2002 International Radon Symposium Proceedings
A Critique of the "EPA Method" for Analyzing and Calibrating Charcoal Canisters for Radon
Measurements
Phillip H. Jenkins, PhD,
BowserMorner, Inc., Dayton, Ohio
ABSTRACT
After the publication of EERF Standard Operating Procedures for Rn222 Measurement Using Charcoal
Cani,ster.s in 1987, an instrument manufacturer marketed an analysis system which containcd the methodology
and calibration factors utilized by the EPA radon laboratory in Montgomery, Alabama for measurement of
radon in air using 4inch charcoal canisters. The system also included a methodology for the analysis of the
smaller charcoal canisters developed at the University of Pittsburgh. The use of this turnkey system for
analyzing charcoal canisters has led to several problcms and misunderstandings related to calibrating and
analyzing charcoal canisters for the measurement of radon in air. This paper describes the "EPA method for
analysis of fourinch, openfaced charcoal canisters, problcms with the method itself, problems due to
misapplication of the method and an alternative to the method.
lntroduc tion
I n 1987 the Eastern Environmental Radiation Facility (EERF) of the U. S. Environmental Protection Agency
(EPA) published a manual (Gray and Windham 1987) documenting its procedure for using fourinch openfaced charcoal canisters to measure indoor radon concentration. The original manual containcd information for
the calibration of charcoal canisters in a chamber where the air was relatively stagnant. Later an addendum was
added to the manual containing calibration factors for canisters in an active air environment, which more
closely resembles a residential indoor environment.
Thc manual describes a mcthod of calibrating openfaced charcoal canisters, and their use for measuring
ambient radon in indoor air, that has come to be called by some the "EPA method." However, it was not
intended that the mcthod described in the manual be construed as a "standard" method, rather that the manual
documented the method that was in use at the EERF. Although this paper criticizes mcthod, the reader should
in no way infer a criticism of how the method was applied at the EERF. Rather this paper is critical of how the
mcthod has been and is being applied, and sometimes misapplied, within the radon testing industry. Further,
the reader should not infer criticism of charcoal canister analysis laboratories in the radon testing industry. The
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author believes that where this method has been misapplied in the industry, the personnel involved were well
intentioned but simply were not educated in the intricacies of the method or the software system that utilizes it.
This paper is an attempt to correct this situation. The reader should also keep in mind that this paper relates
specifically to fourinch openfaced charcoal canisters; although, the discussion presented here could possibly
be applied to other configurations of charcoal detectors.
A Description of the Method
The EERF's approach is similar to that developed and reported by George (George 1984). The radon
concentration in air is determined using the following equation:
where C = Radon concentration (pCi1L)
R = Net count rate (counts per minute or cpm)
R = [Gross count rate (cpm)]  [Background (cpm) of the detector for that day]
Ts = Canister exposure time (min)
E = Detector counting efficiency (cprn/pCi)
CF = Calibration factor (Llmin)
DF = Decay factor from the midpoint of the exposure to the beginning of count
DF = exp[\ (To+ Ts/2)]
where \ = decay constant for ^ ~ n(rnin")
^. = ln(2) / 1112 [tiI2= halflife of ^ ~ n(min)]
To = time from end of exposure to beginning of count (min)
The background count rate (cpm) and the detector efficiency (cpmlpci) arc determined each day that the system
is used. The detector efficiency is determined by counting a standard canister containing ^ ~ nin secular
equilibrium with a known quantity of ^Ra. In this author's opinion, the decay factor, DF, should be defined
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differently, but as long as the above definition is used consistently during both calibration and analysis of
samples, the DF as defined above works as well as other alternatives. This has been discussed in more detail
elsewhere (Jenkins 1991).
The crux of this paper is the calibration factor, CF. Note that this factor has the unit of literdminute (Llmin), as
if it were a flow rate. A reasonable analogy is that [(CF) x (Ts)] could be thought of as the volume of air from
which radon is totally removed, as if air were pumped through a system that removed and trapped the radon.
Some contend that the CF and Ts should be combined into a different calibration factor that has the unit of
volume (Blue and Jarzemba, 1992). But, in this paper the CF remains as used by the EERF.
The CF for a given exposure of a charcoal canister is a function of the sampling time and the relative humidity
of the air to which the canister is exposed. However, it should be made clear "up front" that the parameter that
is measured and used in the calibration and analysis of charcoal canisters is not the relative humidity but rather
the mass of moisture adsorbed during the exposure, which itself is a function of relative humidity. The relative
humidity is not measured during a field exposure. Relative humidity is only a consideration because canisters
must be exposed over a range of relative humidity values for calibration purposes.
In calibrating charcoal canisters, it is necessary to derive a family of calibration factors for various times of
exposure and values of mass of adsorbed moisture. This is typically done by exposing groups of canisters in a
controlled radon chamber for a minimum of three exposure periods covering the time periods that are used for
field measurements and at three values of relative humidity; a low value between 20% and 30%, a medium
value between 45% and 55% and a high value between 70% and 80%.
The EPA manual referenced above describes the calibration of fourinch openfaced charcoal canisters that
were used "as received" from the manufacturer, without heating them to remove the moisture present in them at
the time of manufacture. Each canister was used once and then discarded. Five canisters each were exposed in
the radon chamber at the EERF for exposure times of 1, 2, 3, 4, 5 and 6 days and at relative humidity values of
20%, 50% and 80%. In all, ninety canisters were exposed for the calibration. The CF for each canister was
calculated using the following equation:
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Please note that Equation (2) is merely Equation (1) rearranged to solve for CF instead of for C. The fifteen
values of CF for the canisters exposed for two days are shown in Table 1. A plot of CF as a function of
moisture gain for the twoday canisters contained in the EPA manual is reproduced here as Figure 1. This is
assumed to be a curve of "best fit" to the data shown in Table 1.
According to the algorithm used by the EERF, the first step in determining a value of CF for a canister used for
a field measurement is to select an "Initial CF" value, based on the observed mass of moisture gained during the
exposure, from the curve shown in Figure 1. Although Figure 1 contains CF values only for exposure times of
48 h, this "Initial CF" is taken from this curve regardless of the actual exposure time. If the actual is exposure
time was 48 h, then this "Initial CF" value is the "Final CF" value used in Equation (1).
For exposure times other than 48 h, the algorithm uses a set of "Adjustment Factors" (AF) to adjust the "Initial
CF" to a value that is appropriate for the actual exposure time. The AF values are shown in Table 2. These AF
values are assumed to have been derived from the CF values determined by EERF for canisters exposed for
periods of time other
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Table 1 . Calibration Factors from EERF for twoday canisters in an active environment
Mass of moisture gain (g)
Relative Humidity (%)
5
10
CF (Llmin)
15
Moisture Gain (9)
Figure 1 . Calibration Factors [CF (Llmin) ] for a twoday exposure
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Table 2. Adjustment Factors (AF) from EERF
Exposure Time (h)
AF (Llmin)
AF (Llmin)
AF Wmin)
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than 48 h, but this cannot be verified because EERF published CF values only for the 48h canisters. Note that
the AF values have the unit of Llmin, just as the CF values do. EERF apparentlychose to define the AF values
for 48h canisters to be identical to the CF values for 48h canisters, as can be seen from Tables 1 and 2.
Curves of the AF values are reproduced here as Figure 2. There is one curve each for the three values of
relative humidity; 20%, 50% and 80%.
The algorithm provides a method for determining which of the curves of AF values to use, based on the mass of
moisture the canister gained during the exposure. This method is summarized in Table 3. If the moisture gain
was less than 1.0 g, then the 20% curve is used. If the moisture gain was between 1.0 and 4.0 g, then the 50%
curve is used. If the moisture gain was greater than 4.0 g, then the 80% curve is used. Values of AF for the
actual exposure time and also for an exposure time of 48 h are determined from the appropriate humidity curve.
Then the following equation is used to calculate the final value of CF.
Final CF
=
Initial CF x (AF for actual exposure time) / (AF for 48 h exposure timc) (3)
In other words, the ratio of the AF for the actual exposure timc to the AF for a 48h exposure time becomes a
modifying factor by which the "Initial CF" is multiplied to produce the "Final CF." This final value is then
used in Equation (1) to determine the radon concentration during the exposure.
0
20
40
60
80
100
120
140
160
Exposure Time (h)
Figure 2. Adjustment Factors [AF (Llmin) ] for three values of relative humidity
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Table 3. Selection of AF curve
1
If mass of moisture gained is:
1
Then use curve for:
Because it is the ratio of two Adjustment Factors that is used as a modifying factor to adjust the "Initial CF," it
can be seen that EERF's choice to set the AF values for the 48h canisters equal to the CF values for the 48h
canisters, and to give the AF values the unit of Llmin, was an arbitrary choice. There is no physical meaning to
the AF values' having any unit at all, and they in fact probably should be unitless. Further, the AF values for
the 48h canisters could be set to any arbitrary value, as long as the AF values for the other exposure times are
chosen so that the ratios are appropriate for converting "Initial CF" values to "Final CF" values.
With the prolific use of computers today, the concept of using graphs to determine calibration factors and
adjustment factors seems cumbersome, antiquated and prone to errors. However, the method apparently
worked well for the radon lab at the EERF.
The Problems
I . Use of EERF calibration data bv other laboratories
After the EERF published its procedure and calibration data, a charcoal analysis system was marketed by a
private manufacturer, which incorporated the "EPA method" and EERF's calibration data into its analysis
software for 4inch openfaced canisters. Although the manufacturer no longer supports this turnkey system,
several charcoal laboratories, often with little or no understanding of how the software works, are still using it.
There are several problems with using the turnkey system, but probably the largest one is that some laboratories
used the system, and the builtin calibration factors from EERF, without any calibration of their own charcoal
devices whatsoever. If the charcoal canisters used by the laboratory were exactly the same as those used by the
EERF, and if they were used in exactly the same manner, then perhaps the EERF calibration factors would be
appropriate. However, this should be verified constantly by obtaining and analyzing spiked samples, and this
was often not done. Therefore, the turnkey software system made it too easy for laboratories to analyze
charcoal canisters with a system that was not calibrated, or at least whose calibration was unverified, by the end
user. This is obviously against EPA protocols; however, the protocols are not followed by everyone and are
enforced by only a few states.
2. The method itself
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Besides the obvious problems with determining calibration factors and adjustment factors from graphs, there
appears to be a problem with the "EPA method" itself. There is no problem if the exposure time happens to be
48 hours. The problem is in the use of the "Adjustment Factors" to correct to an exposure time different from
48 hours.
From Figure 2, it is seen that there is not a large difference between the low humidity and medium humidity
curves. So if the algorithm for selecting among the AF curves (Table 3) caused the low humidity curve to be
used, when in reality the medium humidity curve should have been used, or vice versa, then the resulting error
is probably not significant. However, the same cannot be said about the difference between the medium
humidity curve and the high humidity curve in Figure 2. There appears to be a significant difference between
the two curves, but it must be remembered that it is the ratio of the two AF values in Equation (3) that is
important, not the actual AF values themselves. So, what is important to know is how this ratio, or modifying
factor, changes as a function of the humidity to which the canister is exposed, and this cannot be easily
discerned from Figure 2. The plot in Figure 3 shows this ratio of the AF values as a function of exposure time
and relative humidity.
0
20
40
60
80
100
120
140
160
Exposure Time (h)
Figure 3. Ratio of ~djustmentFactors for three values of relative humidity
From Figure 3 it is seen that the ratio of AF values , i.e. the modifying factor in Equation (3), is similar in value
for the low and medium humidity curves regardless of the length of the exposure. However, the modifying
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factors for the medium and high humidity curves are significantly different for exposure times much smaller or
much larger than 48 hours. This is seen more clearly in Figure 4, which shows two curves; the ratio of the
modifying factor for the medium humidity curve to that of the low humidity curve, and the ratio of the
modifying factor for the high humidity curve to that of the medium humidity curve. It is easy to see from
Figure 4 that the change in the modifying factor that occurs at the breakpoint of 1.0 g according to the EERF
algorithm is fairly small regardless of the exposure time and approaches approximately 7%at an exposure time
of 144 h. However, the change in the modifying factor that occurs at the breakpoint of 4.0 g can be significant,
exceeding 10%for an exposure time of 72 h and 30% for an exposure time of 144 h. So, a canister for 72 h or
longer could have a quite different calibration factor applied during the analysis, depending on whether the
moisture gain was 4.0 g (or less) or 4.1 g (or more).

Med to Low
High to Med
0
20
40
60
80
100
120
140
160
Exposure Time (h)
Figure 4. Ratio of Modifying Factors
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3. Between the curves
What arc the chances that the wrong curve in Figure 2 would be sclcctcd by the EERF algorithm? To judge
that, one needs to know the mass of moisture gain for all of the canisters calibrated by EERF; however, EERF
published only the moisturc gains for the 48h canisters as shown in Table 1. Blue and Jarzemba published
values of moisturc gain for all of the exposure times and for the type of canister used by EERF. It is not clear
from thcir publication if these values actually came from EERF through a private communication, or whether
Blue and Jarzemba exposed similar canisters in a controlled atmosphere for the needed time periods.
Regardless, thcir published values are reproduced here as Table 4.
Tablc 4. Observed moisturc gains for EERF canisters
Exposure Time (h)
24
48
72
96
120
144
20%
0.4
0.5
0.5
0.5
0.5
0.5
Observed Mass of Moisture Gain (g)
50%
80%
0.7
4.5
1 .O
7.5
1.2
10.5
1.3
11.9
I .4
13.4
I .5
15.8
As seen in Tablc 4, the mass of moisture gain for the 20% relative humidity condition was for all exposure
times slightly less than 0.0 g. This makes sense because the canisters were used "as received" from the
manufacturer without heating them to remove any moisture that was present at the time of manufacture. The
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2002 International Radon Symposium Proceedings
values of mass of moisture gain published by the EERF, and shown in Table 1, probably should have been less
than 0.0 g.
From Table 4, it is also seen that for the EERF canisters the algorithm for selecting the appropriate AF curve as
outlined in Table 3 makes sense. In every case but one, the mass of moisture gain was less than 1.0 g for the
exposures at 20% RH, between 1.0 and 4.0 g for exposures at 50% RH and greater than 4.0 g for exposures at
80% RH. However, field exposures are not carried out in atmospheres of controlled relative humidity. The
relative humidity is likely not constant during the exposure of a field sample, and probably also is not measured.
Therefore, the relative humidity values given throughout this paper have no meaning other than for the canisters
that were used in the calibration procedure.
It is certainly conceivable that a canister similar to those used by the EERF could be exposed for 72 h or more
to a relative humidity condition that, on the average, was somewhere between 50% and 80%, and yet the mass
of moisture gain was 4.0 g or less. In this case, some AF value between the 50% and 80% curves might be
appropriate, but the "EPA method" and the turnkey software has no choice but to default to the 50% curve. In
which case an error is introduced that could be over 10%. It would be much more desirable to use an algorithm
that was continuous with mass of moisture gain. In other words, an algorithm that would produce an
appropriate value for AF or CF for any value of mass of moisture gain rather than having breakpoints at certain
values. The "EPA method" and the turnkey software do not provide for selecting values between the curves,
although they might be more appropriate.
4. Recycled canisters
Many charcoal laboratories recycle their charcoal canisters by heating them in an oven to remove any adsorbed
moisture and radon before reusing them. This means first of all that the calibration data produced by EERF are
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totally inappropriate for those charcoal canisters. In this case, the canistcrs do not start with a quantity of
moisture in them, so the amount of moisture adsorbed during exposure should be greater than that observed for
the EERF canisters. Therefore, it is essential that charcoal laboratories that recycle their canisters produce
calibration factors for their own detectors, but this has not always been done.
Secondly, the EERF algorithm for sclccting the appropriate AF curve, which is built into the turnkey software,
is not appropriate for recyclcd canistcrs. The breakpoints used in the EERF algorithm are not appropriate for
recycled canisters, but there is no way to change the algorithm in the turnkey software. Even canisters exposed
at a condition of 50% relative humidity can adsorb more than 4.0 g of moisture when thcy start out with no
moisture. So, even if a charcoal laboratory produces a set of calibration factors for their detectors, as long as
the turnkey software is used, it would not always select the appropriate AF curve and significant errors could
result. It is possible modify the table of calibration factors and adjustment factors used by the software to adjust
to more appropriate values for recycled canisters, but this still is not an ideal solution to the situation. No matter
how this is done, significant errors in calculating the calibration factors to be applied to field samples can be
made. The EERF algorithm docs not distinguish between canistcrs that adsorb 4.1 g of moisturc and 20 g of
moisturc; the same calibration factor would be used for both ends of this range. For recyclcd canisters exposed
for 72 hours or more, this can be a significant source of error.
A Better Alternative
With the common use of computers today, there are much better approaches to the analysis of charcoal canisters
than using methods that require looking up values from curves or tables. Further, algorithms that include
adjustment factors and selections among set curves based on moisture gain should be avoided, particularly when
thcy are not appropriate for the type of charcoal canister that is being used. Instead, a model or equation that
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produces calibration factor values that are continuous with the parameters of exposure time and moisture gain
can be developed to fit the calibration data. Such an equation can be used in spreadsheets, computer programs
or even handheld calculators. This approach always results in the best value of CF, based on calibration data,
for any given set of values of exposure time and moisture gain. The details of one goes about analyzing
calibration data to produce a best fit according to a given model are beyond the scope of this paper; however, a
few approaches are described here.
One approach is to use a linear regression model that is second or thirdorder (or higher order) in the
parameters of exposure time and mass of moisture gain to fit the calibration data. Again note that relative
humidity does not enter into the model at all, and only is an issue because canisters must be exposed at different
controlled values of relative humidity in order to produce calibration data. A possible thirdorder linear model
looks like the following:
where T
exposure time (d or h)
M = mass of moisture gain (g)
bo through bg = regression coefficients (constant values resulting from the
regression analysis)
=
A "stepwise" regression procedure can be used to add terms to the model in order of improving the fit to the
calibration data until some criteria is met that causes no more terms to be added to the model. The likely result
of such a procedure is that not all of the terms shown in Equation (4),or some similar linear model, would
remain in the model. In other words, the regression coefficients for one or more terms would not be
significantly different from zero and therefore those terms would not remain in the model. A good fit to the
calibration data is possible with this type of model; however, one must be careful not to extrapolate at all
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outside the range of the calibration data as the values of CF that might result could be totally inappropriate.
This type of model and its limitations arc discussed elsewhere (Jenkins 1991).
Blue and Jarzemba proposed a model that is based on the physical process of adsorption of radon onto charcoal.
This model is nonlinear in the parameters of exposure time and moisture gain. Their model is as follows:
CF
=
a [l  e x p ( 4 t)]  13 [l  exp(6 t)] M
Where t = exposure time
M = moisture gain
a , and 8 are constants based on physical properties of charcoal
Note that this model has a sound physical basis in that the coefficients all have some physical meaning based on
the properties of the charcoal canister. For more details see Blue and Jarzemba, 1992. Blue and Jarzemba
demonstrated that this model fits the EERF calibration data very well.
The model in Equation (5) was tried by this author at BowserMorner, Inc. in the early 1990's and was not
found to be work well for recycled canisters. After some trial and error, a variation on this model was found to
be more satisfactory. This model, which has been used successfully for about 10 years for recycled canisters, is
as follows:
CF
=
bo exp (b, T)exp{b2 M [ l  exp (b3 T)]}
Where T = exposure time (d)
M = moisture gain (g)
bo through b3 are regression coefficients
Note that this model does not have a sound physical basis because the regression coefficients do not necessarily
have any meaning related to the physical characteristics of the charcoal canisters, but rather are merely
regression coefficients; i.e., values that produce the best fit to the calibration data.
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Figure 5 shows graphically a set of calibration data for recycled charcoal canisters exposed in the BowserMorner radon chamber at three relative humidity conditions and for periods of 2 , 3 , 4 , 5 and 6 days (used by
permission, please see acknowledgement below). Each data point on the graph represents the calibration factor
for an individual charcoal canister, calculated using Equation (2). The curves are the lines of best fit for the
entire family of data points taken as a group, using Equation (6) as the model. Although each individual curve
shown in Figure 5 may not seem to be the best fit for the data for that particular exposure period, the family of
curves taken as a whole is the best fit to the entire group of calibration data.
Please note that the scale on the Yaxis of the graph in Figure 5 was intentionally left blank. Further, the
regression coefficients for the equation of best fit to these data arc not reported here. These measures first of all
provide some protection of the data on behalf of the client who paid for the calibration exposures in the first
place, and secondly prevent the misapplication of the data by others as was done with the EERF calibration
data.
The equation of best fit is Equation (6) with the proper values for the regression coefficients that resulted from a
nonlinear regression procedure. This equation can be used in a spreadsheet or computer program to produce the
best value, based on the calibration data, for the calibration factor for a charcoal canistcr for any value of
exposure time and mass of moisture gain, without any approximations.
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2
days
3 days
4
days
5 days
6 days

2
0
2
4
6
8
10 12 14 16 18 20 22 24
Mass Gain (g)
Figure 5. Calibration curves for recycled canisters using a nonlinear model
It can be easily seen from the clusters of data points in Figure 5 that most of the canisters that were exposed to a
relative humidity of approximately 50% had moisture gains exceeding 4.0 g. So, clearly the breakpoint at 4.0 g
used by the EERF algorithm and by the turnkey software is not appropriate for these charcoal canisters.
Conclusion
This author highly recommends that the turnkey software based on the EERF algorithm no longer be used by
charcoal analysis laboratories. Instead, a method based solely on an equation of best fit to the laboratory's
calibration data should be used.
References
Blue, T. E.; Jarzemba, M. S. A Model for Radon Gas Adsorption on Charcoal for OpenFaced Canisters in an
Active Environment. Health Physics, 63(2):226232; 1992.
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2002 International Radon Symposium Proceedings
George, A. C., Passive, Integrated Measurement of Indoor Air Using Activated Carbon. Health Physics, 46:867;
1984.
Gray, D. J.; Windham, S. T. EERF Standard Operating Procedures for Rn222 Measurment Using Charcoal
Canisters. Montgomery, AL: U.S.EPA Office of Radiation Programs; EPA 520587005; June 1987.
Jenkins, P. H. Equations for Calculating Radon Concentration Using Charcoal Canisters. Health Physics,
61(1):131136; 1991.
Acknowledgement
The author is grateful to Mr. David Grammer, RAdata, Inc., Flanders, New Jersey, for permission to use a set of
that laboratory's calibration data as an example in this paper.
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