Radon Detection Monitors

Advisory Report - December 2003

Radon Detection Monitors

Detecting the presence of radon, that colorless, odorless, tasteless radioactive gas that seeps into our homes from the underlying soil, is very simple to do using the many devices designed for this purpose. They come in many shapes and sizes. You can purchase simple test kits from the local hardware store, order them from the internet, or hire a company to do the testing for you. The kits homeowners can order and use themselves come with simple instructions and most of these monitors can be accurate if the test is performed according to the instructions. So how does a homeowner decide which monitor to use? Is one monitor better than another? Can the homeowner really do this and get accurate results or should they hire an expert to do the test? Do the experts have better monitors? All of these questions can be answered once you have been armed with the basic facts. This article will describe the different types of monitors, where and when they are used and their advantages and disadvantages in general terms.

There are basically two main categories of radon detection monitors: passive and active.

Passive Monitors

Passive monitors do not require external power to make them work. That is, they do not need batteries, DC adaptors or electricity. There are four popular monitor types currently in use in the U.S. They are “charcoal canisters,” “liquid scintillation vials,” “alpha track monitors,” and “electret ion chambers”. The first three types of monitor can be deployed by homeowners and are often ordered through mail order. A trained radon technician must hired to deploy the electret ion chamber type.

• Charcoal Canisters are hockey puck-shaped tins filled with charcoal. They are either 3 or 4 inches in diameter and can be open faced or have a filter over the face called a diffusion barrier. Radon gas enters, or diffuses, into the charcoal and remains trapped along with the subsequent radon decay products. The advantages of these are that charcoal canisters are inexpensive, do not require power to operate, can be sent through the mail, can be deployed by anyone and the EPA says “they can be accurate.” On the downside, they are highly sensitive to humidity, must be analyzed by an approved laboratory very soon after deployment, there is no way to tell if it has been tampered with and the results are biased towards the last 24 hours of the deployment period. There are a couple variations on the design, one looks like big tea bag while another type uses a shallow plastic tray instead of a round tin.

• Liquid Scintillation (LS) vials, are similar to charcoal canisters in that they also use charcoal as the radon collection medium. The device is usually in the form of a small plastic vial with a screw cap. LS vials have the same advantages and disadvantages as charcoal canisters, with one additional disadvantage. The amount of charcoal used in these devices is much less than the charcoal canisters, which means that the LS device must be analyzed very quickly after the measurement period is completed.

• Electret Ion Chambers are larger than the charcoal canisters, made of plastic, and contain an electrically charged Teflon disk located inside the main chamber. There is a plunger located on the top that is used to open and close the device. With the plunger in the up position, or open, radon gas is allowed to enter the main chamber through a filter. The radon gas inside the chamber decays and creates electrically charged particles that fall onto the charged Teflon disk which in turn discharge the voltage on the disc by small amounts. The trained radon technician will measure the voltage on the disk before and after deployment. The difference between the two voltages is used to calculate the amount of radon the device was exposed to. The advantages are that results can be given immediately, they do not require power to operate and they can be re-used by the radon technician. On the downside, they are sensitive to humidity, background gamma radiation and altitude. The before and after voltage measurements should be done at the same temperature. Finally there is no indication of whether tampering occurs.

• Alpha Track devices are typically small plastic containers. On the inside of the device, one or more plastic detectors are placed in a holder. Radon gas is allowed to enter the device through small openings covered by a filter. Once the radon gas is inside, it will decay. The small particles thrown off by the radon gas during decay will hit the plastic detector and cause a tiny dent on the plastic surface. After the deployment period, the device is sent to the laboratory where the plastic detectors are removed and chemically etched to make the small dents easier to see under a microscope. The marks are counted and the number of marks is proportional to the amount of radon gas the device had been exposed to. The advantages are that they are inexpensive, do not require power to operate, can be sent through the mail and can be deployed by anyone. The disadvantages are that alpha track detectors can only be used for long term testing and therefore can not be used for real estate transactions, where results are needed quickly. Tampering can not be detected but because these monitors are used by concerned homeowners, tampering is not an issue.

Passive Radon Device Comparison Chart

NOTE: This chart illustrates that there are many differences between passive devices. Because of the limitations of charcoal canister and similar devices, U.S. Inspect can only recommend top quality Alpha Track devices to be used for passive testing.  However, since the length of time that the circumstances of a real estate transaction allows is usually less than the minimum required for effective passive testing, U.S. Inspect recommends that continuous radon monitoring be used for real estate transactions, which provide results in as little as two days.  Read on for more information about continuous radon monitors.

Active (Continuous) Monitors

All active radon monitors require power to function. This can be from batteries, DC adaptor or electricity from the wall. There are three popular devices currently in use in the U.S. They are the “Scintillation Cell/Photomultiplier Tube”, “Ion Chamber” and “Solid State Detector.” Describing how each device works is beyond the scope of this article, but the basic difference among them is that the first type requires that air be actively drawn into the detection cell by a very precise mechanical air pump. The last two devices allow the air and radon to enter their detection chambers passively. The advantage of this kind of monitor is increased accuracy. All continuous monitors are required to record data at least once every hour. Some devices record at shorter intervals with a few as short as every 15 minutes. This level of detail assures not only accuracy but can also guard against tampering. Also, some continuous monitors will monitor their own vital signs to insure that that it is operating within its specified operating parameters. Some of the more advanced continuous monitors also record several other environmental parameters such as temperature, humidity, barometric pressure, physical movement and some even monitor movement of external bodies within the measurement area using advanced infrared detection systems. These advanced monitors help to determine not only that the test data is valid, but also if the test was tampered with. Additionally, continuous monitors can also help to identify variances, which can help identify radon in water problems. On the downside, continuous radon monitors are expensive and must be operated by trained radon technicians. They also require power to operate.

The Protocols, Short Term and Long Term

There are two testing protocols: short term and long term. Short term testing is defined as any test lasting between 48 hours and 90 days. Typically, short term tests are used during real estate transactions, when time is short and results are required prior to making a buying decision. The Continuous Radon Monitor is used exclusively for short term testing and is the preferred choice when accurate results are required showing variations over time. It is also the only device type that can detect test tampering.

The three passive monitors commonly used for short term real estate transactions are the charcoal canister, liquid scintillation vial and the electret ion chamber.

NOTE: There is one very important protocol that must be followed when using any passive monitor for real estate transactions. The radon technician or homeowner MUST deploy two devices, placed 4 inches apart, in the lowest livable level of the home. Since passive devices will only give a single average result, with no indication of variations over the deployment period, two devices are required to validate the result.  On the other hand, the real estate testing protocol only requires that one continuous monitor be used for a real estate transaction.

For homeowners who are interested in finding out what their annual average radon level is, EPA protocols encourage you to perform an initial short term (screening) test followed by a confirming short term or long term test. The choice of second test term depends on the result of the first screening test. Below 10 pCi/L (pico cures per liter), perform a follow-up long term test. Above 10 pCi/L, perform a short term follow-up test.

There are only two long term passive devices available. They are the Long Term Electret Ion Chamber and the Alpha Track monitor. Of these, there is only one that can be deployed by the homeowner, the Alpha Track monitor. Why is a long term test preferred over a short term test? Longer testing periods provide a more accurate indication of the annual average level of radon in the home. The EPA action level is based on the annual average level and has been set at 4 pCi/L. Radon levels can vary in the home during different parts of the year. Only a long term test can more accurately determine what the annual average will be. Ideally your test would last for one year. The alpha track monitor is by far the best alternative for the concerned homeowner. It is the only homeowner-deployable monitor that can do this.

Get the Facts

For more information on radon and radon testing protocols, there are two EPA publications that outline the correct testing protocols and answer the questions homeowners might have about radon and radon testing. The Citizens Guide to Radon is aimed at the concerned homeowner. The Home Buyers and Sellers Guide to Radon is aimed at the real estate transaction. Both are available on the EPA website.

Click here for more information about Radon.

Contributed by Martin Smith
Director of Training and Residential Operations
U.S. Inspect, LLC