Radon Measurement Methods

Listed below are brief descriptions of each of the 16 radon measurement methods that have been identified by the U.S. EPA and were used in EPA’s former RPP. The following descriptions are divided into methods appropriate for measuring radon gas and radon decay products, respectively.

1.  AC - Activated Charcoal Adsorption

For this method, an airtight container with activated charcoal is opened in the area to be sampled and radon in the air adsorbs onto the charcoal granules. At the end of the sampling period, the container is sealed and may be sent to a laboratory for analysis.

The gamma decay from the radon adsorbed to the charcoal is counted on a scintillation detector and a calculation based on calibration information is used to calculate the radon concentration at the sample site. Charcoal adsorption detectors, depending on design, are deployed from 2 to 7 days. Because charcoal allows continual adsorption and desorption of radon, the method does not give a true integrated measurement over the exposure time. Use of a diffusion barrier over the charcoal reduces the effects of drafts and high humidity.

2.  AT - Alpha Track Detection (filtered)

For this method, the detector is a small piece of special plastic or film inside a small container. Air being tested diffuses through a filter covering a hole in the container. When alpha particles from radon and its decay products strike the detector, they cause damage tracks. At the end of the test the container is sealed and returned to a laboratory for reading.

The plastic or film detector is treated to enhance the damage tracks and then the tracks over a predetermined area are counted using a microscope or optical reader. The number of tracks per area counted is used to calculate the radon concentration of the site tested. Exposure of alpha track detectors is usually 3 to 12 months, but because they are true integrating devices, alpha track detectors may be exposed for shorter lengths of time when they are measuring higher radon concentrations.

3.  UT - Unfiltered Track Detection

The unfiltered alpha track detector operates on the same principle as the alpha track detector, except that there is no filter present to remove radon decay products and other alpha particle emitters. Without a filter, the concentration of radon decay products decaying within the “striking range” of the detector depends on the equilibrium ratio of radon decay products to radon present in the area being tested, not simply the concentration of radon. Unfiltered detectors that use cellulose nitrate film exhibit an energy dependency that causes radon decay products that plate out on the detector not to be recorded.

This phenomenon lessens, but does not totally compensate for the dependency of the calibration factor on equilibrium ratio. For this reason, EPA currently recommends that these devices not be used when the equilibrium fraction is less than 0.35 or greater than 0.60 without adjusting the calibration factor. EPA is currently evaluating this device further to determine more precisely the effects of equilibrium fraction and other factors on performance. These evaluations will lead to a determination as to whether to finalize the current protocol or remove the method from the list of Program method categories.

4. LS - Charcoal Liquid Scintillation
This method employs a small vial containing activated charcoal for sampling the radon. After an exposure period of 2 to 7 days (depending on design) the vial is sealed and returned to a laboratory for analysis. While the adsorption of radon onto the charcoal is the same as for the AC method, analysis is accomplished by treating the charcoal with a scintillation fluid, then analyzing the fluid using a scintillation counter. The radon concentration of the sample site is determined by converting from counts per minute.
5. BC - Blind Continuous Radon Monitor

A Blind CRM is one for which the data collected cannot be accessed (viewed, downloaded or printed) by the field technician in any manner until AFTER it has been transmitted to an analysis lab for processing and report generation.  Typically, the raw data collected by the CRM is transmitted via a direct upload to the analytical laboratory where it is processed, converted to a test report and reviewed for anomalies before being electronically delivered to the field technician and the client on his/her behalf.  To be clear, if the field user can access and see the test results on a display screen, smartphone, computer, or printed paper BEFORE the laboratory has processed and reviewed the data, it is NOT a blind CRM.

6. CR - Continuous Radon Monitor

This method category includes those devices that record real-time continuous measurements of radon gas. Air is either pumped or diffuses into a counting chamber. The counting chamber is typically a scintillation cell or ionization chamber. Scintillation counts are processed by electronics, and radon concentrations for predetermined intervals are stored in the instrument’s memory or transmitted directly to a printer.

7. EL - Electret Ion Chamber: Long-Term

For this method, an electrostatically charged disk detector (electret) is situated within a small container (ion chamber). During the measurement period, radon diffuses through a filter-covered opening in the chamber, where the ionization resulting from the decay of radon and its progeny reduces the voltage on the electret. A calibration factor relates the measured drop in voltage to the radon concentration. Variations in electret design determine whether detectors are appropriate for making long-term or short-term measurements. EL detectors may be deployed for 1 to 12 months. Since the electret-ion chambers are true integrating detectors, the EL type can be exposed at shorter intervals if radon levels are sufficiently high.

8. ES - Electret Ion Chamber: Short-Term

For this method, an electrostatically charged disk detector (electret) is situated within a small container (ion chamber). During the measurement period, radon diffuses through a filter-covered opening in the chamber, where the ionization resulting from the decay of radon and its progeny reduces the voltage on the electret. A calibration factor relates the measured drop in voltage to the radon concentration. Variations in electret design determine whether detectors are appropriate for making long-term or short-term measurements. ES detectors may be deployed for 2 to 7 days. Since electret-ion chambers are true integrating detectors, the ES type can be exposed at longer intervals if radon levels are sufficiently low.

9. GC - Grab Radon/Activated Charcoal

This method requires a skilled technician to sample radon by using a pump or a fan to draw air through a cartridge filled with activated charcoal. Depending on the cartridge design and airflow, sampling takes from 15 minutes to 1 hour. After sampling, the cartridge is placed in a sealed container and taken to a laboratory where analysis is approximately the same as for the AC or LS methods.

10. GB - Grab Radon/Pump-Collapsible Bag

This method uses a sample bag made of material impervious to radon. At the sample site, a skilled technician using a portable pump fills the bag with air, then transports it to the laboratory for analysis. Usually, the analysis method is to transfer air from the bag to a scintillation cell and perform analysis in the manner described for the grab radon/scintillation cell (GS) method below.

11. GS - Grab Radon/Scintillation Cell

For this method, a skilled operator draws air through a filter to remove radon decay products into a scintillation cell either by opening a valve on a scintillation cell that has previously been evacuated using a vacuum pump or by drawing air through the cell until air inside the cell is in equilibrium with the air being sampled, then sealed. To analyze the air sample, the window end of the cell is placed on a photomultiplier tube to count the scintillations (light pulses) produced when alpha particles from radon decay strike the zinc sulfide coating on the inside of the cell. A calculation is made to convert the counts to radon concentrations.

12. SC - Three-Day Integrating Evacuated Scintillation Cell

For this method, a scintillation cell is fitted with a restrictor valve and a negative pressure gauge. Prior to deployment, the scintillation cell is evacuated. At the sample site, a skilled technician notes negative pressure reading and opens the valve. The flow through the valve is slow enough that it takes more than the 3-day sample period to fill the cell. At the end of the sample period, the technician closes the valve, notes the negative pressure gauge reading, and returns with the cell to the laboratory. Analysis procedures are approximately the same as for the GS method described above. A variation of this method involves use of the above valve on a rigid container requiring that the sampled air be transferred to a scintillation cell for analysis.

13. PB - Pump-Collapsible Bag (1-day)

For this method, a sample bag impervious to radon is filled over a 24-hour period. This is usually accomplished by a pump Programmed to pump small amounts of air at predetermined intervals during the sampling period. After sampling, analysis procedures are similar to those for the GB method.


14. CW - Continuous Working Level Monitoring

This method encompasses those devices that record real-time continuous measurement of radon decay products. Radon decay products are sampled by continuously pumping air through a filter. A detector such as a diffused-junction or surface-barrier detector counts the alpha particles produced by radon decay products as they decay on this filter. The monitor typically contains a microprocessor that stores the number of counts for predetermined time intervals for later recall. Measurement time for the Program measurement test is approximately 24 hours.

15. GW - Grab Working Level

For this method, a known volume of air is pulled through a filter, collecting the radon decay products onto the filter. Sampling time usually is 5 minutes. The decay products are counted using an alpha detector. Counting must be done with precise timing after the filter sample is taken. The two counting procedures most commonly used are the Kusnitz and the Tsivoglou methods described in the Indoor Radon and Radon Decay Product Measurement Device Protocols.

16. RP - Radon Progeny (Decay Product) Integrating Sampling Unit

For this method, a low-flow air pump pulls air continuously through a filter. Depending on the detector used, the radiation emitted by the decay products trapped on the filter is registered on two thermoluminescent dosimeters (TLDs), an alpha track detector, or an electret. The devices presently available require access to a household electrical supply, but do not require a skilled operator. Deployment simply requires turning the device on at the start of the sampling period and off at the end.

The sampling period should be at least 72 hours. After sampling, the detector assembly is shipped to a laboratory where analysis of the alpha track and electret types is performed using procedures described for these devices (AT, EL, and ES) elsewhere in this appendix. The TLD detectors are analyzed by an instrument that heats the TLD detector and measures the light emitted. A calculation converts the light measurement to radon concentrations.


Provides only the average concentration over a time period ranging from 2 days up to a year

  • Activated Charcoal
  • Liquid Scintillation
  • Alpha tracks
  • Electret ion chambers

Activated Carbon (AC)

  • Adsorbs radon gas by molecular diffusion into carbon grains
  • Adsorbed gas decays into short-lived RDPs
  • Bi-214 & Pb-214 RDPs are gamma-ray emitters
  • Rn-222 concentration is determined by counting gamma emissions of those RDPs

Advantages of Charcoal Devices

  • Convenient and economical
  • Can be used for a 2-day (48 hour) test
  • Easily mailed to the lab
  • Passive – no power required
  • Results can be provided within a few days

Limitations of Charcoal Devices

  • Limited to short-term tests
  • Biased to the Rn concentration of the last 12-24 hours of the deployment period
  • Sensitive to excessive humidity
  • Sensitive to excessive airflow
  • Provide no indication of severe changes in the Rn concentration during the test
  • Detection of tampering is difficult unless other tamper detection measures are deployed

AC – Points to Remember

  • Optimal exposure period for open-face canisters is two days
  • Can be biased to Rn concentration of the last 12-24 hours of deployment, particularly open-face canisters
  • Diffusion barrier cans must be left out longer (3-5 days or 5-7 days, depending on type), but more uniformly integrate changes in the radon concentration over time
  • Diffusion barrier canisters are less sensitive to humidity and air flow than open-face cans

Liquid Scintillation (LS) Charcoal

  • Small vial of a few grams of activated charcoal upon which radon can be adsorbed.

How Lab Analyzes Liquid Scintillation Devices

  • At lab, charcoal is transferred into liquid scintillation fluid, counted in a liquid scintillation counter.

Alpha Track Detectors (AT)

  • Small, alpha-sensitive plastic chip or cellulose film in a small, filtered container
  • Radon passively diffuses thru filter & decays
  • RDPs that plate-out on the chip release alpha particles that cause damage scars to the plastic (alpha tracks)
  • Calibrated based on tracks per unit area per pCi/l-day (counted with microscope)
  • Designed for long-term measurements of 3 months up to 1 year

Advantages of Alpha Track Devices

  • Relatively Inexpensive
  • Convenient
  • Easily mailed
  • Unobtrusive
  • Needs no external power
  • Can integrate over long periods

Limitations of Alpha Track Devices

  • Need longer deployment period
  • Potential for precision errors, especially at low radon concentrations

Electret Ion Chamber (ES, EL)

  • Bottle-like chamber of electrically conductive plastic coupled with a charged Teflon electret disk
  • Radon atoms diffuse thru filter; subsequent Poloniums release alpha particles as they decay
  • Alpha particles knock electrons from oxygen & nitrogen atoms, creating ion pairs
  • Freed electrons (negative ions) that reach electret surface deplete some its charge
  • Calibrated based on volts lost per pCi/l-day

Advantages of Electret Ion Chambers

  • Can be used for either short-term or long-term measurements, depending on which electret you choose
  • Electrets can be re-used until voltage drops below 200 volts
  • Results can be provided immediately upon pick-up; can be analyzed by the tester*
  • Unobtrusive
  • Need no external power

Limitations of Electrets

  • Sensitive to external gamma – should be corrected for
  • Sensitive to altitude – may need to be corrected for
  • Touching the electret surface or dropping it can inadvertently deplete voltage
  • Pre & Post voltage should be measured at the same temperature
  • Excessive humidity can adversely affect voltmeter – don’t take it on the raod

How Electrets Are Analyzed

  • Read initial voltage of electret BEFORE deployment
  • Read final voltage AFTER deployment
  • Electret should be positioned at same orientation & room temp each time it’s read
  • Average Rn concentration is function of volts lost per pCi/L/day


  • Provides interval readings as well as the average concentration for the time deployed
  • Hourly readings record the extent to which radon concentrations varied during the deployment period
  • Sophisticated electronics may include other tamper detection features as well

Types of Continuous Radon Monitors (CR)

  • Alpha Scintillation Monitors
  • Pulsed Ionization Chamber Monitors
  • Solid State Silicon Monitors

Alpha Scintillation Monitors

  • Scintillation flask & photomultiplier tube (PMT) counting system with timing circuitry
  • Radon passively diffuses or is pumped into cell
  • Polonium produced by Rn decay plate-out on zinc-sulfide phosphor coating on cell walls
  • Alpha particles striking the walls create a light spark (scintillation) that is amplified by PMT and recorded by microprocessor as an alpha count
  • Calibrated based on alpha counts per hour/pCi/l

Pulsed Ionization Monitors

  • Ion chamber with electrometer & data logger
  • Radon atoms that diffuse thru filter, & subsequent Poloniums release alpha particles as they decay
  • Alpha particles knock electrons from oxygen & nitrogen atoms, creating ion pairs
  • Negative ions pulse against the ion chamber’s positively charged pole; positive ions pulse against the chamber’s negatively charged pole or surface
  • Pulses are recorded by microprocessor
  • Calibrated based on ion counts per hour per pCi/l

Solid State Silicon Monitors

  • Ambient air diffuses thru filter into detection chamber
  • Alpha particles from radon & subsequent RDPs are sensed by the silicon detector
  • Data logger counts the alphas sensed
  • Calibrated based on counts per hour / pCi/l

Advantages of Continuous Radon Monitors

  • Many models have good precision
  • If detector is reasonably efficient, can reliably track variations in the radon concentrations
  • Ability to record changing concentrations can indicate potential tampering of closed-house conditions
  • Many units also record interval humidity & barometric pressure
  • Data can be downloaded immediately

Limitations of Continuous Radon Monitors

  • Cost more to purchase
  • Some models have a very low detector efficiency (2.5 CPH per pCi/L)
    • Large error in hourly readings
    • Creates what appears to be wide Rn variations from hour to hour
    • Inability to accurately track changing radon concentrations

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