The Value of Hourly Radon Readings
EPA’s Science Advisory Board recommendations for CRMs

In early 1992, the EPA requested their Science Advisory Board (SAB) to review the scientific basis of the real estate testing options proposed in the original draft of Homebuyer’s and Seller’s Guide to Radon and to weigh in on methods to discourage tampering.
The SAB noted that radon concentrations vary with time of day, with weather, by season, and with the occupants’ HVAC use patterns. They also recognized radon measurement methods vary in their precision, ability to provide an integrated sample over time, susceptibility to tampering, and time periods for which they may be usefully deployed.
While acknowledging simultaneous measurements would improve precision and sequential measurements should deliver a more accurate representation of the seasonal average, their comments stressed the importance of employing methods like hourly Continuous Radon Monitor (CRM) readings “to reduce or eliminate inadvertent or deliberate interference with the measurement devices(s) or violation of closed house conditions to ensure the integrity of the results.”
Ultimately, the 1993 EPA Protocol for Radon and RDP Measurements in Homes provided three options for testing within the time constraints of a real estate transaction. Sequential Test Option 1 defined the process for placing and interpreting results of a single passive device for each of two consecutive deployment periods; Simultaneous Test Option 2 outlined procedures for placing two collocated passive devices for a single deployment period; and Single Test Option 3 laid out requisites for utilizing results of a single CRM for making a mitigation decision.

Option 3 specified use of an “active CRM with the capability to integrate and record a new result at least hourly.” The Protocol explains “shorter integration periods and more frequent data logging afford greater ability to detect unusual variations in radon concentrations.” It stipulated, “If the monitor cannot integrate over a period of one hour or less, then an additional (secondary) passive or active measurement device must be used.”
The Honeywell Professional Radon Monitor was the first really affordable and therefore popular CRM used by home inspectors. The detector response function averaged about 2.5 counts per hour per pCi/L of radon, and it could record and display at 4, 8, 12, or 24-hour intervals. It did not provide a reading in one-hour intervals because the count rate was too low to yield a reliable result in only one hour of counting time, unless the radon concentration just happened to be high enough to sufficiently improve the statistics. Since it didn’t meet the requirements of the Single-Test Option for real estate transactions, users were supposed to place another test device alongside it.
To be clear, that doesn’t mean these monitors couldn’t produce a very reliable result after 48 hours of sampling. The Honeywell monitor had been evaluated and accepted by the EPA Radon Measurement Proficiency Program and individually calibrated CRMs using the same sensor technology are capable of measuring within 3 to 5% of a NIST-traceable target radon concentration after 24 hours of data collection. The detector is just not capable of producing reliable readings for each single hour.
Page 18 of the 1993 Protocol states, “Instruments with greater efficiency or sensitivity, or a high signal-to-noise ratio can achieve results with a smaller uncertainty than instruments with low efficiency, poor sensitivity, or low signal-to-noise ratio. Greater efficiencies, sensitivities, or a high signal-to-noise ratio may also facilitate tampering detection by being more sensitive to fluctuations in radon levels.”
It then declares, “There have been recommendations for setting minimum efficiency standards for active devices at 16 counts per hour per hour per pCi/L. EPA plans to conduct research to establish minimum standards in the future for all categories of devices, passive as well as active detectors.”
EPA’s plans to establish minimum CRM counting efficiency standards never came to fruition; Honeywell and eventually other manufacturers of CRMs with low-efficiency detectors simply programmed their data loggers to record at one-hour intervals to meet the Protocol, even though their interval readings have a large uncertainty and a relatively high minimum detectable concentration.
More than 25 years after EPA announced its intentions, the 2019 ANSI-AARST MS-QA standard accepted the status quo by defining a continuous monitor as “an electronic device that is first, capable of automatically recording a retrievable time series of numeric measurements of radon concentration averaged over time intervals of 1 hour or less; second, has a minimum detectable concentration (MDC) of no greater than 4 pCi/L for a 1-hour measurement; and third, has a calibration factor (counting efficiency) of at least 2 counts per hour per picocurie.”
What has been sacrificed? The integrity of the one-hour readings. When the uncertainty of each hourly calculation is so large, the primary advantage of using a CRM is negated.
Opinions of authors are solely their own and do not necessarily represent the opinions or positions of ASHI, its agents, or editors. Always check with your local governmental agency and independently verify for accuracy, completeness, and reliability.
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