Radon Chambers - National Radon Safety Board




The radon chamber accreditation program is the part of the radon program that ensures that providers of quality assurance exposures of devices and instruments that measure radon and radon decay products meet standard criteria to ensure high quality in their services. Quality assurance exposures include calibrations, spikes, proficiency tests and performance evaluations. Quality in these services is essential to the radon testing industry, because the radon chamber operators provide the reference upon which radon and radon decay product measurements are based. To achieve an adequate degree of quality, radon chambers must meet certain design and operating characteristics.

A secondary radon chamber is one that meets the requirements of this document, and specifically inter-compares with a primary reference, such as the EPA radon laboratory in Las Vegas. A secondary chamber can perform quality assurance exposures of devices including calibrations, spikes and proficiency tests. An application for Secondary Radon Chambers is available on-line.

The size and sophistication of radon chambers can vary greatly depending on their intended uses. Chambers that are relatively small and simple can be used for quality assurance exposures for some types of devices or instruments that measure radon only. Some devices, such as those that use activated charcoal, require temperature and humidity control. A chamber that is used to test radon decay product monitors must have an aerosol generator to produce an adequate concentration of particles in the air to which the radon decay products can attach. Also, the size of the chamber must be adequate to prevent large losses of radon decay products from the air due to plateout. Larger chambers are necessary in cases where the instruments themselves are large and where personnel must enter the chamber to set up and operate the instruments.

The vast majority of measurements made in the U.S. are for radon itself rather than for radon decay products. However, several radon testers use monitors that measure radon decay product concentration. Chambers that are used for calibrating or evaluating them must have the appropriate level of sophistication. This document addresses requirements for two categories; a basic level for testing devices that measure radon only, and a more sophisticated level for testing devices that measure radon and/or radon decay products.

Indoor concentrations of 220Rn, or thoron, can be significant and can be a threat to health as well as a source of interference in the measurement of radon. However, few people have the equipment and expertise to measure thoron concentration. Further, there is no EPA Guideline concentration for thoron as there is for radon. For these reasons, the chamber requirements for quality assurance exposures for thoron measuring devices are not addressed in this document.

The criteria in this document are intended for quality assurance exposures of devices used by NRSB certified radon testers and mitigators. The measurements conducted by these radon practitioners are typically carried out in an indoor environment in residences, schools or commercial buildings. This document does not address criteria for testing devices that are used in the varied environmental conditions that occur outdoors, or in harsh industrial environments.

To assess the radon exposure of the general public and to deal effectively with a measurement quality assurance plan, the reliability and comparability of the data must be assured. This can be accomplished by initial calibration of radon instruments in a standard test facility under nearly the same conditions as their intended use in the field. Annual calibrations and proper quality control procedures will ascertain high quality in the measurements.


Criteria for a basic level of test chamber for devices that measure radon only are as follows:

1. Chamber Size: The size of the radon chamber should be large enough to accommodate different types of instruments for single or replicate exposures and for different times of exposure. For practical and economical reasons the chamber should be larger than 1 m3 in volume, and preferably greater than 5 m3. Ports must be provided for sampling from the outside whenever needed. A walk-in type chamber is preferred, because many commercial devices must be placed inside and operated by personnel inside the chamber. The radon chamber must be large if all types of devices (small passive and large electronic) are to be placed inside for calibration or evaluation. For only small passive devices, a chamber 1 – 2 m3 in volume is adequate when placing the devices through portholes or through a pass-box arrangement. However, depending upon the specific design of the chamber, there may be a limit to the quantity of charcoal devices, which adsorb radon from the air, that can be placed in a chamber of this size without affecting the ability to control the radon concentration in the chamber.

2. Chamber Environmental Conditions: The atmosphere inside the radon chamber should be environmentally controlled to simulate conditions of habitation and to test instruments under different and controlled conditions that may affect their performance. It may be necessary to test at several different radon concentrations, thus requiring a variation in ventilation rate or control of the source of radon entering the chamber. For testing devices that are sensitive to the velocity of the surrounding air, such as open-face charcoal canisters, it is essential to have control of the flow of air in the chamber interior. The radon chamber should have the required environmental conditions to test for the effects on instruments due to temperature, humidity and gamma radiation; conditions that are likely to be encountered in field situations. These effects can only be tested in a chamber with variable and well-controlled conditions. The test conditions for temperature and humidity should range from 65 – 80 F and 20 – 75%, respectively.

3. Radon Source: The radon generator should consist of pure, dry 226Ra. Radium in a liquid solution can present contamination problems and should be avoided. The activity of the source must be properly sized depending upon the design of the chamber, but should typically be in the range of somewhat less than 1 microcurie ( Ci) up to about 10  Ci. With the proper design, a useful range of radon concentrations between 2 and 50 pCi/L can be achieved. Keeping the test chamber at slight positive pressure will produce a more uniform radon atmosphere inside. Handling and shielding of the radon generator system can be accomplished easily with dry 226Ra sources. Care must be taken to avoid exposure to radon from leakage from the chamber or from the exhaust air in the immediate vicinity of the chamber. Uranium tailings and naturally enriched soils as radon generators should be avoided because of uncertainties in their characterization and origin.

4. Traceability to National Standards Materials: Radon traceability from the radium standard alone cannot be readily established because of the uncertainty of the emanation of radon from the source and other variables such as dilution, leakage, etc. In a secondary chamber such as described here, traceability is established with internal standard methods and instruments that have been calibrated in a primary calibration facility that has established traceability to NIST or another official Standards Laboratory. At present, in the absence of a commercial primary radon facility, EPA is preparing to assume the role of the primary laboratory for radon traceability purposes in one of its chambers in Las Vegas.

5. Chamber Monitoring Instruments: The conditions in the test chamber should be monitored continuously, logging the data at least hourly for radon concentration, temperature and humidity. The radon-monitoring device must be calibrated and traceable to a primary standard for continuous operation and be capable of providing the hourly and the average radon value over different periods of exposure. The sensitivity of the chamber-monitoring device must be comparable to, or better than, that of the device(s) being tested.

6. Qualifications of Radon Chamber Operators: The performance of any service provider is only as good as the person behind it. The calibration and evaluation of different types of devices requires an operator with a broad background and knowledge about them. The operator must be knowledgeable in the principles of operation of each device, including its performance characteristics and protocol application. At a minimum, the director of chamber operations should have a college degree in one of the sciences with a good grasp of nuclear detection methods, statistics and quality assurance.


This category of chamber is more sophisticated than that for chambers intended for testing devices that measure radon only. These more sophisticated chambers can be used not only for the measurement of radon, but also for the measurement of individual radon decay product concentrations, the collective radon decay product concentration in the unit of Working Level (WL), the concentration of particles in the air and the particle size distribution.

The main characteristics of a sophisticated radon and radon decay product chamber include all of the characteristics described above for a “radon only” chamber with the following additions or modifications:

1. Chamber Size: The chamber must be large, with a minimum volume of 10 m3, to prevent significant losses of radon decay products from the air due to plateout and reduction of radon concentration when testing large quantities of charcoal devices. It is preferable that the chamber be of the walk-in type to allow the entry of personnel and the placement and operation of large instruments.

2. Radon Source: The radon generator should consist of pure, dry 226Ra. The source should be shielded to reduce exposure to personnel and possible undesired exposure of devices to an elevated field of gamma radiation. The activity of the source must be chosen based on the design and operation of the chamber; however, it should typically be in the range of 5 to 20  Ci. It should be possible to control the radon concentration in the range of 4 to 50 pCi/L and the concentration of radon decay products in the range of 0.001 to 0.4 WL with equilibrium factors ranging from 0.1 to 0.8

3. Particle Generator: Because radon decay product instruments may be sensitive to particle size and concentration, a particle generator, particle counting equipment and particle sizing equipment are needed to evaluate the conditions under which the instruments are exposed. A particle generator and a particle counter are required to achieve and control concentrations in the range of 2000 to 100,000 particles/cm3 inside the test chamber. When the effects of plateout on instruments that measure radon decay product concentration are investigated, filtration may be required to achieve the low particle concentration and equilibrium factor that are necessary for such testing. Particle concentrations in homes can vary significantly with time and are usually very low at night. This factor can significantly affect a measurement of radon decay product concentration conducted over a period of 48 hours.

4. Chamber Monitoring Instruments: The radon decay product concentration must be monitored continuously on an hourly basis. The instrument that is used to monitor the radon decay product concentration in the chamber must have been tested in a chamber at a primary laboratory with known radon decay product concentrations under different equilibrium conditions, or its measurements must be normalized to a series of radon decay product grab measurements collected throughout the test period using instruments that are traceable to primary methods and have been tested at a primary laboratory.


Chambers that are approved for conducting Radon Measurement Proficiency (RMP) tests must follow the procedures developed by the Radon Measurement Device Performance Testing Panel of the NRSB. Under no condition is it acceptable for a chamber operator to conduct an RMP test on a device or instrument that is manufactured by a company that is affiliated with the chamber facility. Further, such affiliations may produce potential conflicts of interest when RMP tests are conducted for clients who may be competitors with the chamber facility or its affiliated company. Therefore, clients should be made aware of any such potential conflicts of interest before RMP tests are conducted.


A. C. George

A tertiary radon chamber is a facility that must among other things, successfully intercompare with an accredited secondary chamber in order to qualify for accreditation. Accuracy of 10% is expected. A tertiary chamber may be used for initial evaluations of devices by manufacturers, for spiking and calibration as required by US EPA Radon Protocols, AARST standards and the Radon Proficiency Program. Calibration in a tertiary radon chamber is not for detailed device evaluation as required for device approval.

The use of tertiary chambers for testing or calibrating devices for the measurement of radon decay product concentration is not addressed in this document as there is little demand for this kind of service. Tertiary radon chambers should not be used for proficiency testing and detailed device evaluation as required for device approval purposes because more stringent standards and capabilities are required, found in primary and secondary accredited radon chambers. If proficiency testing was allowed, in a tertiary radon chamber, there could be potential for conflict of interest in manufacturers or vendors testing devices that they may sell or rent to their clients.

The criteria for tertiary radon chambers are less stringent than those of secondary and primary radon chambers. In terms of size, they can be much smaller. They must be specific for the device being evaluated, spiked or calibrated. If several devices are tested simultaneously, a large volume chamber ranging from 500 1500 liters is required to accommodate all of them. A large volume chamber is required for spiking or calibrating charcoal devices and multiple units of continuous radon monitors. The concentration of radon inside the chamber must be maintained homogeneous , well defined and either kept stable or varied in some deliberate manner for some applications. For the evaluation or calibration of a single device a smaller chamber could be adequate as long as the radon concentration is homogeneously mixed and well defined during the test period.

Active continuous radon monitors that use a pump to sample test air through a probe, or a single entry point such as a valve on a scintillation cell, may require a very small volume chamber. If a monitor is being evaluated or calibrated by pumping air through it, it can be placed outside the radon chamber and pump intake air from inside the chamber through a port and recycle the air back into the chamber. This may not be feasible for many types of monitors because accessing the exhaust port may be very difficult or impossible.

Active continuous radon monitors that use a pump to sample test air through a probe (point source), require essentially no volume. In this case, the radon generator airflow rate must be sufficiently higher than the combined air-flow rate of the two devices one being tested and one being the reference device. As an example the RAD 7 sniffer, has a probe that can be connected to the output of the radon generator. The tertiary radon facility used for evaluating or calibrating this type of monitor would be evaluated on case-by-case basis.

If a tertiary radon chamber is used to test or calibrate devices that are affected by conditions of temperature and humidity, such as charcoal devices and some continuous monitors, then the chamber must be capable of controlling these conditions over some reasonable range typically found indoors. For example, the tertiary chamber should be capable of maintaining the temperature and relative humidity at values ranging from 65 oF to 80 oF and 30% and 70%, respectively. The tertiary chamber should be capable of maintaining well defined conditions for a period of time sufficient for the type of test being conducted.

Typically, radon devices are exposed in the field for a minimum of 48 hours. Chamber tests of short-term devices, are expected to be conducted over a period in the range of 2-7 days. Tests of long-term devices could be conducted over a period ranging from weeks to months depending on the concentration of radon in the test chamber.

A tertiary chamber must be monitored continuously to measure the average concentration of radon as a function of time over the period of a test or calibration. A certified continuous radon monitor is the most appropriate monitor for measuring the radon concentration in the chamber. The continuous monitor must successfully pass a blind intercomparison test at least annually in an accredited secondary or primary chamber. Documentation must be maintained to show proof of traceability to an accredited radon chamber. The sensitivity, or precision error of the device used to monitor the chamber must be at least as good as, and preferably better than the devices being tested or calibrated. It is advisable to use duplicate continuous radon monitors if possible to determine precision and whether the performance of either one of the collocated devices is acceptable. If the two traceable radon monitoring devices do not agree within the prescribed precision limits, then there is an option to investigate and find the source of the problem.

Although, a continuous radon monitor is preferred, properly calibrated charcoal canisters or electret ion chambers, reserved specifically for use in tertiary radon chambers, may be used to monitor the radon concentration. These devices must first pass successfully blind intercomparison tests at least annually in an accredited secondary radon chamber.

If grab samples such as scintillation cell devices are used to monitor a radon chamber during calibrations, they must record data, as a minimum on hourly basis. Their calibration must be traceable to a documented reference laboratory intercomparison in a secondary or primary accredited radon chamber.

The operator of a tertiary radon chamber must ensure that personnel and equipment are protected from radiation exposure and contamination from the radon generator source. The radon sources used in tertiary radon chambers must meet the radiation regulations which differentiate between licensed and exempt radium sources. Appropriately sealed and shielded 226Ra sources in dry form are commercially available and are preferred. However, in-house prepared radon generator sources that use small activities of radium or natural ores such as uranium ore or tailings are sometimes used to avoid State licensing or certification.

If such a source is used it must be < 1 microcurie in strength and be adequately sealed against leakage into the nearby environs. Usually, double filtration will keep the dry medium in its container but allow the radon to emanate freely into the test chamber or into the device being tested. The operator must demonstrate and document that the contribution of thoron is negligible, if a source such as a natural ore is used that potentially could contain 224Ra. (In other words, if someone is using a commercially available 226Ra source, there is no need to demonstrate and document that the contribution of thoron is negligible.) The exhaust air from the radon generator and /or chamber must be vented into an unoccupied environment usually outdoors, where it is diluted very quickly. Also consideration of the point of exhaust must be taken to avoid reentry indoors.

A worker safety program must be in place to protect personnel from radon and radon decay products that may be leaking into occupied areas. A personnel or area radon monitor such as a long-term alpha track detector can be used near the tertiary chamber for quarterly or annual determination of exposure to radon. Documentation of exposure must be available and maintained for a minimum of five years. Other applicable safety considerations, such as electrical safety, fire safety, safety from tools and confined space environments must be addressed and maintained at all times.

An operator of a tertiary radon chamber must have a Quality Assurance Plan, Standard Operating Procedures and Worker Safety Plan in place when applying for certification. The personnel responsible for the operation of the tertiary chamber and for the acquisition of data and results must be qualified by means of participating in radon courses and by being successfully certified as radon specialist. As a minimum, a radon measurement specialist must be supervising the entire operation.

There must be operating procedures documenting the steps taken to provide the appropriate exposure environment for the calibration of a specific device and what criteria are used to determine the successful calibration of the device and what criteria were used to determine its successful calibration. There must also be procedures documenting how the device that is used to monitor the tertiary chamber is calibrated and compared with an accredited secondary radon chamber as a minimum once a year. The accreditation organization reviews the documented information and acts accordingly to see that the required criteria were addressed.

NRSB cannot anticipate every tertiary facility or application thereof and should review each application on a case-by case basis to ensure that the tertiary facility meets the appropriate criteria required for the stated application.