United States
              Environmental Protection
              Agency
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
              Research and Development
EPA/600/SR-95/149     April 1996
EPA      Project Summary
               Design and Testing of Sub-Slab
               Depressurization  for Radon
               Mitigation in  North  Florida  Houses:
               Part  I  -  Performance and  Durability
              C. E. Roessler, R. Morato, R. Richards, H. Mohammed, D. E. Hintenlang,
              and R. A. Furman
                A demonstration/research  project
              was conducted to evaluate sub-slab
              depressurization (SSD) techniques for
              radon mitigation in North-central Florida
              where the housing stock is primarily
              slab-on-grade and the sub-slab medium
              typically  consists of  native soil and
              sand. Objectives included developing
              and testing the use of a soil depressur-
              ization computer  model as  a design
              tool,  optimization of SSD design for
              North Florida houses, and observation
              of the performance and durability  of
              the installed systems.
                Between May 1989 and August 1990,
              SSD systems were designed and in-
              stalled in nine houses—seven  with
              simple rectangular floor plans and two
              with more complex L-shaped designs.
              Installations included a single-suction-
              point system in one house and  two-
              suction-point/single-fan systems  in
              eight houses. The installation in one of
              the larger L-shaped houses  consisted
              of a single-suction-point system in ad-
              dition to a two-suction-point/single-fan
              system. All systems used small diam-
              eter, nominal 50-mm (2-in.) piping.
                All houses were equipped with  con-
              tinuous radon monitors and integrat-
              ing radon monitors were also deployed.
              All houses were visited on  a regular
              schedule  for measurements and obser-
              vations.
                The mitigation successfully reduced
              indoor radon concentrations originally
              on the order of 10 to 30 pCi/L to post-
              mitigation values of <4 pCi/L in all nine
              houses. Levels were reduced to values
 on the order of 2 pCi/L or less in three
 houses.
  Installation experiences demon-
 strated the importance of avoiding
 "short-circuit" air flow leakage near
 suction points, providing drainage for
 moisture that condenses in the system
 during cooler weather (even in Florida),
 and sealing around discharge ducts at
 roof penetrations to prevent re-entry of
 exhausted sub-slab gases.
  System manipulations indicated that
 a single suction point was sufficient on
 two houses with 160 to 170 m2 (1700 to
 1800 ft2) slabs, but that passive ventila-
 tion is not likely to be effective for this
 type of sub-slab medium.
  During the  limited time available for
 durability  observations  (3  to 18
 months), the systems retained effec-
 tiveness in  maintaining reduced indoor
 radon concentrations, no  fans failed,
 and no  structural  effects were ob-
 served.
  This Project Summary was developed
 by EPA's National Risk Management
 Research Laboratory's Air Pollution
 Prevention  and Control Division, Re-
 search Triangle Park, NC, to announce
 key findings  of the research project
 that is fully documented in a separate
 report of the same title (see Project
 Report ordering information at back).

 Background
   This work was conducted in North
 Florida because  of the presence of el-
 evated indoor  radon1 levels and the need
 to investigate  mitigation techniques sue-

-------
cessful for the  housing  stock and  condi-
tions represented.
  In late  1986, results  from a  statewide
indoor radon survey identified  two focal
areas of elevated indoor radon in Florida:
one in the Bone Valley phosphate mining
region of West  Central  Florida  (Polk,
Hillsborough, and  surrounding counties);
and the other in the North Florida  Haw-
thorn  formation region—with the greatest
affected  populations in the Gainesville-
Ocala area  (Alachua and  Marion  coun-
ties). Several other studies have confirmed
the presence of indoor radon levels rang-
ing  from  <1 to about  200 pCi/L in this
area.
  In Florida, the housing stock is primarily
of slab-on-grade construction with several
variations of floor-wall joining. There is a
small  percentage of crawl-space and slab/
crawl-space combination  houses  (both
open  and enclosed crawl  spaces),  and
there are very few houses with basements.
  The U.S.  Environmental Protection
Agency (EPA) has suggested that soil de-
pressurization  is  the   most successful
method of limiting indoor radon;  thus sub-
slab depressurization (SSD) appeared to
be  a  promising  mitigation method for
Florida houses. However, at the time this
project was initiated, most  mitigation ex-
perience  in the  U.S. had been with base-
ment  houses.  Furthermore, the sub-slab
materials  commonly used in Florida  con-
struction  consist of native soil  and sand.
These would be expected  to have  lower
air permeabilities than the coarse gravels
commonly used under basement slabs in
regions of the U.S.  where SSD  has been
highly successful.  This suggested  that
more  complex and  more robust systems
might be  required  to successfully control
radon in construction typical of Florida.
  Radon mitigation  demonstration work in
Florida was  begun with the  1987 initiation
of the EPA-sponsored Florida Radon Miti-
gation Project - Phase I  in Central Florida
(Polk county). In late 1987, EPA also spon-
sored a University of Florida (UF) project
to identify elevated radon  houses  that
might serve as candidates for  a parallel
North  Florida mitigation project.  During the
1987-88 winter, screening measurements
(charcoal  collector method)  were made in
nearly 400 Gainesville  and  Ocala vicinity
slab-on-grade  houses  in neighborhoods
designated on the  basis of  geological po-
tential for elevated radon. In these screen-
ing  measurements on this selected group
of houses, about 70% of the indoor radon
concentrations exceeded  4 pCi/L  and
about 20% of the total exceeded 20 pCi/L.
  The North  Florida  effort continued  with
the  August 1988 initiation of the research
and demonstration project, Florida Radon
Mitigation Project Phase II - North Florida.

Objectives
  This project had a demonstration objec-
tive and  a series of  research objectives.
The demonstration objective was to dem-
onstrate  mitigation methods that are ef-
fective for the substrate and construction
type characteristic of the  North  Florida
region. Initial emphasis was on sub-slab
depressurization (SSD). The project  had
three  research objectives:
   1.     Develop tools for design of SSD
         systems. This includes testing the
         use of a soil depressurization
         computer model2.
   2.     Optimize SSD design for North
         Florida houses.
   3.     Observe the short- and long-term
         performance and durability of the
         installed SSD system in  this  en-
         vironment

  This project involved the following work
areas:
   1.     Select and characterize a candi-
         date pool of houses.
   2.     Mitigate a subset of these houses
         —select houses, design mitigation
         systems, and install mitigation
         systems.
   3.     Monitor:
         3a. Collect baseline data prior
             to mitigation.
         3b.   Monitor initial post-mitigation
             performance.
         3c.   Conduct special studies on
             installed systems for the  pur-
             pose of system optimization.
         3d.   Evaluate durability of in-
             stalled systems-continue
             monitoring and observations
             for the duration of the
             project.

Diagnostic Methods
  From the data obtained in a house iden-
tification  study, 12 elevated  radon houses
were selected as potential candidates for
1The terms "radon" and "Rn" are used to designate the
 radon isotope, radon-222; and "radium" is used to
 designate radium-226.
2 Further development and testing of a computer model
 previously developed at UF for simulating sub-slab
 pressures and flows during the operation of soil de-
 pressurization systems was authorized. The work,
 which included expanding the model, developing it as
 an SSD design tool, and validation, is presented in Part
 II of this report.
the  mitigation  demonstration.  These
houses  were visited, and the  EPA diag-
nostic  measurements  were  performed.
These  diagnostic  observations included
descriptive information, sub-slab measure-
ments, radon measurements, and  house
dynamics observations.
  Sub-slab  measurements included de-
termination of soil gas radon by "sniff and
"grab" sampling, sub-slab communication
testing, calculation of effective  permeabil-
ity, and sampling of the sub-slab material.
Indoor  radon  measurements  included
short-term measurements  of concentra-
tions in the living  space and  also  mea-
surements of radon  in the  building  shell.
House dynamics measurements included
indoor/outdoor and indoor/sub-slab  pres-
sure differential measurements under vari-
ous conditions and blower door pressur-
ization/depressurization tests.

Mitigation System Design and
Installation
  The design procedures are described in
the Part II report. Briefly,  potential suction
points were located on the basis of acces-
sible, unobtrusive locations—usually in in-
terior closets. The UF soil depressuriza-
tion computer model  was then used as a
design tool. For an  initial set of five houses
selected  for mitigation in  1989, the  model
was  used to simulate pressure  fields un-
der  proposed designs.   Suction system
pressures and  flows were predicted by
superimposing the sub-slab "system curve"
on the respective fan performance curves
of candidate fans. For each house, the
number of suction points, their locations,
and the fan size were selected from the
combination giving a  pressure field  cover-
age believed to be adequate to overcome
inflow of radon-bearing  soil gas. Subse-
quently, the  computer model was used  to
develop  soil  depressurization system
guidelines for the  Florida radon-resistant
building  code.  For  a second  set  (four
houses)  selected for mitigation in  1990,
system designs were specified  using the
evolving code guidelines.
  To save  cost,  reduce  space require-
ments, and facilitate  installation, nominal
50-mm (2-in.) polyvinyl chloride (PVC) pip-
ing was specified for the  major runs of the
SSD systems rather than the nominal 100-
mm (4-in.) piping reported in the literature
for previous  mitigation projects. It was an-
ticipated  that,  because of  the low  flows
associated  with  the low permeability
Florida sub-slab medium, flow-related pres-
sure losses in  the smaller piping  would
not be large enough to  compromise the
effectiveness of the system.

-------
  At each suction point,  sub-slab fill and/
or soil was  removed  to form  a  roughly
hemispherical pit, approximately 0.5 to 0.9
m (20 to 36 in.) in diameter. Nominal 100-
mm  (4-in.) PVC piping  with a  cleanout
branch to serve  as an access port was
installed through the slabs. The  remain-
der of the suction  system consisted  of
nominal 50-mm (2-in.) PVC piping. Suc-
tion piping was run vertically from the pit
to the attic. Fans  were located in the attic.
For the systems  with two suction  points,
lateral  piping was  run from the  vertical
risers to a tee located under the suction
fan. For the  single-suction-point systems,
the  vertical piping was run directly up  to
the  fan.
  Systems were  installed by the research
team. Electrical hookup was provided by
licensed electrical contractors.

Monitoring

Approach, Parameters, and
Measurements
  At each  house,  monitoring  was con-
ducted during  three time periods: 1) the
baseline data collection period, 2) the sys-
tem installation and tuning  period,  and  3)
the  post-installation  performance/durabil-
ity monitoring period. Data collection con-
sisted of a combination  of 1) continuous
multi-parameter data acquisition (in a sub-
set  of four houses), 2) continuous radon
monitoring, 3)  integrated radon monitor-
ing, and 4) point measurements and ob-
servations in conjunction with site visits.
  The  continuous recording data-acquisi-
tion systems which  were installed in the
subset of four  houses (referred to as "in-
strumented houses") consisted of data log-
gers with sensors for pressure differential
(outdoor vs.  indoor  and  sub-slab  vs.  in-
door), indoor radon, temperature (indoor
and outdoor), rainfall, and wind speed and
direction. Data were sampled  every  30
seconds and summed or averaged,  and
hourly  sums or averages were stored  in
memory.
  Indoor  radon was monitored continu-
ously in all houses,  either  as part of the
data logging system (hourly averages)  in
the  instrumented houses or by a stand-
alone continuous radon  monitor  (4-hour
averages) in the other houses. Integrating
radon monitors (electret  ionization cham-
bers) were also used.
  During  site visits to the  houses, pres-
sures and flows were measured  in the
suction lines near the suction point,  and
"sniff and "grab" sample measurements
were made of radon concentrations in the
sub-slab  and/or  exhaust air.  Qualitative
observations were made of the system
and house condition.

Baseline Measurements
  Baseline data  collection was targeted
for at least a month-long period prior  to
installation of the SSD system. Measure-
ments  included indoor  radon  concentra-
tion by integrating detectors, indoor radon
concentration by  continuous  monitoring,
pressure  differentials  (in some  instru-
mented houses),  and  weather data (in
some instrumented houses).

Post-Installation Performance/
Durability Monitoring
  Following installation  and  tuning of the
mitigation system, continuous  data acqui-
sition systems (instrumented  houses)  or
continuous radon  monitors  (non-instru-
mented houses) were operated, integrat-
ing  radon monitors were deployed,  and
periodic house visits were performed.
  Post-installation  monitoring  was  con-
ducted according to the following general
three-stage schedule:
   • Stage  1  Monitoring (in  service <6
    months)-Continuous and/or integrat-
    ing indoor radon  monitoring was per-
    formed and  houses were visited bi-
    weekly to observe system operation,
    measure  pressures and  flows,  and
    service radon monitoring equipment.
  •  Stage 2 Monitoring  (in service 6 to 12
    months)—Houses without data loggers
    were visited monthly. For houses with
    data loggers,  data acquisition  was
    continued, data were reviewed,  and
    visits were performed as necessary.
  •  Stage 3 Monitoring (in service  >12
    months)—As  a longer-term follow-up,
    visits were conducted approximately
    every 6 months  to  inspect the  sys-
    tems, measure pressures and flows,
    and deploy radon monitors for a week-
    long  measurement.
   Performance and durability were evalu-
   ated in terms of:
  •  System Performance and Interaction
    with  the  Sub-slab   Medium—System
    pressures and flows, noise and vibra-
    tion, and requirements for adjustments
    and maintenance.
  •  Condition  of the Sub-slab Environ-
    ment-Effective permeability calculated
    from pressures  and flows, and ex-
    haust air and/or  sub-slab  radon  con-
    centrations.
  • Effectiveness-Indoor radon concen-
    trations.
  • Structural Effects-Observations  for
    evidence  of subsidence, heaving,
    cracking, separation of joints, etc.
  In addition,  responses were made  to
homeowner questions or homeowner-iden-
tified problems.

Results and Discussion

House Characterization
  Diagnostics were performed on  12
Gainesville and  Ocala  vicinity slab-on-
grade houses during the last week of No-
vember 1988.

Installation of Demonstration
SSD Mitigation Systems
  SSD  systems were  installed  in nine
houses:  six  in  Gainesville  and three  in
Ocala  (Table  1). House floor plans  in-
clude seven  rectangular and two  more
complex, L-shaped designs. The  installa-
tions  include one  house with a single-
suction-point system, seven with two-suc-
tion-point/single-fan systems, and a house
with both a  two-suction-point/single-fan
system  and  a single-suction-point/single-
fan system.  Four houses  were  instru-
mented for continuous data acquisition.
  Five of the systems were installed be-
tween May and November 1989, three in
Gainesville  and two in Ocala. These
houses  were  all  of simple, single rectan-
gular  slab configuration with  slab areas
ranging from 158 to 195 m2 (1700 to 2100
ft2).The system at the smallest house con-
sists of a single suction  point and a single
fan; all of the others are two-suction-point/
single-fan systems. The Gainesville houses
were  equipped with continuous data ac-
quisition systems.
  During the summer of 1990, systems
were  installed in two additional  houses
with simple rectangular  slabs (149 to 181
m2 or 1700 to 2100 ft2)  and in two larger
(195 to  203  m2 or  2100 to  2200 ft2  )
houses  with L-shaped floor plans. All  of
these systems had two suction points con-
nected to a single fan. The system in the
largest  house also  had  a  third  suction
point  with a  second fan. A  continuous
data acquisition  system was  installed  in
one of the rectangular houses.

-------
Table 1.
 House
         Summary of Mitigation Installations
         North Florida Project
   Slab,
  m2(ff)
                                   Operation
                                    Date
                        Indoor Rn,
                          pCi/L
                                                 Unmitigated
                               System on
 Rectangular Slabs (7 houses):
 Ocala-1
 Ocala-2
 Gainesville-1*
 Gainesville-2*
 Gainesville-3*+

 Gainesville-4*
 Ocala-3
167 (1800)
164(1760)
164(1760)
194 (2087)
158 (1700)

181 (1950)
149 (1608)
 L-Shaped Slabs (2 houses):
 Gainesville-5#
 Gainesville-6
195 (2100)
203 (2188)
May 1989
May 1989
Jul 1989
Nov 1989
Oct 1989

May 1990
Aug 1990
Jul 1990f
Jul 1990f
16
10
11
25
 9

11
30
25
26
2.5
2.0
3.5
2.5
2.0

2.6
2.0
2.5
2.5
* Continuous data acquisition systems (4 houses).
f Although Gainesville-5 & -6 were turned on July 1990, they required further adjustment and
  became successful Oct 1990.
  System Types:
  + Gainesville-3: Single-suction-point system
  # Gainesville-5: Dual installation
               (Two-suction-point/single-fan system
               plus single-suction-point/single-fan system)
  All others: two-suction-point/single-fan systems
                                     1 house
                                     1
                                     7

                                     9 houses
Mitigation Results
  The mitigation successfully reduced in-
door  radon concentrations  originally on
the order of 10 to 30 pCi/L to post-mitiga-
tion values of < 4 pCi/L in all nine houses.
Levels were reduced  to values  on the
order of 2 pCi/L or less in three  of the
houses.

Design and Installation
Experiences

Mitigation Design
  As  indicated above, the UF soil depres-
surization  model  was  used as a design
tool in placing suction points and sizing
system components. The results of this
work are presented in the Part II report.

Moisture Condensation
  The  early  installations  had  some
undrained low points in the  horizontal pip-
ing runs in the attics; with  the advent  of
cool weather  in   November 1989, water
condensation from the moist exhausted
air essentially blocked these systems and
compromised  their effectiveness.  This
                         problem was overcome by installing drain
                         lines from the moisture traps.

                         Sub-Slab Leakage
                           It was observed that air leakage  near
                         the suction point can compromise the sys-
                         tem effectiveness.  For example,  in one
                         case, "short-circuit" flows  from  a leakage
                         crack  near one suction point  of  a  two-
                         point, single-fan system resulted in exces-
                         sive flows  at that suction  point, an imbal-
                         ance of the system, a compromised pres-
                         sure field, and unsatisfactory effectiveness.
                         Caulking the crack resulted in satisfactory
                         performance.  Subsequent failure  of the
                         silicone caulking resulted in degraded per-
                         formance; this  was  remediated by
                         recaulking  with urethane elastomer. Other
                         experimental work and simulation with the
                         computer model indicated that leakage at
                         points  more remote from the suction point
                         has much less influence on effectiveness.

                         Re-entrainment
                           An adventitious experience indicated the
                         potential for re-entrainment problems. Fol-
                         lowing the initial installations in two houses,
                         indoor radon levels were >:10 pCi/L when
the systems were operating. Attic levels
of 10's of pCi/L were found in subsequent
radon  monitoring.  Investigation revealed
that the roof penetration was  not sealed
around the vent pipe, apparently provid-
ing the opportunity  for discharged sub-
slab gases to enter the attic and be drawn
into the house ventilation system. Sealing
the roof penetrations reduced  radon con-
centrations in the attics and indoors to <4
pCi/L  (Table 1).

Optimization Studies

Pipe Sizing
   Nominal 50-mm  (2-in.) suction  piping
was installed as planned. For most of the
cases  (61% of the suction  holes), flows
were sufficiently low that calculated pres-
sure losses  due to flow were  <15 Pa/10
m, and for 90% of the holes losses were
calculated to be <100 Pa/10 m. In the two
cases  of the highest flows where  calcu-
lated  losses were >100 Pa/10 m,  actual
pressures on the order  of -300 Pa (-277
to -328 Pa) were observed. The systems,
involving  these suction points in combina-
tion with  a  second  suction  point, were
effective  in reducing indoor radon levels
by factors of 3 to 10, resulting in  indoor
radon levels of 3.5 pCi/l or less for these
houses.  The  use of the smaller  piping
permitted  savings in cost, space,  and in-
stallation  effort.

Suction Points
  The effectiveness of single-suction-point
operation was tested in several of houses
with two-hole systems by operating these
systems  for a period  of time with one or
the other suction line valved  off.  These
experiments indicated that:
   1. Two suction  points  successfully
     maintained levels below  4 pCi/L for
     slab areas up to 2100 ft2.
   2. A single  suction point was sufficient
     on three houses with 1700 to 1800
     ft2 slabs.

Passive Ventilation
  The potential effectiveness  of passive
sub-slab  ventilation was tested in several
of the houses  by monitoring indoor radon
with  the  fans off and the suction lines
open. These  experiments  indicated  that
passive venting (fan off, vent line open)
was not  effective for a  packed sand/soil
sub-slab  medium.

Durability
  Special questions were posed concern-
ing durability for systems operating under
Florida conditions. Would continued  op-
eration impact the sub-slab environment

-------
in  a manner that affects the continued
effectiveness of the system? If there were
effects on the sub-slab environment, would
these have structural effects on the build-
ing? Would continued performance of the
fans be compromised by the low flow and
high temperature in Florida  installations?
  As of the end of 1990,  the 1989 instal-
lations had been  monitored  for post-miti-
gation  periods on  the order of 13 to  19
months. Insufficient time  had elapsed  for
significant durability monitoring on the 1990
systems which had been installed during
the period  May through  August.  During
the limited observation  period (3  to  18
months), the following were observed:
   1.  With the transient exceptions noted
      below, the  systems exhibited rela-
      tively constant performance  and  re-
      tained  their effectiveness in main-
      taining reduced indoor radon con-
      centrations.
   2.  In one case, failure  of silicone caulk-
      ing  of a leakage crack near a suc-
      tion  point  resulted  in increased
      "short-circuit" flows. This was
      remediated by re-caulking with ure-
      thane  elastomer,  a  more  durable
      material.
   3.  With the advent of  cold  weather,
      condensation formed  in horizontal
      attic runs that were not self-drain-
      ing. This resulted in an audible gur-
      gling noise, reduced flow, and  in-
      creased fluctuations  in indoor  ra-
      don concentrations. This condition
      was remediated by installing traps
      and drains.
   4.  No fan failures were observed—any
      effect of  low flow  on fan life was
      not  expressed during the  available
      observation  period.
   5.  No structural effects were observed.
   6.  With the exception  of the "gurgling"
      associated  with the water conden-
      sation before the installation of traps
      and  drains,   there  were   no
      homeowner complaints of noise or
      other annoyances.
Conclusions and
Recommendations

Design Considerations
   1.  SSD was effective for North-central
      Florida  slab-on-grade  houses  of
      both simple rectangle and L-shaped
      floor plans.
   2.  For the sub-slab media found  in
      this region, low flows permitted use
      of smaller diameter, nominal 50 mm
      (2 in.) piping.
   3.  Two suction points were success-
      ful for slab areas up  to  200 m2
      (2100ft2).
   4.  A single suction  point  was  suffi-
      cient on three houses with single-
      level, rectangular slabs with areas
      on the order of 160 to 170 m2 (1700
      to 1800ft2).
   5.  Experiments  with installed  active
      systems  (fan off, vent  line  open)
      indicated that passive ventilation is
      not  likely  to  be effective  for this
      type of sub-slab medium.

Installation Considerations
   1.  Air leakage near the suction  point
      can compromise system effective-
      ness; leakage at points farther from
      the  suction point has much less
      influence on effectiveness.
   2.  Even in Florida, moisture can con-
      dense in the system during cooler
      weather; it is important to avoid low
      points in horizontal attic  runs and
      to install traps and drains if water
      trapping points cannot be avoided.
   3.  It is important to seal around the
      discharge duct at the roof penetra-
      tion to prevent re-entry of the ex-
      hausted sub-slab  gases. Examina-
     tion  for other sources of re-entrain-
     ment is also warranted.
Performance and Durability
  The following conclusions are limited by
being based on short observation times-
3 to 18 months:
  1.  Pressure and flow values  in  SSD
      systems may exhibit some tempo-
      ral variability; documentation of per-
      formance from point measurements
      should be based on averages from
      a series of measurements taken on
      different days.
  2. On a near-term basis, SSD sys-
     tems as installed  in this project re-
     tain effectiveness in  maintaining  re-
     duced indoor radon concentrations.
  3. Continued integrity of sealing of po-
     tential short-circuit air flow sources
     near suction  points is essential to
     continued effectiveness. System
     maintenance should  include inspec-
     tion of such sealing.
  4. During cooler weather, unintended
     trapping of moisture  condensation
     in horizontal  attic runs can compro-
     mise system performance. Mainte-
     nance should include inspection  for
     such inadvertent effects.
  5. Fan failures have not been  identi-
     fied as a problem in  the short term
     (based on observing a small num-
     ber of systems).
  6. Structural effects  have not been
     identified in the short term.
  7. Other than for the noises associ-
     ated with the water condensation
     before correction, these systems
     have not generated homeowner
     complaints.
  Short-term  durability  information would
be enhanced by following all houses for at
least a year,  and long-term durability in-
formation would be gained by following all
the houses even longer.

-------
 C. Roessler, R. Morato, R. Richards, H. Mohammed, D. Hintenlang, and R. Furman
   are with the University of Florida, Gainesville, FL 32611.
 David C. Sanchez is the EPA Project Officer (see below).
 The complete report consists of two volumes entitled "Design and Testing of Sub-
   Slab Depressurization  for Radon Mitigation in North  Florida Houses: Part I.
   Performance and Durability."
 "Volume I. Technical Report" (Order No. PB96 -103 585;  Cost: $21.50,  subject to
   change)
 "Volume II. Data Appendices," (Order No. PB96-103 593;  Cost: $35.00, subject to
   change)
 The above reports will be available only from:
         National Technical Information Service
         5285 Port Royal Road
         Springfield, VA 22161
         Telephone: 703-487-4650
 The EPA Project Officer can be contacted at:
         Air Pollution Prevention and Control Division
         National Risk Management Research Laboratory
         U. S.  Environmental Protection Agency
         Research Triangle Park, NC 27711
United States
Environmental Protection Agency
National Risk Management Research Laboratory (G-72)
Cincinnati, OH 45268

Official Business
Penalty for Private Use $300
     BULK RATE
POSTAGE & FEES PAID
         EPA
   PERMIT NO. G-35
EPA/600/SR-95/149

-------