Answer to Question #11641 Submitted to "Ask the Experts"

What causes my pocket digital radon monitor to spike for 24 hours after a commercial jet starts its descent? On my Corentium Digital Radon Monitor, the short-term average reading at the start of the flight was 0.74 becquerels per cubic meter (Bq m^-3), but while landing at Seattle-Tacoma Airport it went up to 160 Bq m^-3 and stayed high for 24 hours. The same thing happened when I landed in San Diego.

You have asked an interesting question. I have not personally used the Corentium detector to which you refer, so I cannot offer any firsthand observations. The detector behavior that you describe, in which the reading appears to increase when the aircraft lands, is peculiar. I have not heard previously of this response change, and I do not have a specific certain answer to explain it. I will make a few comments that occur to me.

As described by the manufacturer, the instrument uses a passive, diffusion-type chamber to allow air carrying radon to penetrate to the detector, which is described as using alpha spectrometry as the detection technique. The specific type of detector is a semiconducting silicon photodiode, used with appropriate software to separate radon alpha response from other alpha-emitting progeny influences.

When a modern-day airplane flies at typical altitude of 11,000 to 12,000 meters (m), even though the cabin is pressurized, the cabin pressure is usually lower than what is experienced at ground level; from what I could find, typically such pressures would be equivalent to the atmospheric pressure at about 2,000 m. Additionally, although some aircraft try to humidify dry air at altitude, many do a poor job, and many others do nothing; as a result, cabin air at cruising altitude is often quite dry.

When an airplane descends to ground level, the chamber of the radon detector will be exposed to air that is both more dense and often much more humid than the air it experienced at high altitude. In fact, because of the lower pressure in the diffusion chamber at high altitudes, air and humidity at lower altitudes will be actively driven into the chamber by the pressure differential. Depending on specific air conditions and chamber temperature, it is possible that excess humidity could produce condensation in the instrument, including in the electronic circuitry and the detector, leading to possibly improper functioning and false readings. The manufacturer warns against using the detector in the presence of high moisture. Depending on the extent of condensation in different situations, the erroneous results could take significant time to resolve, which would be consistent with your observations.

Another possible reason for transient increases in readings after landing in an aircraft could be electrical interference related to a relatively sudden increase in local electromagnetic radiation associated, possibly, with typical multiple cell phone users activating their phones to inform others of their arrival. However, this would not explain the reading remaining high for 24 hours as you found in your cited instances.

There may be other possible factors that affected your detector, but I cannot further define any likely candidates.

George Chabot, PhD, CHP