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The measurement of radon decay products and the devices to do so are
recognized by the US EPA. The following excerpts from US EPA documents are
provided as a guide to the radon professional. For brevity sake, only
relevant portions have been reproduced below. The entire documents can be
downloaded from EPA's website at
http://www.epa.gov/radon/pubs/index.html.
US EPA, Office of Radiation and Indoor Air (ORIA)
(6609J), 402-K-00-008, July 2000
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Radon Test Results Reported in
Two Ways
Your radon test results may be reported in either
picoCuries per liter of air (pCi/L) or working
levels (WL). If your test result is in pCi/L, EPA
recommends you fix your home if your radon level is
4 pCi/L or higher. If the test result is in WL, EPA
recommends you fix the home if the working level is
0.02 WL or higher. Some states require WL results
to be converted to pCi/L to minimize confusion. |
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Los resultados de la prueba del
radón se informan de dos maneras
Los resultados de la prueba del radón pueden
informarse en picocuries por litro de aire (pCi/L) o
en niveles operativos (WL). Si el resultado de la
prueba es en pCi/L, la EPA recomienda que arregle la
vivienda si el nivel de radón es de 4 pCi/L o más.
Si el resultado de la prueba es en WL, la EPA
recomienda que arregle la vivienda si el nivel
operativo es de 0.02 WL o más. Algunos estados
exigen que los resultados en WL sean convertidos a
pCi/L para minimizar la confusión. |
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Office of Air and Radiation (6609J), EPA 402-R-93-003, June 1993
The following figure shows one of the EPA's testing strategies. Note
the * reference to radon decay product measurements throughout the chart, where
0.02WL is a recognized criteria.
Expressing Radon Decay Products in Terms of Equilibrium
Equivalent Radon (EER)
Section 4.7 Reporting Radon Results
Measurement results reported in the units that
the device measures. Any measurement results based on radon gas (pCi/L of air)
should be reported to no more than one decimal place, e.g., 4.3 pCi/L. Any
measurement result based on radon decay products (WL) should be reported to no
more than three decimal places, e.g., 0.033 WL. Any conversions from WL to pCi/L
or from pCi/L to WL should be presented and explained clearly. If the WL value
is converted to a radon concentration, it should be stated in the report to the
homeowner that this approximate conversion is based on a 50 percent equilibrium
ratio (unless the actual equilibrium ratio is determined). In addition, the
report should indicate that this ratio is typical of the home environment, but
that any indoor environment may have a different and varying relationship
between radon and its decay products.
The following verbiage is from the US EPA's
"Indoor Radon and Radon Decay Product Measurement Device Protocols," EPA
402-R-92-004, July 1992 (revised)
3.2 Protocol for Using Radon Progeny
Integrating Sampling Units (RPISU or RP) to Measure Indoor Radon Decay Product
Concentrations
3.2.1 Purpose
This protocol provides guidance for using
radon progeny integrating sampling units (RPISU or RP) to produce accurate
and reproducible measurements of indoor radon decay product concentrations.
Adherence to this procedure will help ensure uniformity in measurement
programs and allow valid intercomparisons of results. Measurements made in
accordance with this protocol will produce results representative of
closed-building conditions. Measurements made under closed-building
conditions have a smaller variability and are more reproducible than
measurements made when the building conditions are not controlled. The
investigator should also follow guidance provided by the EPA in "Protocols
for Radon and Radon Decay Product Measurements in Homes" (U.S. EPA 1992c) or
other appropriate EPA measurement guidance documents.
3.2.2 Scope
This protocol covers, in general terms, the
equipment, procedures, analysis, and quality control objectives for
measurements made with RPs. It is not meant to replace an instrument manual
,but rather provides guidelines to be incorporated into standard operating
procedures by anyone providing measurement services. Questions about these
guidelines should be directed to the U.S. Environmental Protection Agency.
3.2.3 Method
3.2.3.1 Thermoluminescent Dosimeter (TLD)
RP. There are three types of RPs. The TLD type contains an air sampling
pump that draws a continuous, uniform flow of air through a detector
assembly. The detector assembly includes a filter and at least two TLDs. One
TLD measures the radiation emitted from radon decay products collected on
the filter, and the other TLD is used for a background gamma correction.
This RP is intended for a sampling period of 48 hours to a few weeks.
Analysis of the detector TLDs is performed in
a laboratory using a TLD reader. Interpretation of the results of this
measurement requires a calibration for the detector and the analysis system
based on exposures to known concentrations of radon decay products.
3.2.3.2 Alpha Track Detector (ATD) RP.
A second type of RP consists of an air sampling pump and an ATD assembly.
The air sampling pump draws a continuous, uniform flow of air through a
filter in the detector assembly where the radon decay products are
deposited. Opposite to the side of the filter where the radon decay products
are deposited is a cylinder with three collimating cylindrical holes. Alpha
particles emitted from the radon decay products on the filter pass through
the collimating holes and through different thicknesses of energy-absorbing
film before impinging on a disc of alpha track detecting plastic film
(LR-115 or CR-39). Analysis of the number of alpha particle tracks in each
of the three sectors of the film allows the determination of the number of
alpha particles derived from radium A (Po-218) and radium C' (Po-214). This
feature allows the determination of the equilibrium factor for the radon
decay products. This type of RP is intended for a sampling period of about
48 hours to a few weeks.
Etching and counting of the alpha track
assembly is carried out by mailing the detector film to the analysis
laboratory. Interpretation of the results of this measurement requires a
calibration for the detector and the analysis system based on exposure to
known concentrations of radon decay products.
3.2.3.3 Electret RP. The
electret RP is similar in operation to the TLD-type RP, except that the TLD
is replaced with an electret. The current model of this device contains a
one-liter-per-minute constant air flow pump and collects the decay products
on a 11.4 cm2 filter. As the radon decay products that are
collected on the filter decay, negatively charged ions generated by alpha
particle radiation are collected on a positively-charged electret, thereby
reducing its surface voltage. This reduction has been demonstrated to be
proportional to the radon decay product concentration. For more general
information on electrets, the reader should refer to Section 2.3.
RPs are true integrating instruments if the
pump flow rate is uniform throughout the sampling period. The electret must
be removed from the chamber and the electret voltage measured with a special
surface voltmeter both before and after exposure. To determine the average
radon concentration during the exposure period, the difference between the
initial and final voltages is divided first by a calibration factor and then
by the number of exposure days. A background radon concentration equivalent
of ambient gamma radiation is subtracted to compute radon concentration.
Electret voltage measurements can be made in a laboratory or in the field.
3.2.4 Equipment
The three types of RP sampling systems
include a sampling pump and the detector assembly. Sampling with the TLD-type
RP requires either a fresh detector assembly or fresh TLD chips to be
inserted in the detector assembly. Using the electret-type RP requires a
sufficient charge on the electret. Sampling with the ATD-type RP requires a
fresh detector disc (LR-115 or CR-39). An air flow rate meter should be
available for checking flow rates with the RP, and spare filters should be
available as replacements as needed.
3.2.5 Predeployment Considerations
The plans of the occupant during the proposed
measurement period should be considered before deployment. The RP
measurement should not be made if the occupant will be moving during the
measurement period. Deployment should be delayed until the new occupant is
settled in the house.
The RPISU should not be deployed if the
user's schedule prohibits terminating the measurement at the appropriate
time.
Prior to installation in the building, the
pump should be checked to ensure that it is operable and capable of
maintaining a uniform flow through the detector assembly. Extra pump
assemblies should be available during deployment in case a problem is
encountered.
Arrangements should be made with the occupant
of the building to ensure that entry into the building is possible at the
time of installation, and to determine availability of a suitable electrical
outlet near the sampling area in the selected room.
3.2.6 Measurement Criteria
The reader should refer to Section 1.2.2 for
the list of general conditions that must be met to ensure standardization of
measurement conditions.
3.2.7 Deployment and Operation
3.2.7.1 Location Selection. The
reader should refer to Section 1.2.3 for standard criteria that must be
considered when choosing a measurement device location.
In addition, the air intake (sampling head)
should be placed at least 50 centimeters (20 inches) above the floor and at
least 10 centimeters (four inches) from surfaces that may obstruct flow.
3.2.7.2 Operation. The RP
should be installed and, if possible, the air flow rate checked with a
calibrated flow meter. The location, date, starting time, running-time meter
reading, and flow rate should be recorded on the detector assembly envelope
and in a log. The RP should be observed for a few minutes after initiating
measurements to ensure continued operation. The occupants should also be
informed about the RP and requested that they report any problems or pump
shut-down. The occupants should be aware of the length of time the RP will
be operated, and an appointment should be arranged to retrieve the unit. The
criteria for the standardized measurement conditions (Section 1.2.2) should
also be told to the occupants.
The sampling period should be at least 48
hours, and may need to be longer, depending on the type of RP head. A longer
operating time decreases the uncertainty associated with the measurement
result.
3.2.8 Retrieval of Devices
Prior to pump shut-down, the flow rate should
be measured with a calibrated flow meter (if possible) and the unit should
be observed briefly to ensure that it is operating properly. The detector
assembly or detector film should be removed for processing and the date,
time, running-time meter reading, and flow rate should be recorded both on
the envelope and in a log book. The filter should be checked for holes or
dust loading and any other observed conditions that might affect the
measurement. If TLDs or film discs are to be removed from the detector
assembly, removal should be delayed for at least three hours after sampling
is completed to allow for decay and registration of radon decay products on
the filter.
3.2.9 Documentation
The reader should refer to Section 1.2.4 for
the list of standard information that must be documented so that data
interpretation and comparison can be made.
In addition, the serial numbers of the RPs,
TLDs, film discs, or electrets must be recorded.
3.2.10 Analysis Requirements
Analysis of the film from the ATD-type RPs
requires an analysis laboratory equipped to etch and count alpha track film.
Analysis of TLD-type RPs requires a TLD
reader. The TLD reader is an instrument that heats the TLDs at a uniform and
reproducible rate and measures simultaneously the light emitted by the
thermoluminescent material. The readout process is controlled carefully,
with the detector purged with nitrogen to prevent spurious emissions. Prior
to analyzing the RPISU dosimeters, the TLD reader should be tested
periodically using dosimeters exposed to a known level of alpha or gamma
radiation. TLDs are prepared for reuse by cleaning and annealing at the
prescribed temperature in an oven.
Analysis of the electret-type RPs requires a
specially-built surface voltmeter for measuring electret voltages before and
after exposure. For more information on analysis requirements, the reader
should refer to Section 2.3.10 (Electret Ion Chamber Radon Detectors) of the
Radon Measurement Device Protocols.
3.2.10.1 Sensitivity. The lower
limit of detection (LLD [calculated using methods described by Altshuler and
Pasternack 1963]) should be specified by individual suppliers for RP
detectors exposed according to their directions. The LLD will depend upon
the length of the exposure and the background of the detector for materials
used. The LLD should be calculated using the results of the laboratory
control devices.
3.2.10.2 Precision. Precision
should be monitored and recorded using the results of the duplicate detector
analyses described in Section 3.2.11.3. This method may achieve a
coefficient of variation of 10 percent at radon decay product concentrations
of 0.02 WL or greater. An alternate measure of precision is a relative
percent difference, defined as the difference between two duplicate
measurements divided by their mean; note that these two measures of
precision are not identical quantities. It is important that precision be
monitored continuously over a range of radon concentrations and that a
systematic and documented method for evaluating changes in precision be part
of the operating procedures.
3.2.11 Quality Assurance
The quality assurance program for an RP
system includes five parts: (1) calibration, (2) known exposure detectors,
(3) duplicate (collocated) detectors, (4) control detectors, and (5) routine
instrument checks. The purpose of a quality assurance program is to identify
the accuracy and precision of the measurements and to ensure that the
measurements are not influenced by exposure from sources outside the
environment to be measured. The quality assurance program should include the
maintenance of control charts (Goldin 1984); general information is also
available (Taylor 1987, U.S. EPA 1984).
Users of electret-type RPs should follow the
quality assurance guidance given for electret ion chamber devices in Section
2.3 of this document.
3.2.11.1 Calibration. Every RP
should be calibrated in a radon calibration chamber before being put into
service, and after any repairs or modifications. Subsequent recalibrations
should be done once every 12 months, with cross-checks to a recently
calibrated instrument at least semiannually. Calibration of RPs requires
exposure in a controlled radon-exposure chamber where the radon decay
product concentration is known during the exposure period. The detector must
be exposed in the chamber using the normal operating flow rate for the RP
sampling pumps. Calibration should include exposure of a minimum of four
detectors exposed at different radon decay product concentrations
representative of the range found in routine measurements. The relationship
of TLD reader units or etched track reader units to working level (WL) for a
given sample volume and the standard error associated with this measurement
should be determined. Calibration of the RPs also includes testing to ensure
accuracy of the flow rate measurement.
3.2.11.2 Known Exposure Devices.
Anyone providing measurement services with RP devices should submit
detectors with known decay product exposures (spiked samples) for analysis
at a rate of three per 100 measurements, with a minimum of three per year
and a maximum required of six per month. Known exposure detectors should be
labeled in the same manner as the field detectors to assure blind
processing. The results of the known exposure detector analysis should be
monitored and recorded, and any significant deviation from the known
concentration to which they were exposed should be investigated.
3.2.11.3 Duplicate (Collocated)
Detectors. Anyone providing measurement services with RP devices
should place duplicate detectors in enough houses to test the precision of
the measurement. The number of duplicate detectors deployed should be
approximately 10 percent of the number of detectors deployed each month or
50, whichever is smaller. The duplicate detectors should be shipped, stored,
exposed, and analyzed under the same conditions. The samples selected for
duplication should be distributed systematically throughout the entire
population of samples. Groups selling measurement services to homeowners can
do this by making two side-by-side measurements in a random selection of
homes. Data from duplicate detectors should be evaluated using the
procedures described by Goldin (section 5.3 in Goldin 1984), by Taylor
(Taylor 1987), or by the EPA (U.S. EPA 1984). Whatever procedures are used
must be documented prior to beginning measurements. Consistent failure in
duplicate agreement may indicate a problem in the measurement process and
should be investigated.
3.2.11.4 Control Detectors. TLD-type
RPs use a TLD that is shielded from the gamma radiation emitted by the
material on the filter. This TLD is incorporated in the detector assembly to
measure the environmental gamma exposure of the sampling detector. The two
TLDs are processed identically and the environmental gamma exposure is
subtracted from the sample reading. Electret-type RPs also require an
environmental gamma background correction.
3.2.11.4.1 Laboratory Control Detectors.
The laboratory background level for each batch of assembled TLDs should be
established by each supplier. Suppliers should measure the background of a
statistically significant number of unexposed thermoluminescent assemblies
that have been processed according to their standard operating procedures.
To calculate the net readings used to calculate the reported sample radon
concentrations, the analysis laboratory subtracts this laboratory blank
value from the results obtained from the field detectors.
Similarly, the laboratory background level
for each batch of ATD-type RPs should be established by each supplier of
these detectors. Suppliers should measure the background of a statistically
significant number of unexposed detector films that have been processed
according to their standard operating procedures. The analysis laboratory
will subtract this laboratory blank value from the results obtained from the
field detectors before calculating the final result.
Users of electret-type RPs should follow
similar control detector procedures discussed in section 2.3.11.1.
3.2.11.4.2 Field Control Detectors
(Blanks). Field control detectors (field blanks) should consist of a
minimum of five percent of the detectors deployed each month or 25,
whichever is smaller. Users should set these aside from each shipment, keep
them sealed, label them in the same manner as the field detectors, and,
where applicable, send them back to the analysis laboratory as blind
controls with one shipment each month. These field blank detectors measure
the background exposure that may accumulate during shipment or storage. The
results should be monitored and recorded. If one or a few of the field
blanks have concentrations significantly greater than the LLD established by
the supplier, it may indicate defective material or procedures. If the
average value from the background control detectors (field blanks) is
significantly greater than the LLD established by the supplier, this average
value should be subtracted from the individual values reported for the other
detectors in the exposure group. The cause for the elevated field blank
readings should then be investigated.
3.2.11.5 Routine Instrument Checks.
Proper operation of all analysis equipment requires that their response to a
reference source be constant to within established limits. Therefore,
analysis equipment should be subject to routine checks to ensure proper
operation. This is achieved by counting an instrument check source at least
once per day during operation.
Pumps and flow meters should be checked
routinely to ensure accuracy of volume measurements. This may be performed
using a dry-gas meter or other flow measurement device of traceable
accuracy.
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EPA Protocols