A comparison of consumer-grade electronic radon monitors

• RSIC National Laboratories ambient radon concentrations: 20–30 Bq m⁻³
• Temperature: 21 °C–22 °C
• Humidity: 20%–30% RH
• Duration: 7 d

During the ambient levels check, the consumer-grade electronic radon monitors' measurements were compared against co-located calibrated C-NRPP-approved continuous radon monitors. This test was intended as a general check on the operation of the monitors. All monitors involved in the study passed the ambient check with average radon levels reporting between 24 and 29 Bq.

During testing, all monitors were placed in the National Radon Chamber and exposed simultaneously to the same radon concentrations and environmental conditions. Monitors were spaced apart a minimum of 10 cm.

The radon monitors included in the study are listed in table 1. The table lists specifications including the manufacturer's claimed accuracy, and the measurement technology and detection method used by each monitor.

Performance evaluation of the devices was based the Performance Classification Scheme used by Public Health England in their 'PHE Inter-comparison of Passive Detectors' [6]. For each model of radon monitor tested, the following performance data was calculated:

• Accuracy—relative percent error between the average radon concentration measured by each monitor and the radon chamber reference radon concentration
• Precision—relative standard deviation for the results measured for each model of individual radon monitor tested

Accuracy

$$\begin{equation*}Relative\,percent\,error\,\left( \% \right) = \,\frac{{\left( {Measured\,mean - Reference\,\;value} \right)}}{{Reference\;\,value}} \times 100\% \end{equation*}$$

Precision

$$\begin{equation*}Relative\,standard\,deviation\,\left( \% \right) = \,\frac{{Standard\,deviation}}{{Measured\,mean}} \times 100\% \end{equation*}$$

From the calculated relative percent error and relative standard deviation for each monitor, an overall percentage of measurement error was calculated.

Percentage of measurement error

$$\begin{equation*}Measurement\ error\,\left( \% \right) = \,\sqrt {{{\left( {Relative\,percent\,error} \right)}^2} + {{\left( {Relative\,standard\,deviation} \right)}^2}} \end{equation*}$$

Measurement error was used as a measure of performance because it includes consideration for variations in both precision and accuracy. In order to provide a simple reporting format that will allow consumers to compare the performance of different monitors, each monitor type was assigned a grade based on its measurement error during each of the test conditions. The grades are listed in table 2 and a summary of all the results is presented in table 4.

The conditions inside the radon test chamber during round 1 are a reasonable approximation of the conditions inside a Canadian home during the winter.

• Radon concentration target: 200 Bq m⁻³
• Average radon concentration measured: 206 ± 28 Bq m⁻³
• Temperature: 18 °C–22 °C
• Relative Humidity: 20%–50% RH
• Duration: 7 d

The average radon concentration achieved during Round 1 of testing was measured using the AlphaGuard DF2000 reference system. While there were fluctuations over the course of seven days of testing, the average concentration achieved was 206 ± 28 Bq m⁻³. This average concentration and the calculated uncertainty are shown in figure 1 using the placement and width of the horizontal blue band.

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As can be observed in figure 1, eight individual units across four different monitor types measured the radon levels in the chamber above the actual radon level. While one device type had two units measuring lower in the range, no devices measured below the actual radon levels.

The performance grades assigned for round 1 are listed in table 4.

The conditions inside the radon test chamber during round 2 were a reasonable approximation of the conditions inside a Canadian home during the summer, provided the home is not equipped with air conditioning. Air conditioning systems not only cool the air inside a home; they also reduce the humidity. The conditions inside homes with air conditioning will therefore be closer to the conditions inside the radon test chamber during round 1 all year round.

• Radon concentration target: 200 Bq m⁻³
• Average radon concentration measured: 208 ± 28 Bq m⁻³
• Temperature: 30° C
• Relative Humidity: 70% RH
• Duration: 7 d

The average radon concentration achieved during Round 2 of testing was measured using the AlphaGuard DF2000 reference system. While there were fluctuations over the course of seven days of testing, the average concentration achieved was 208 ± 28 Bq m⁻³. This average concentration and calculated uncertainty are shown in figure 2 using the placement and width of the horizontal blue band.

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As can be observed in figure 2, three individual units across two monitor types measured the radon levels in the chamber below the actual radon level, and three individual units across two monitor-types measured radon levels above.

In comparing figure 2 to figure 1, there is a marked difference in the reported radon levels for several of the monitors. Given that the radon level inside the chamber remained consistent between Round 1 and Round 2, it would appear that the changes in temperature and relative humidity affected several of the monitors.

Under-reporting the radon level is of particular concern when using consumer-grade electronic radon monitor, since this can lead to a false sense of security for the homeowner. The performance grades assigned for round 2 are listed in table 4.

In the third round of testing, the radon level was raised to 400 Bq m⁻³, while temperature and humidity were 18 °C–22 °C and 20%–50% RH respectively, just as they were in Round 1.

• Radon concentration target: 400 Bq m⁻³
• Average radon concentration measured: 410 ± 42 Bq m⁻³
• Temperature: 18 °C–22 °C
• Relative Humidity: 20%–50% RH
• Duration: 7 d

The average radon concentration achieved during Round 3 of testing was measured using the AlphaGuard DF2000 reference system. While there were fluctuations over the course of seven days of testing, the average concentration achieved was 410 ± 42 Bq m⁻³. This average concentration and calculated uncertainty are shown in figure 3 using the placement and width of the horizontal blue band.

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As can be observed in figure 3, two individual units from the same monitor type measured the radon levels in the chamber below the actual radon level, and eleven units across five monitor types measured above.

The performance grades assigned for round 3 are listed in table 4.

In the fourth round of testing, the radon level was raised to 1000 Bq m⁻³, while temperature and humidity remained at 18 °C–22 °C and 20%–50% RH respectively.

• Radon concentration target: 1000 Bq m⁻³
• Average radon concentration measured: 1075 ± 79 Bq m⁻³
• Temperature: 18 °C–22 °C
• Relative humidity: 20%–50% RH
• Duration: 7 d

The average radon concentration achieved during Round 4 of testing was measured using the AlphaGuard DF2000 reference system. While there were fluctuations over the course of seven days of testing, the average concentration achieved was 1075 ± 79 Bq m⁻³. This average concentration and calculated uncertainty are shown in figure 4 using the placement and width of the horizontal blue band.

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The performance grades assigned for round 4 are listed in table 4.

Figures 1 through 4 provide a visual representation of the accuracy and precision of each monitor type during each round of testing. The measurement error and corresponding performance grade of each type of monitor was also calculated for each set of conditions; as summarised in table 4 below.

For homeowners, using a consumer-grade electronic radon monitor to measure the radon levels in their home is a very different experience than using a passive long-term radon detector, which has historically been the most accessible homeowner testing option. While an electronic monitor can be a helpful tool, it is important to understand its limitations. In considering the results of this research, communicating the implications of measurement error to homeowners was found to be of primary importance.

It is important to consider the measurement error when interpreting the radon level reported by a consumer-grade electronic radon monitor. This is a new element for homeowners to consider, since for decades the only radon measurement options for homeowners were to use a passive long-term radon detector and return it to a laboratory for analysis, or to hire a radon measurement professional. With either of these options, homeowners receive a written report that clearly indicates the margin of error. Now, when using an electronic radon monitor, the margin of error is easily ignored since the radon monitors displays a very specific number without displaying any calculated uncertainties.

While the margin of error may not commonly be considered, it can make an important difference in the reported radon level and the associated perception of risk, especially near the Canadian guideline level of 200 Bq m⁻³. As illustrated in figure 5, if a monitor were to indicate a radon level of 185 Bq m⁻³, a homeowner might easily feel reassured that the radon level is comfortably below Canada's guideline level of 200 Bq m⁻³. However, even if this monitor had scored a grade of A (as defined in table 3 of this paper), it would mean that the margin of error could be up to plus or minus 10%. Considering that 10% of 185 Bq m⁻³ is 18.5, when taking that margin of error into account, the monitor is in fact reporting that the radon level is between 166.5 Bq m⁻³ and 203.5 Bq m⁻³.

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It is worth noting that none of the consumer-grade electronic radon monitors included in this study scored a grade of A during both Round 1 and Round 2, which were the two sets of test conditions at the Canadian guideline level of 200 Bq m⁻³. Therefore, when considering figure 5, the range of radon levels as indicated by Grade B is probably the most realistic starting point for most applications near the guideline level of 200 Bq m⁻³.

Once again, when compared to the more traditional radon measurement method of installing a long-term passive radon detector and returning it to the laboratory for a report after three months' time, the fluctuation of radon levels is more of a concern for the homeowner using an electronic radon monitor because these monitors provide readings within hours or days rather than months.

When looking at a digital read-out, it can be alarming to see a sudden spike in radon levels. It is important for homeowners to understand that while radon levels may occasionally spike upwards or dip downwards, it is the average annual exposure level that is a concern. Health Canada recommends that homeowners base any decision to mitigate their home on a long-term average radon level, which is determined by testing for 91 d or more. Most of the digital monitors reviewed in this study display the average radon level over a given time period in addition to the 'current' radon level. However, since the electronic radon monitors begin to report a radon level within minutes, hours, or days, homeowners may now be increasingly tempted to base decisions on shorter testing periods.

Those homeowners who do make decisions based on shorter testing periods will fall into one of three categories: those whose short-term radon test is higher than their annual average, those whose short-term radon test is indicative of their annual average, and those whose short-term radon test is lower than their annual average. The risk to the first group of homeowners is that they decide to install a radon mitigation system even though this system may not have been judged necessary were the annual average radon level known. The second group have, purely by chance, gained a representative measure of their annual average radon level, though they have no way of knowing this before completing a long-term test. The third group is the group most at risk, because they risk concluding that their annual average radon levels are below the guideline level, when in fact this in not the case.

Without further research, it is impossible to know for sure how many homeowners consider their homes to have a 'safe' radon level after any given short-term testing period using an electronic radon monitor indicating below the Canadian guideline level. It might seem natural to assume that any homeowner who has made the investment in a consumer-grade electronic radon monitor will leave it in place for at least a three-month period as recommended by Health Canada. However, a growing number of Canadian homeowners are not purchasing these consumer-grade electronic radon monitors but are instead borrowing them from a library or other loan program [14]. These loan programs inherently lead almost exclusively to one-time, short-term testing, as the loan period rarely extends to three months. In addition, the loan period may occur any time throughout the year given that the borrowers can only obtain the device during the season when their turn comes up on the waiting list [15]. Given these considerations, more research into the effectiveness of radon monitor loan programs is merited.