness did not depend on age or mitigator
as illustrated in Fig. 5 (p G 0.05).
The most commonly used benchmark as the achiev-
(USEPA 2010). This valuewas also used in mitigation cost
effectiveness analyses (USEPA 1992; Ford et al. 1999).
However, more than 90% of the long-term post-mitigation
radon concentrations measured in the present survey were
lower than that. The median radon concentration in these
as the median regional outdoor radon concentration at 1 m
radon concentration was more than 2,000 Bq mj3had less
than15Bqmj3after mitigation.So,atleast inthis sample,
the long-term goal of the 1988 Indoor Radon Reduction
Act was met for many houses, in that the indoor air was as
free from radon as the ambient air outside buildings.
Fig. 3. Distribution of short-term radon measurements made shortly
after mitigation as reported by homeowners (grey box) and those
measured 0.5 to 7 y after mitigation in the present work (black
Fig. 4. Cumulative probabilities for long-term radon concentrations
at the primary (grey square) measurement site and secondary sites
Fig. 5. Post-mitigation radon concentration distributions grouped
by mitigator and age. Radon distributions of systems whose age was
less than 2 y are shown with the dark black boxes, and older system
distributions are shown as grey boxes. Median values of long-term,
house-average radon are shown as horizontal lines within the boxes
whose boundaries include 50% of the results. No old systems were
measured for mitigators REJB or RRGV.
244 Health PhysicsSeptember 2012, Volume 103, Number 3
A geographically more diverse survey, conducted
shortly after the establishment of U.S. mitigation standards,
did report a higher post-mitigation radon distribution than
the present work (Brodhead 1995). The 1995 survey in-
cluded 86 mitigators and 226 houses that had been miti-
gated within a year of the survey. Most of the houses were
located east of the Mississippi. Since mitigators generally
guarantee reducing the radon below the action level, it is
not surprising that both studies found similar percentages
of houses below this level; 94% in the 1995 study and
97% in the present study. However, a higher percentage,
of Minnesota mitigation systems reduced radon below
lower benchmarks. For example, 91% of the mitigated
Minnesota houses had radon below 74 Bq mj3compared
to only 70% in the 1995 study. This result suggests that
mitigation systems can reduce the risk more than previ-
ously assumed and be more cost effective.
Short-term post-mitigation measurements
Post-mitigation screening measurements reported by
homeowners and those measured in 2008 are moderately
measurements are also correlated with the long-term post-
mitigation radon at the primary site and the house-average
radon. These correlations are statistically significant (p G
0.05 level) but too weak (r È 0.5 to 0.2) to establish ac-
curate predictive relationships.
Screening tests seem to be adequate for periodic post-
mitigation assessment of a home’s long-term average ra-
don concentrations relative to the 150 Bq mj3action
level. Table 2 shows that a single post-mitigation screen-
ing test correctly classified the long-term livingspace radon
relativeto the action level 92% of the time. The probability
that a test result below the action level (negative result)
came from a homewhose long-term radon levelwas below
the action levelwas 98%. Only two of the homeswith long-
term home average radon above the action level had screen-
ing results below the action level. The probability that a
was above the action level was only 20%. However, the
95% confidence interval of this probability is large (3% to
50%) because only two of 10 positive tests belonged to
houses with long-term radon above the action level. Table 2
shows that post-mitigation screening tests do better at di-
agnosing homes than pre-mitigation tests in unmitigated
houses from the same geographic region (Steck 2005).
Three factors may be responsible for this improved
diagnostic performance of post-mitigation screening tests.
First, the lower radon concentrations decrease the chance
that temporal or instrumental variations will cause short-
term measurements to misclassify the radon at the primary
site. Support for this possibility comes from a comparison
of the diagnostic performance of the current results with
similar median, 48 Bq mj3(White 1994). The correct clas-
sification rate for the U.S. set of unmitigated houses was
92% and 93% in the post-mitigation Minnesota houses.
However, in two groups of unmitigated Minnesota houses
170 Bq mj3), the correct classification rate was only 55%
and 80%, respectively (Steck 2005). Second, mitigation
systems eliminated the large difference in radon concentra-
primarysiteresult islikely tobe closer tothe house-average
radon than a pre-mitigation result. Finally, anecdotal evi-
dence suggests that the magnitude of post-mitigation tem-
poral radon variation is much less than pre-mitigation
variation. For example, the daily average radon concentra-
tion variations shown in Fig. 6 from the primary site in the
author’s house are much lower after mitigation than during
the same periods during two different years prior to mitiga-
tion. The daily averages were calculated from hourly
measurements of calibrated continuous radon monitors:
before mitigation, using a Pylon AB-5 with PRD (Pylon
Table 2. Diagnostic performance indices of screening tests to
predict long-term, house average radon in mitigated and unmitigated
Performance indices for
Correct classification rate
Predictive value positive resultc
92% (86 Y 96)b
20% (3 Y 50)
98% (94 Y 100)
54% (40 Y 70)
40% (30 Y 60)
80% (50 Y 90)
aMitigated results from current study; unmitigated houses from Steck (2005;
b95% confidence interval.
cProbability that a positive screening test result will be from a house with
radon above the action level.
dProbability that a negative screening test result will be from a house with
radon below the action level.
Fig. 6. Variation of the daily average radon in one house during the
same time-of-year before mitigation in 2003, 2004, and after miti-
gation (14 December 2010) in 2010.
245 Reducing radon risk in Minnesota homescD. J. STECK
Weissbadener Strasse 20, D-01159 Dresden, Germany).
lessthan2%.The day-to-dayvariationaveraged50 and100
and 10 Bq mj3(36%) after mitigation in 2010.
Risk and risk reduction
The risk of radon-related lung cancer depends on the
energy delivered to the lungs. More of this energy comes
from short-lived alpha-emitting radon decay products than
radon. Reducing the external radon gas source through
active soil ventilation is not likely to distort the ratio be-
tween airborne radon decay product energy and radon con-
surrogate for risk reduction in homes.
The potential for risk reduction in a population de-
pends on the number of people who are exposed to radon
levels that can be effectively and efficiently lowered. The
percentage of cumulative risk for exposures up to a specific
radon concentration can be calculated from the product of
the radon concentration probability distribution and the
lifetime risk, which depends on radon concentration, in-
tegrated up to that specific radon concentration divided by
the integral over the entire range of radon concentrations.
This percentage can be useful to estimate the potential for
risk reduction at different action levels in different regions.
An example of the difference between the cumulative per-
centage of riskfor the U.S.andMinnesota (MN) populations
is lognormally distributed with a median of 26 Bq mj3
higher; 120 Bq mj3(Steck 2005). The cumulative risk
below 100 Bq mj3is roughly 50% in the U.S., while in
Minnesota about 20% of the cumulative risk comes from
homes below 100 Bq mj3. Only 25% of the U.S. risk is
above the 150 Bq mj3action level, but most of the risk in
Minnesota (65%) falls above the action level. Since miti-
gation at this action level reduced radon concentrations on
average by 90%, the potential for efficient risk reduction is
high in Minnesota.
Radon mitigation’s health impact can be assessed in-
directly using radiation dose reduction. The average dose
rate reduction was 8 mSv yj1in the mitigated Minnesota
homes when calculated using the current effective dose
conversion factor recommended by UNSCEAR (2009).
be calculated directly from epidemiological studies of radon-
related lung cancer. An estimate for the absolute lifetime risk
coefficient per unit radon exposure for a population can be
risk, age and gender distribution, smoking prevalence, and
by the EPA for the U.S. population estimated that the life-
time lung cancer mortality risk is 1.5 ? 10j4per Bq mj3
radon concentration in the house. The estimated uncer-
tainties in this coefficient are likely to be a factor of two
or three with the major uncertainty coming from the risk
model (Pawel and Puskin 2004). Recent residential expo-
sure epidemiologic results may help reduce this uncertainty
(Krewski et al. 2005; Darby et al. 2005; WHO 2009). That
lifetime risk coefficient was used in a Monte Carlo simu-
lationto model the risk reduction distribution in Minnesota,
assuming that the Minnesota age and smoking character-
istics are similar to the national average. A simple model
by assuming a static population in single-family homes
over the lifetime (74 y) of each resident. Since the number
of occupants and the radon concentration in single-family
homes are spatially inhomogeneous, the estimate of the po-
tential risk reduction was done on a county-by-county basis.
Bayesian estimates were used to calculate the distribution
of pre-mitigation long-term radon concentrations in the
living spaces and the fraction of homes above the action
level (Steck 2005). The fraction of single-family homes
above the action level, along with the number homes and
number of occupants per home from the 2010 census, pro-
duced an estimate that 1.2 million Minnesotans currently
live in homes with radon above the action level (USDCCB
measurements in the present study. The simulation calcu-
lated the risk reduction for each mitigation to generate an
estimate of the risk reduction distribution. An average
lifetime risk reduction of about 4% resulted from the av-
erage radon concentration reduction of 300 Bq mj3.
The estimated number of lung cancers that could be
prevented in each county is mapped by county in Fig. 8.
Notethat the shadingin Fig. 1is different from the shading
in Fig. 8 where both population density and radon dis-
tributions are reflected. Preventing a radon-related lung
Fig. 7. Cumulative percentage risk for a population at exposures
below the specific radon concentration. Example: 50% of the radon-
related risk for the U.S. population occurs at concentrations below
80 Bq mj3.
246 Health PhysicsSeptember 2012, Volume 103, Number 3
Puskin 2004). Approximately 50,000 of the Minnesotans
who currently reside in high radon homes could have
their lives extended by mitigating their homes with ac-
tive soil ventilation.
Radon concentrations in most single-family homes
were reduced from concentrations above 150 Bq mj3to
concentrations close to those in outdoor air using active
soil ventilation systems installed by private-sector contrac-
tors. Short-term screening measurements appear to be ad-
equate diagnostic tests for post-mitigation status relative
to the current radon action level. If mitigation systems as
effective as those observed in this study were widely im-
plemented and sustained in Minnesota homes with radon
above the action level, an estimated 50,000 people could
have their lives extended for nearly two decades by pre-
venting radon-related lung cancer. Radon mitigation using
active soil ventilation would likely result also in substantial
positive public health benefits in other radon-prone regions
with similar characteristics.
AcknowledgmentsVSpecial thanks to these mitigators who made this proj-
ect possible by freely sharing their client lists: William and Robert
Carlson, Healthy Homes LLC; Randy Weestrand, Radon Removal Inc.; Jack
Bartholomew, Radon, Energy and Ventilation Services; Will Rogers, Radon
Relief; and Gary Vaness, Radon Reduction Inc. Thanks to Henry Schuver
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