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| Method no.: |
ID-208 |
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| Matrix: |
Air |
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| OSHA Permissible Exposure Limit: |
100 pCi/L (29 CFR
1910.1096(c)(1)) |
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| Collection Procedure: |
A passive monitoring device known as
an E-PERM electret is used to collect and to measure
the ionized particles produced by radon gas. |
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| Recommended Sampling Time: |
2 - 7 days |
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| Analytical Procedure: |
The voltage on an electret is read
before and after the electret is exposed to the workplace
atmosphere. The voltage difference corresponds to the quantity of
radon in the workplace atmosphere. |
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| Manufacturers Quoted
Detection Limit: |
0.7 pCi/L for short-term electret
with a two day exposure and 0.4 pCi/L with a seven day
exposure. |
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Manufacturers Quoted
coefficient
of variation: |
The coefficient of variation is
10% (at one sigma) at radon concentrations of 4 pCi/L or greater at
the minimum recommended exposure periods. (8.1.) |
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| Method Classification: |
Partially Validated Analytical
Method |
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| Chemist: |
Dixon Cook |
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| Date (Date Revised): |
April, 1992 (February, 1993) |
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Commercial manufacturers and products mentioned in this method
are for descriptive use only and do not constitute endorsements by
USDOL-OSHA. Similar products from other sources can be
substituted.
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1.
Introduction
This method
describes the collection and analysis of radon gas using a standard
S chamber E-Perm monitoring device. Workplace
atmospheres are passively measured within the device by taking
readings of the voltage difference before and after the exposure
period.
1.1 History (8.2.
& 8.3.)
Within the Salt Lake Technical Center, radon gas was
previously collected on charcoal canisters and analyzed by a
private contractor using the method of scintillation counting.
1.2. Principle
E-PERMs are passive devices
requiring no power to function. They are integrating detectors and
can be used to determine the average radon concentration in the
working environment where the device is located during the
measurement period. The E-PERM electret radon chamber
(see diagram 1) consists of a plastic shell which has a
spring-loaded plastic cap and a replaceable holder at the bottom
which holds the electret. The electret has a Teflon surface which
has a fixed voltage induced on it. When the E-Perm sampler is
open, radon gas will diffuse into the shell through small holes at
the top and the particulate radon progeny will be trapped by the
filter. The negative ions released during the nuclear decay of
radon gas will move to the surface of the electret causing a
reduction in its surface voltage. The amount of voltage reduction
is directly related to the time integrated average radon
concentration to which the electret was exposed.
1.3.
Advantages and Disadvantages
1.3.1. This method is extremely convenient for both
the industrial hygienist and the analyst. No liquids are used
either in the sampling or the analysis.
1.3.2. The
standard S chamber E-Perm cannot be worn as a personal sampler
and is to be used as an area sampler only.
1.3.3. The
analysis can be completed in a few minutes and no digestion or
expensive equipment is needed.
1.3.4. The E-Perm has
adequate sensitivity for measuring workplace radon exposures.
1.3.5. There are no errors introduced due to
temperature, humidity, air draft, or concentration variations.
Corrections for background radiation are available for each
state and are applied to each measurement.
1.3.6. The
E-Perm does not measure radon daughters created outside the
chamber. Since radon daughters are particulates and cannot pass
through the filter, only radon gas can enter the chamber. The
radon will decay into its daughters and the daughters inside the
chamber will reduce the voltage on the electret.
1.3.7.
This method is much cheaper than other methods since an electret
can be re-used and since the measurement can be completed at the
OSHA Laboratory and no private contractor need be used.
1.3.8. The standard S chamber E-Perm does not have
adequate sensitivity for measuring an 8 hour workshift and must
be deployed for at least 2 days. 1.4. Uses and
Occurrence (8.1.)
Radon is not used commercially. Radon is formed by the
radioactive decay of certain naturally-occurring
uranium and thorium isotopes to radon 222 and eventually to lead
206. Because radon diffuses from the soil and from the domestic
water supply, it has become a concern in homes and workplaces. In
regions with large deposits of radioactive materials in the
ground, radon gas seeps into buildings and decays into radioactive
radon daughters. In the 1970's the construction of
energy-efficient buildings resulted in elevated concentrations of
radon and other pollutants, many times over that found outdoors.
(8.2.)
The only known health effect associated with exposure to
elevated levels of radon is an increased risk of developing lung
cancer. The risk of developing lung cancer increases as the level
of radon and the length of exposure increase. In 1985 the US-EPA
evaluated the existing data on the lung cancer risk from radon
exposure, and recommended that residents consider taking some kind
of remedial action if the radon level in their houses exceeded 4
picocuries (pCi) of radon per liter of air. The EPA projects that
if a person is exposed to 4pCi/L of radon and radon progeny for a
lifetime there is a 1-5 % chance that the person will die of lung
cancer. The EPA does not consider this a safe level of radon
concentration, but it is lower than that proposed by some other
organizations. The National Council on Radiation Protection &
Measurements (NCRP) study says that 8 pCi per L of radon in air is
an acceptable level for homes, and others believe the action level
should be as high as 20 pCi/L (8.3.).
1.5. Physical and Chemical Properties (8.4.)
| Radon: |
CAS No. 7647-01-0 |
| Atomic formula: |
Rn |
| Atomic weight (longest-lived
isotope): |
222.0 |
| Specific gravity: |
9.73 g/l |
| Melting point: |
-71.°C |
| Boiling point: |
-61.8°C | 2. Working Range and Detection Limit
2.1. An electret is a Teflon disk
across which a voltage of approximately 750 volts has been
applied. The dynamic range for a electret in an "S" chamber has a
200 V to 750 V range limitation which translates to a limitation
of about 270 pCi/L-days (see calculation of radon concentration in
section 7). In other words, the voltage would fall from 750 V to
200 V if exposed for one day to about 270 pCi/L or if exposed for
10 days to about 27 pCi/L. The upper range can be extended by
exposing the electrets for shorter than the recommended times. (8.1.)
2.2. The manufacturer's quoted minimum measurable levels
for radon analysis at 25% error using the E-PERM
short-term electrets in an S-chamber are
0.7 pCi/L for a two day measurement, and 0.4 pCi/L for a seven day
measurement. The minimum measurable level for an electret may be
improved by using a longer sampling time.
2.3. The
coefficient of variation is 10% (at one sigma) at radon
concentrations of 4 pCi/L or greater at the minimum recommended
exposure period of 2 days. (8.1.)
3. Method
Performance (8.1.)
3.1. The error associated with the system component
imperfections, which includes uncertainty in chamber volumes,
electret thicknesses and other component parameters, was
experimentally measured to be about 5% for the
E-PERMs by Rad Elec Inc.
3.2. The error in
the electret voltage reading can be as much as 1 volt which gives
a percent error of 100 X 1.4 / (I-F). Where I = initial voltage
and F = final voltage.
3.3. The maximum error introduced
by using the EPA-listed state average background values to correct
measurements made in various locations within a state is about 0.1
to 0.2. pCi/L. Background error can be minimized by using the
precise gamma background level at the site of measurement. However
even if there is a 20% error from the state average level, the
error introduced is only about 0.2 pCi/L.
3.4. The total
error is the square root of the sum of the squares of individual
error components listed in 3.1. to 3.3. ET =
(El2 + E22 +
E32 )1/2,
where El is 5%, E2 is the error referred to in 3.2. and E3 is the
background error referred to in 3.3.
3.5. The error
depends upon the type of E-PERM device and the measurement period.
3.6. No validation of this method was performed by the
OSHA-SLTC - a full validation was performed by Rad Elec, Inc. (8.1.).]
4.
Interferences
4.1. No interferences exist. 5. Sampling
5.1. Equipment
5.1.1. E-PERM ion chamber: The "S" (standard-volume)
chamber consists of a plastic shell which has a
spring-loaded plastic cap.
5.1.2.
Electrets: Removable plastic discs containing an electrically
charged wafer of Teflon. The white surface electrets are
short-term samplers which can be used to sample for
one to 14 days. [Chambers and electrets are available from Rad
Elec, Inc., 7499 Whitepine Rd., Richmond, VA 23237.]
5.2. Sampling Procedure
5.2.1. The measurement should not be made if the
occupant is planning remodeling; making changes in the heating,
ventilating and air conditioning system; or performing other
modifications that may influence the radon concentration during
the measurement period. The E-PERM should not be
deployed if the occupant's schedule prohibits terminating the
measurement at the time selected for returning it to the
Laboratory.
5.2.2. The building should be closed, with
all windows and external doors shut (except for normal entrance
and exit) for at least 12 hours prior to and during the sampling
period. For this reason, measurements should be made during the
winter whenever possible.
5.2.3. E-PERMs should be
deployed into workplaces as soon as possible after their initial
voltage is measured. Until an E-PERM is deployed, its electret
cover should remain in place over the electret to minimize
background effects.
5.2.4. To select the proper location
of an E-PERM within a room, the following must be considered.
The E-PERM must not be disturbed during the
measurement period. The E-PERM should not be placed
near drafts caused by high volume air conditioning vents,
windows, and doors. Avoid locations near excessive heat, such as
direct strong sunlight. The E-PERM should be placed
flat on a shelf or table at least 50 centimeters (20 inches)
above floor level and with the detector's top face at least 10
centimeters from other objects. Nothing should impede air flow
around the E-PERM. The E-PERM should
not be placed close to the outside walls of the building. (8.1.)
5.2.5 When retrieving the E-PERM, care should be taken
to inspect the device for damage during handling. The
information called for on the 91A should be accurately recorded.
The E-PERM serial number should be recorded in a
log book along with a description of the location in the
building where it was placed. The most important information is
the day and time the E-PERM was opened and the day
and time the E-PERM was closed.
5.2.6.
After sampling is completed be sure the E-PERM is securely
closed. Wrap the E-PERM top-to-bottom
with a sample seal (OSHA 21 or equivalent).
5.2.7.
E-PERMs should be deployed for a 2 to 7 day measurement period.
The E-PERM is turned off by screwing down the
"pop-up" lid on the top of the canister.
5.2.8 Ship the samples to the laboratory using
appropriate packing materials. The E-PERM must be sent to the
laboratory as soon as possible, preferably within a few days
following exposure termination. 6. Analysis
6.1 Precautions
6.1.1. Refer to instrument and Standard Operating
Procedure (8.1.)
manuals for proper operation. 6.2. Equipment
6.2.1. A standard volume E-PERM (SSTB or SSTG).
6.2.2. An instruction sheet for the industrial
hygienist, and a shipping container for returning the E-PERM(s)
to the laboratory.
6.2.3. Electret Reader from Rad Elec,
Inc. for reading the electret voltages before and after
exposure. (The reader is portable and can be used either in the
field or in the Laboratory.)
6.2.4. Set of reference
electrets.
6.2.5. Personal computer capable of using 5
inch diskette.
6.2.6. Software supplied by Rad Elec.,
Inc. on a 5 inch diskette for a personal computer.
6.3. Reagents - None are required.
6.4.
Analytical Procedure (8.1.)
Analyze the samples in accordance with the Standard
Operating Procedure. (8.1.)
All E-PERMs should be analyzed in the Laboratory as
soon as possible following removal from the workplace.
6.4.1. The analyst must take every precaution to
assure E-PERM custody continuity throughout the analysis. In
particular, extreme care must be taken to assure that the
identification number of each exposed electret remains traceable
to the E-PERM shell identification number in which it was
exposed whenever an electret is removed from an E-PERM.
6.4.2. Before using an E-PERM in the field, the initial
voltage must be taken. If an initial voltage is less than 200
volts, the E-PERM must be discarded.
6.4.3. Place the
closed E-PERM into the circular electret receptacle on the
read-out instrument. Rotate the E-Perm in the electret reader to
assure that it is well seated in the receptacle.
6.4.4.
Open and close the shutter repeatedly until the same voltage
reading appears twice in sequence (this usually takes 3
openings). The twice repeated voltage observed in this sequence
is the true electret blank voltage. The voltage must be between
-001 and +001 volts before taking a reading, if not, re-seat the
E-Perm and reread the voltage.
6.4.5. Carefully unscrew
and remove the bottom piece from the E-PERM. The bottom piece
holds the electret (the Teflon disk on the bottom piece). Do
not touch the electret surface or the
electret will discharge.
6.4.6. Carefully place the
electret face down into the circular electret receptacle on the
read-out instrument. Rotate the electret a little to assure that
it is well seated in the receptacle.
6.4.7. Open and
close the shutter repeatedly until the same voltage reading
appears twice in sequence. The twice repeated voltage observed
in this sequence is the true electret voltage. This value must
be recorded in the proper place in the analyst's notebook. The
E-PERM is now ready to be sent into the field.
6.4.8.
Upon receiving the E-PERM from the industrial hygienist, read
the final voltage from the electret following the previous
procedure. After reading the final voltage, carefully remove the
electret and replace it in the storage mode in an E-PERM
(closed) or with its shipping cover.
6.4.9. Check the
accuracy of the analysis by reading the E-PERM reference
electrets once during the analysis and check against previous
readings. Reference values should agree within 5% of their
stated values. Record reference values in your notebook.
7.
Calculations
7.1. In order to determine the average radon
concentration (C) during the exposure period, the following
equation is to be used:
Where:
| C |
= |
Average radon concentration in pCi/L |
| CV |
= |
initial electret voltage minus final
electret voltage |
| K |
= |
The calibration factor (supplied by
manufacturer) |
| d |
= |
The number of days exposure |
| B |
= |
a correction for background gamma
radiation - generally 1 pCi/L. More accurate background
corrections can be made by using the background gamma
correction by state table found in 8.1.,
Part I Appendix-3, Page 5. |
7.2. The concentration of radon in each air sample
is expressed in pCi/L.
7.3. Reporting Results
7.3.1. Report results to the industrial hygienist as
pCi/L radon, using two significant figures.
7.3.2. The
estimated detection limit is reported when no analyte is
detected. The detection limit for a short-term
electret in an "S-Chamber" (assuming 25% error) is
0.7 pCi/L with an exposure of 2 days and 0.4 pCi/L with an
exposure of 7 days (8.1.,
Part I Appendix-4, page 3).
8. References
8.1. E-PERM system Manual, Rad Elec, Inc.
8.2. Radon and Radon Daughter Field
Measurements, George, Andreas C., Environmental Measurements
Laboratory, US-DOE, Presented at the NBS Seminar on
Traceability for Ionizing Radiation.
8.3.
Radon Tagged as Cancer Hazard By Most Studies,
Researchers, C&EN Feb. 6, 1989, Pg 7.
8.4. CRC Handbook of Chemistry and
Physics, 52nd. Edition, The Chemical Rubber Co., 1971-1972.
8.5. Chemical Hygiene
Plan, OSHA Laboratory. |
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