FOUR INCHES! TOO CLOSE, TOO FAR, JUST RIGHT?
Robert K. Lewis
Commonwealth of Pennsylvania
Department of Environmental Protection
Bureau of Radiation ProtectiodRadon Division
Harrisburg, PA, USA

INTRODUCTION
The July 1992 Indoor Radon and Radon Decay Product Measurement Device Protocols, EPA
402-R-92-004 (EPA 1992) provides five measurement device location selection criteria that
should be followed when placing a device within a room for measurement of the radon
concentration. These criteria include: (1) a place where the device will not be disturbed and
there is adequate room for the device, (2) away from drafts caused by heating, ventilating and
air conditioning vents, doors, fans, and windows, and areas of high heat, such as near fireplaces
or in direct sunlight, and areas of high humidity should be avoided, (3) the device should be at
least three feet from windows and other potential openings in the exterior wall and if there are no
potential openings the device should be at least one foot from the exterior wall, (4) the device
should be at least 20 in. above the floor and at least four inches from other objects, and (5) the
device should not be placed in a kitchen, laundry room, closet or bathroom.
These location selection protocols along with the measurement condition protocols help to
ensure a more standardized and reproducible testing environment within the home or building
environment from one test to the next and from one home to another. They in part also help to
produce a worst-case scenario-testing environment.
The EPA protocols do leave open some room for interpretation. If a device is lifted off of a shelf
to dust under would this invalidate or even introduce any additional error into the result? If a
device is placed on top of a television set that happens to be on for several hours each night,
would that be considered excessive heat? Would a different result be obtained if a device were
placed 30 in. above the floor instead of 20 in.? EPA did not consider this to be a significant
difference! Would two devices that were touching each other interfere with the diffusion of
radon into one or the other of the devices?
Some of these protocol requirements may be more sensitive to deviation than others. For
instance, Gray and Windham 1989, found a 24% over response for open-faced charcoal canisters
exposed to an air velocity of 30 linear feet per minute. They advised strict conformance to
protocols for this device. Charcoal canisters are also sensitive to temperature and humidity. At
50% relative humidity a raise in temperature from 50' to 80ÂF resulted in a 26% decrease in

charcoal collection efficiency (Ronca-Battista and Gray, 1987). High humidity will also
decrease charcoal collection efficiency, particularly after two days of exposure. Thus the reason
for avoiding kitchens, bathrooms, and laundry rooms. However, deviations for some devices to
some of the requirements may not have any adverse effect on test results. A change in the most
recent EPA protocols (1992) for placement of the device from the floor from 30 in. to 20 in. was
not thought to be critical. Additionally, would placing two devices closer than four inches to one
another or to any other object adversely affect their result?
The intent of this experiment was to examine one small part of the current device placement
protocols. This examination is by no means exhaustive with only four devices examined under a
limited number of sceneries. Hopefully, others will provide additional information on this topic
as well as other research related to the device protocols. This type of information may be very
useful to future AARST working groups involved in the update, revision, or adoption of new
radon device standards.
Diffusion
According to kinetic theory, the molecules in a gas are continually moving and colliding with
each other. This motion and these collisions produce a random and uniform distribution of a
substance in a given volume. This distribution will follow a concentration gradient, whereby the
gas will diffuse from a region of high concentration to a region of lower concentration. The rate
at which this diffusion occurs is dependent upon temperature and the molecular or atomic mass.
For the specific case of Radon-222, atoms in a typical basement environment will be traveling at
about 400 mph, but only over very short distances. This point is brought up only so that it is
realized that radon gas diffusion will fairly quickly produce a uniform concentration within the
basement environment. Point sources such as open bottom sump pits and HVAC system air
flows could certainly cause perturbations to this uniform distribution. However, as Dr. Kearney
said "except for thermal, radioactive decay, and non-steady-state ("puffs") effects, radon
concentration in an enclosed volume is, for health physics purposes, uniform. Consider the
uniformity of the salinity of ocean water, top to bottom, side to side. Elementary kinetic theory
explains it all" (Health Physics Newsletter, 1991, Vol. 19, No. 3).
It would seem reasonable to assume that diffusion would work to produce a uniform exposure to
various radon test devices whether they were four inches apart or touching, or whether they were
touching something else such as a table top or some sort of enclosure.
Materials and Methods
A series of radon gas exposures was carried out on different radon measurement methods, both
passive and active, in both a well-controlled radon chamber and a basement environment.
During all exposures a control device(s) were deployed strictly according to EPA radon testing
protocols. The control device(s) served to provide the reference radon value against which the
other device results were compared.

Various deviations of the protocols were tested including devices touching one another, devices
less than four inches from other objects, devices in closed containers, devices in buckets, and
devices in sealed Tyvek Bags (Figures 1 & 2). The closed containers were plastic boxes provide
by Rad Elec, Inc. They are designed to hold two E-Perms in the open position. There is a foam
insert within the box that holds the two E-Perms in place. The interior volume of the box is 240
cubic in., with 67 cubic in. taken up by the foam insert. With the lid of the box closed there are
two circular openings on each side to allow for air movement into and out of the box. These two
openings provide for 1.6 sq. in. of open space. The Tyvek Bags are 10" wide by 14.5" long with
a sealable flap. Tyvek, manufactured by DuPont, is a 100% high-density polyethylene fiber. It
is vapor-permeable. Tyvek is virtually non-linting, plus its tight pore structure keeps even the
smallest particulates from dusting through the package. The bucket used was a 1.5 gal. standard
plastic bucket.
Devices tested were F&J R40VDB four-inch diffusion barrier charcoals, Rad Elec short-term
electret ion chambers, Landauer's Radtrak Alpha Track Detectors (ATD), and Pylon AB-5lPRD.
Devices tested in the radon chamber were kept under constant conditions of temperature, relative
humidity, and radon concentration throughout the exposures. Two separate exposures were
carried out in the chamber at concentrations of 10 and 16 pCiIL, 68OF, 3 1% relative humidity,
and 9 microR1hour. Two separate exposures were also carried out in the basement environment
at around 8 pCiIL, 70°F 37% and 45% relative humidity, and 8 microR1hour. Radon
concentration, temperature, and relative humidity did vary somewhat in this basement
environment during the exposure period.
Results and Discussion
Four separate exposures were performed; the first exposure took place in the radon chamber and
deployed two E-Perms in the open box, two E-Perms in the closed box, two E-Perms in only the
foam insert, two E-Perms in the sealed Tyvek Bag, eight E-Perms in the bucket, four E-Perms
touching one another, and four E-Perms as controls. The second exposure took place in the
basement environment and consisted of all the same scenarios as above except for the four EPerms touching. The third exposure took place in the radon chamber and deployed four
charcoals touching one another, four charcoals in sealed Tyvek Bag, two charcoals touching in a
closed box, eight charcoals in 1.5 gal bucket, four charcoals as controls, four ATDs touching,
four ATDs touching and in sealed Tyvek Bag, four ATDs touching and in closed box, four ATD
controls, four Pylon AB-51PRD's as controls, and four Pylon AB-5lPRDs with detectors
from
each other. The fourth and final exposure took place in the basement environment and deployed
two charcoals as controls, two charcoals in closed box, and two charcoals in closed box but with
foam insert removed.
%'I

Listed in Tables 1-4 are the configurations, sample size, and mean radon concentration plus or
minus one standard deviation from the four different exposures.

An analysis of variance was used to judge whether the population means were statistically
different from one another or not. The analysis of variance does not tell which specific
populations means are different, just that they are different or not. A paired T-test was used to
look at the data from the Pylons and the two charcoal configurations in the fourth exposure, since
there were just two population means and there was equal sample size.
Table 5 provides the primary conclusion from this study. A difference in the population means
was only seen in the third and fourth exposures involving charcoal canisters, all other exposures
with all other devices showed no difference of the population means under the various
configurations.
Conclusions
Even an intuitive examination of Tables 3 and 4 show that charcoals contained in a closed, small
volume (240 cubic in.) box underestimate the radon concentration as compared to the controls by
35% and 47%, respectively. It is possible that due to the limited volume there may be a
depletion zone around the charcoals that is limiting uptake.
Given adequate air space all test devices, even charcoals, did not seem to be effected by violation
of the four inch separation distance as required by EPA protocols. This finding would seem to
be particularly consistent with devices such as alpha tracks, E-Perms, and continuous
monitors that do not have adsorptive properties.
Based on this initial study, placement of alpha track detectors, E-Perms, continuous monitors, or
charcoal canisters within four inches of "other" objects would not appear to adversely effect
measurement results. "Other" objects should be kept in the context of the present study.
Charcoal canisters should not be enclosed in a "limiting volume".
The question that remains is, "should the test device protocols be changed, should they be left
alone, or could there be specific requirements germane to only certain test devices?" Further
study will be needed to provide the answer.
Additional, interesting protocol questions remain, should crawl space vents be closed or open
during testing, what is a good definition of lowest-livable level, and is the requirement for 10%
duplicates necessary?
Note: The mention of specific product names within this report does not represent endorsement
by the Department. These products are commonly used in the industry and were available to the
Radon Division for this study.

References
Gray, D. and Windham, S., EPA Develops New Canister, Radon News Digest, April 1989.
Health Physics Newsletter, March 1991, Volume XIX, No. 3, page 12.
Marion, J. B., General Physics with Bioscience Essays, John Willey & Sons, Inc., 1979.
Ronca-Battista, M. and Gray, D., The Influence of Changing Exposure Conditions on
Measurements of Radon Concentrations with Charcoal Adsorption Technique, USEPA.
United States Environmental Protection Agency, Indoor Radon and Radon Decay
Product Measurement Device Protocols, EPA 402-R-92-004, July 1992.

Table 1, First Exposure, Radon Chamber, ST E-PERMS

1

Configuration
E-Perm box, open
E-Perm box, closed
Foam only
Sealed Tyvek Bag
1.5 Gal Bucket
Touching
Controls

Sample Size
2
2
2
2
8
4
4

Mean (pCi/L)
11.3
11.2
11.3
10.4
11.3
11.2
11.2

+I- 1 Std. Dev.

0.6
0.8
0.2
0.8
0.4
0.5
0.2

Table 2, Second Exposure, Basement, ST E-PERMS
Configuration
E-Perm box, open
E-Perm box, closed
Foam only
Sealed Tyvek Bag
1.5 Gal Bucket
Controls

Sample Size
2
2
2
2
8
4

Mean (pCi/L)
8.7
8.3
8.0
8.3
7.8
8.0

+I- 1 Std. Dev.
0.3
0.8
0.4
0.4
0.6
0.5

Table 3, Third Exposure, Radon Chamber, CharIATDlCont. Mont.
Configuration
Touching (Char.)
Touching in sealed
Tyvek B& (Char.)
Touching in closed
E-Perm box (Char)
1.5 Gal Bucket
(Char.)
Controls (Char.)
Touching (ATD)
Touching in sealed
Tyvek bag (ATD)
Touching in closed
E-Perm box (ATD)
Controls (ATD)
W separation
(Pylon)
Controls (Pylon)

Sample Size
4
4

Mean (pCi/L)
15.7
16.7

+I- 1 Std. Dev.
0.5
0.8

2

11.2

0.6

8

14.6

1.2

4
4
4

15.9
15.9
16.1

0.8
2.8
3.3

4

17.2

1.7

4

15.3
15.6

4.5
0.1

4

15.6

0.2

Table 4, Fourth Exposure, Basement, Charcoals
Configuration
E-Perm box, closed
(Char.)
Controls

Sample Size

Mean (pCi/L)

2

5.2

+/- 1 Std. Dev.
0.1

2

8.4

0.3

Table 5, Results of Statistical Tests on Population Means
Exposure
First
Second
Third
Third
Third
Fourth

Number of
Configurations
7
6
5
4

2
2

Test Device
ST E-PERMS
ST E-PERMS
Charcoal
ATD
Cont. Monitor
Charcoal

Any Difference
in the Means?
No
No
Yes
No
No
Yes

Probability of
Type I Error
0.05
0.05
0.05
0.05
0.025
0.025

Figure 1

Figure 2