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

POST-MITIGATION RADON CONCENTRATIONS
IN MINNESOTA HOMES
Daniel J. Steck1
Physics Department, St. John’s University
Collegeville, MN 56321
ABSTRACT
Real radon risk reduction requires that mitigation systems maintain low radon
concentrations for years. In most states, the actual radon reductions are not known since
systematic or representative sampling of post mitigation radon is not routinely done or
archived. Uncertainty about the accuracy of post-mitigation screening tests, aging effects
on system performance and follow-up testing maintenance plague calculations of the
long-term effectiveness of current mitigation techniques. Effectiveness calculations
usually assume an average post-mitigation concentration of 2 pCi/L; an assumption that
is unconvincing to some public policy makers. To investigate mitigation system
performance in Minnesota, 150 homeowners, selected from the clients of six professional
mitigators, were sent detectors for one screening test and two long-term tests. These
homeowners reported an average pre-mitigation radon concentration of 10.3 pCi/L (380
Bq m-3). Long-term radon concentrations measured during the winter and spring of 2008,
six months to 7 years post-mitigation, averaged 0.8 pCi/L. A model calculation suggests
that if this kind of effective mitigation were applied across the state, tens of thousands of
Minnesotans could be spared lung cancer mortality. The cost per life saved by mitigation
would be less than the comparable cost of medical treatment alone.
INTRODUCTION
Long-term exposure to elevated radon (222Rn) concentrations has been linked to increased
lung cancer risk. When radon concentrations in a home exceed 4 pCi/L (150 Bq m-3), the
USEPA recommends that the house be mitigated. Real radon risk reduction requires that
mitigation systems maintain low radon concentrations for years. The most common
mitigation system, particularly in the Upper Midwest, is active soil depressurization
(ASD). This method relies on a pressure difference between the soil gas underneath the
house and the atmosphere to remove the radon-laden soil gas. To be effective, an ASSD
system needs to maintain a substantial pressure difference with a fan and well-sealed
suction piping. The performance of a system is usually tested shortly after installation,
often with a short-term test left with the homeowner at when the system is installed.
Mitigation system performance can change through fan failure, blockages, and leaks.
Follow-up radon measurements by the homeowner are recommended by the EPA, but in
most states, the actual radon reductions are not known since systematic or representative
sampling of post mitigation radon is not routinely done or archived.

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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

Uncertainty about the accuracy of post-mitigation screening tests, aging effects on system
performance and follow-up testing maintenance plague calculations of the long-term
effectiveness of current mitigation techniques. Yet, the long-term post-mitigation radon
concentrations play a pivotal role in estimates of the lives saved by radon reduction
programs. There is little published data on the performance of professionally-installed
mitigation systems. Early unpublished reports of systems installed by research and
development teams in a limited number of houses suggested that 2 pCi/L might be
achievable in many homes (USEPA 1992). Two studies done in the early 1990’s
suggested that most mitigation systems could lower radon concentrations to below 2
pCi/L (Brodhead et al. 1993, Brodhead 1995). In the years since, mitigation system
effectiveness calculations usually assume an average post-mitigation concentration of 2
pCi/L; an assumption that is not well supported by a representative sample of radon
measurements in houses from all regions that have been mitigated by current practioners
using modern techniques. Public policy makers who are contemplating new laws and
regulations for radon risk reduction prefer data from systematic and unbiased samples
rather than self-reported data from mitigators or radon detector manufacturers.
In an earlier radon survey, 18 mitigated houses were measured, by chance, during a
general survey (Steck 2005). This group of houses had an average radon concentration of
2.9 pCi/L in their living spaces and 28% of them exceeding the USEPA 4 pCi/L action
level. However, the 12 houses that had been professionally mitigated had an average
radon concentration of 1.7 pCi/L and only 8% exceeded the action level.
The present study aims to assess the performance of professionally-installed mitigation
systems in a radon-prone state. The assessment is based on long term radon
measurements from an unbiased sample of single family homes whose mitigation system
is more than 6 months old. These results, when combined with radon measurements from
unmitigated homes, can be used to estimate the potential that mitigation has for saving
lives in existing Minnesota homes.

METHODS
Research funds for this study were obtained from a private source to provide privacy for
the data of both the professional contractors and homeowners who participated. Ten
professional mitigation contractors, selected from the list that the Minnesota Department
of Health maintains (MDH 2008) were sent enrollment letters requesting their
cooperation in this research project. The ten were selected to have a good sample of
experienced and new mitigators with both urban and rural clients. Five agreed to provide
client contacts for systems that were installed in single family homes during the period
from 6 months to 7 years prior to January 2008. From these client lists, invitations were
sent to 300 homeowners who were selected to reflect a good mixture of new and older
systems in urban and rural locations. Sixty-seven invitations were returned as
undeliverable. One hundred sixty six homeowners agreed to participate by returning the
post card questionnaire (3 returned the card but declined measurements). The
questionnaire contained questions about the operating status of the system, pre and post
mitigation radon measurements and practices.

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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 shows the locations of these homes and the county average radon concentration
measured during an earlier study (Steck 2005).Each home received a radon detector
packet that contained an Air Check short-term detector (AC) for a short-term, screening
measurement (ST) and two Landauer RADTRAK® alpha track detectors (ATD) for longterm measurements (LT). Detector packets were sent between 1 February and 15 March
2008. The homeowners were instructed to perform a screening measurement as soon as
possible at the location where they had made an earlier pre-mitigation screening
measurement. An ATD was also to be placed at this location, referred to as the Primary
site. An additional ATD was to be placed in a frequently occupied room usually on
another level which was referred to as the Secondary site. If possible, the Secondary site
was to be a bedroom. The ATDs were returned after mid June 2008. Hence the
measurements spanned one half of the winter season (closed house) and the spring
(mixed open and closed).

Average Rn in
Lowest Living Space
0 pCi/L - 1 pCi/L
1 pCi/L - 2 pCi/L
2 pCi/L - 3 pCi/L
3 pCi/L - 4 pCi/L
4 pCi/L - 8 pCi/L
8 pCi/L - 16 pCi/L

Figure 1 Measurement locations and county average radon concentrations
A quality assurance program using duplicates (10%), spikes (8%), and blanks (5%) was
followed to characterize the AC and ATD detector performance.

RESULTS
One hundred and sixty-six homeowners returned questionnaires describing their home,
mitigation system status, and radon measurement practices. The median age of the
mitigation systems was 2 years (average age 2.3 years) and they ranged 0.5 to 7 years.
Homeowners reported pre-mitigation average radon of 380 Bq m-3(10.3 pCi/L) and 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

post-mitigation average radon of 44 Bqm-3 (1.2 pCi/L). Table 1 summarizes the selfreported radon distribution statistics.
Table 1 Self reported pre- and post mitigation radon concentrations at the primary site

Pre mitigation Rn
Post mitigation Rn
Post mitigation Rn: Screen
Post mitigation Rn: Long term

Number
128
104
88
8

Median
Bq m-3 (pCi/L)
300 (8.0)
35 (1.0)
33 (0.9)
46 (1.3)

Average
Bq m-3(pCi/L)
380 (10.3)
45 (1.2)
44 (1.2)
44 (1.2)

Seventy-six percent of the respondents did a post-mitigation measurement. Ninety-two
percent of those measurements were short-term. For the 40 systems that were more than 2
years old, the average number of years since the last post-mitigation radon measurement
is 2.6 years and median is 3 years.
Ninety-one percent of the homes had a living space in the basement. That location was
used as the primary measurement site in this survey. The other 9% used the first floor as
their primary measurement site. Secondary measurement sites were primarily on the first
floor with only 6% on the second floor or higher.
Complete radon measurement results are available for 129 homes. Both types of
detectors met the QA performance standards. The pertinent radon measurement
distribution statistics are given in Table 2. The average of the long-term radon
measurements at the primary and secondary sites is used as the statistic to assess
mitigation effectiveness. Since many of the individual radon results were reported to be
less than the instrumental lower level of detection (LLD), Figure 2 shows that the radon
concentration distributions were neither strictly normal nor lognormal. However, above
the LLDs the radon results were better described by a lognormal distribution than a
normal distribution.
Table 2 Radon concentration measurement statistics

Screen at primary site
Long term at primary site
Long term at secondary site
Long term house average
a

Number
137
132
126
133

Median a
Bq m-3 (pCi/L)
28 (0.75)
11 (0.30)
13 (0.35)
15 (0.40)

Average
Bq m-3(pCi/L)
52 (1.42)
31 (0.84)
30 (0.80)
31 (0.83)

Distributions were lognormal above the instrumental LLD

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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
100

0.7

90
80

t
n
u
o
C

0.6

70

0.5

60

0.4

50
40

0.3

30

0.2

20

0.1

10
0

0

1
2
3
4
5
6
Long term house average Rn pCi/L

0.0
7

P
r
o
p
o
r
t
i
o
n
p
e
r

Figure 2 A histogram of the long-term average radon concentrations.
B
a
r

Figure 2 shows the post-mitigation radon distribution for the average long-term radon
concentration measured in each house. Only 3% of the houses still had home average
radon concentrations above the USEPA reference level, 150 Bq m-3(4 pCi/L), while 6%
had at least one of the measurement results above that reference value. The fraction of
homes with average radon above 110 Bq m-3(3 pCi/L) and 75 Bq m-3(2 pCi/L) were 6%
and 9% respectively; while 8% and 16% had at least one of the measurements that
exceeded those lower reference values.

DISCUSSION
The post-mitigation radon concentrations observed in the present work are in general
agreement with the early 1990’s studies in New Jersey and nationwide (Brodhead et al.
1993, Brodhead 1995). The nationwide survey, conducted for AARST included 86
mitigators mostly from the east (Brodhead 1995). He measured the long-term radon
concentrations in 226 houses which had been professionally-mitigated within the past
year or so. He found that 70% of the houses had post-mitigation radon concentrations less
than 2 pCi/L and 94% had concentrations less than 4 pCi/L.
Even though the Minnesota mitigation systems had been operating longer (average age
2.3 years) their performance was slightly better than the 1995 nationwide sample, with
90% less than 2 pCi/L and 97% less than 4 pCi/L. In fact, the median radon concentration
in these mitigated houses is about the same as the regional outdoor concentration (Steck
et al. 1999). The post-mitigation radon concentration was not strongly correlated with
the self-reported pre-mitigation (R2 ~0.1). The post-mitigation concentration did not
significantly differ from mitigator-to-mitigator nor depend strongly on the age of the
system.
Only about 60% of the homeowners did their own follow-up measurement. Short-term
measurements accounted for 90% of these homeowner post-mitigation tests. The
correlation between short-term measurements and long term average in the house in this
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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

study was similar to the correlation in unmitigated Minnesota homes R2 ~0.4 to 0.6
(Steck 2005). A single screening test correctly classified the true living space radon 92%
of the time. The predictive value of a positive test screen was 20% and a negative screen
was 98%. These performance figures were substantially better than observed in
unmitigated Minnesota homes (Steck 2005). Had the screening measurement been used
to assess the success of the mitigation system, 8% of the measurements would have
reported concentrations above 150 Bq m-3(4 pCi/L) which is quite similar the national
results. Two of those homes had long-term home average radon above the reference value
and 8 did not (false positive). Two of the homes with long-term home average radon
above the reference value had screening results below the reference value (false
negative). As long as the possibility for occasional false readings is kept in mind, shortterm measurements appear to be adequate for post-mitigation assessment.
Responsible mitigators perform one or more post-mitigation tests shortly after the system
is installed. Most of these measurements are short-term. Informal reports and online
discussions of mitigator or agency follow-up tests suggest that the effective performance
observed in the present study is not unusual. To see if that was the case for a specific
mitigator, 200 job sheets from one of the experience mitigators (RW) were analyzed. The
randomly selected job sheets contained handwritten pre- and post-mitigation radon
results. The short-term radon concentration distribution of the 20 mitigator’s clients who
participated in the current study was virtually identical to the distribution of the 200 from
the mitigator’s records.
To estimate the risk reduction that is possible through mitigation, the results of this study
were coupled with the results from a random sample of radon concentrations in living
spaces of unmitigated houses from across Minnesota. See Figure 1 (Steck 2005). Since
both radon and population are highly spatially varied, the analysis was carried out on a
county-basis. Bayesian estimated geometric means and standard deviations were
calculated for each county. A Monte Carlo simulation which used the 4 pCi/L action
level was used to generate the average radon reduction if mitigation systems achieved
and sustained an average radon concentration of 1 pCi/L. This simple model assumes a
static population and sustained mitigation performance over a 74 year lifetime. The risk
reduction was calculated by multiplying the population in single family homes, extracted
from the 2000 census data, with the radon reduction and the EPA lifetime risk estimates
(USEPA 2003). The estimated potential lives saved by mitigation are roughly 50,000 in
Minnesota. The potential lives saved by county are shown in Figure 3.
Many public health policy decisions hinge on the cost-effectiveness of the proposed
action. For some rule-making, the EPA uses a value of a statistical life saved based on the
willingness of individuals to pay for protection (EPA reference SLA). Recently, that the
reduction of that value to $6.9 million per life saved was lamented in media reports. This
value might serve as a reasonable benchmark as the upper limit of expenditures for
avoiding radon-related lung cancer through mitigation systems. The cost per life saved by
medical treatment alone can provide a lowest reference value for expenditures. Using the
American Cancer Society’s lung cancer incidence and survival data and the National

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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

Cancer Institutes’ cost for medical treatment, the cost per life saved by medical treatment
in the first year post-diagnosis is estimated to be approximately $150,000.

Potential for lives saved by
mitigating single family homes
from above 4 pCi/L to 1 pCi/L
0 - 100
100 - 250
250 - 500
500 - 1000
1000 - 10000

Total potential current residents
lives saved ~ 50,000

Figure 3 The spatial distribution by county of potential lives saved by mitigating existing
single family houses.
The cost per life saved by mitigation systems can be estimated from costs required to
identify homes above the action level, the costs to install the system, the costs to operate
the system, the maintenance costs and the lives saved per installed mitigation system.
The costs and lives saved will have a range of variation and uncertainty that depend on
the region. If all current Minnesota single family homes (2.5 residents per house) above
the action level where mitigated to a 1 pCi/L, then the cost per life saved by mitigation
would be less than or comparable to the cost per life saved using medical treatment
during the first year post-diagnosis. This conclusion was based on the following
assumptions: each measurement costs roughly three times the wholesale cost of a longterm detector ($35) and installation costs are in the range from $1000-$1800. Six
replacement fans ($120) were believed to be needed over the 70 year operational period.
Annual operating costs based on fan wattages (20 to 80W) and annual heat penalty
($100) ranged from $120 to $170. The heat penalty was based on the leakage air rate
from the recent EPA soil moisture and ASD study (Turk and Hughes 2007) along with
local heating loads. A more sophisticated analysis of the annual energy costs for central
Minnesota (Mooreman 2008) suggest a wider range ($70 to $500) and higher central
estimate ($300). Using the central cost estimate, the cost per life saved is then roughly the
same for mitigation or medical treatment. Even at the high end of the operating costs,
mitigation costs per life saved are still less than 1% of the EPA’s value of a statistical life
saved. In addition, if mitigation fails to prevent the cancer, the medical treatment option

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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

is still available. Even if the action level were lowered to 3 pCi/L, mitigation is still a
cost-effective preventive measure.

CONCLUSIONS
Like most studies, this one could have been improved with a wider sample of houses and
mitigators. Nevertheless, it shows that dramatic radon reductions are possible in many
Minnesota homes using current practices and technologies. If the kind of effective
mitigation encountered in this study were widely implemented throughout the state, tens
of thousands of Minnesotans could be spared lung cancer mortality. Since the cost per
life saved by mitigation would be less than the comparable cost of medical treatment, it
would be wise public health policy to support radon mitigation for homes.

ACKNOWLEDGMENTS
Special thanks goes to these community-minded and courageous mitigators who shared
their client lists, thus making this project possible: 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.,. I also wish to thank Brendon Murn for assistance with the mitigation
effectiveness model and Leo Moorman for helpful discussions about operating costs in
our region.

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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

REFERENCES
American Cancer Society
http://www.cancer.org/docroot/STT/stt_0_2006.asp?sitearea=STT&level=1
Accessed 6/21/2008
Brodhead B, Clarkin M, Brennan T. Initial results from follow-up measurements of New
Jersey homes mitigated for radon. Proceedings of the 1993 International Radon
Symposium, Denver, CO available at http://aarst.org/radon_research_papers.shtml
Brodhead B. Nationwide survey of RCP listed mitigation contractors. Proceedings of the
1995 International Radon Symposium, Nashville, TN available at
http://aarst.org/radon_research_papers.shtml
Minnesota Department of Health. List of Minnesota Radon Mitigation Service Providers:
http://www.health.state.mn.us/divs/eh/indoorair/radon/mitigation.html ; Accessed 12
January 2008
Mooreman L. Yearly Energy Losses of Leaky Radon Mitigation Systems. Proceedings of
the 18th Annual International Radon Symposium, Las Vegas, NV. September 2008
National Cancer Institute Table L2. Estimates of National Expenditures for Medical
Treatment for the 15 Most Common Cancers
http://www.cancer.gov/search/results.aspx?keyword=medical+cost+estimates+for+comm
on+cancers&old_keyword=medical+cost+estimates+for+common+cancers&pageunit=10
&type=rp&first=11&page=2 Accessed 7/29/2008
Steck DJ. Spatial and temporal indoor radon variations. Health Phys 62:351–355; 1992.
Steck DJ, Field RW, and Lynch CF. Exposure to atmospheric radon. Environmental
Health Perspectives; 107: 123-127; 1999
Steck DJ. Residential Radon Risk Assessment: How well is it working in a high radon
region? Proceedings of the 15th Annual International Radon Symposium, San Diego CA.
September 2005
Turk B and Hughes J. Exploratory Study of Basement Moisture During Operation of
ASD Radon Control Systems Contractor Report to: U.S. Environmental Protection
Agency, 2007 http://www.epa.gov/radon/pdfs/moisturestudy/study_main.pdf
Accessed 7/29/2008
U.S. Environmental Protection Agency . Technical support document for the 1992
Citizen’s guide to radon. Washington D.C.; U.S. Government Printing Office; 400-K92011; May 1992

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Symposium Las Vegas NV, September 14-17, 2008. AARST © 2008

U.S. Environmental Protection Agency. Assessment of Risks from Radon in Homes EPA
402-R-03-003; 2003 http://www.epa.gov/radon/risk_assessment.html Accessed
7/29/2008.

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