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24 May 2012

Answer to Question #3511 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:

I have an electric air cleaner that has negative ion needles that the air passes last when exiting the cleaner. When I put my radiation detector about one inch away from these negative ion needles the detector reads up to 0.70 µSv/hr even when the air cleaner is not on. Why is this? Are there any health concerns I should be aware of? I am using a Gamma Scout detector.

First, you should review the response to Q3475. Through an inverse square reduction in dose rate, your 0.70 Sv/hr dose rate would be 0.005 µSv/hr at one foot. This is a fraction of the typical 0.05 to 0.10 µSv/hr background radiation level from cosmic rays and terrestrial gamma photon radiation. I wouldn't worry about possible exposure at this level. A Gamma Scout detector is a Geiger counter-type radiation survey meter, and if it is not in compliance with American National Standards Institute, Inc., health physics instrument standards, it may be subject to electric field interference.

There is a potential for radon decay products to build up in these devices and, as a result, a slight increase in external radiation exposure is possible very near the device. This is particularly so in houses with high radon levels. However, the source of the radon decay products is in the room air, not the device. To exclude this possibility, you should have your house tested for radon.

Also, you might unplug the air cleaner and let it discharge any potential electrical storage circuits and then survey again. If you still get a reading, borrow a different instrument, and see if you get a similar result. If you do, I have to wonder if the ion generator needle might have a radioactive material in the tip, for example, natural thorium. I believe thoriated tungsten would have a lower work function for electron emission (negative ion generation). If you find that the tip is radioactive, we'd be interested in knowing.

David J. Allard, CHP
Edward F. Maher, DSc, CHP

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