Answer to Question #154 Submitted to "Ask the Experts"

Question Number 1:

It's not clear what is meant by this question. 2' x 2' refers to the outer dimensions of the HEPA filter housing. The outer 0.75" - 1" is covered by a flange. As such, the cross sectional area would be 4 sq. feet minus the flange area which would be approximately 2/3 sq. ft.

However, the filter itself is folded over, which means that the actual area of the filter itself in two dimensions is much larger. Since each fold is about 5/16th of an inch apart, the number of folds would be 22" (24" - 2" for the flanges) divided by 5/16. The depth of each fold depends on the depth of the filter which varies—typical housing thicknesses are 11.5" and 5 7/8" but the filter fold would be about 1 inch less because the fold doesn't extend all the way to the edge of the housing.

So, the approximate area of a typical 2' x 2' x 11.5" HEPA filter, flattened out, would be about 56 sq. feet. If the filter is 5 7/8 inches thick, the area would be about 32 sq. feet. These are ballpark values.

Question Number 2:

I've never done this kind of a measurement, but the most likely way it would be done is to make an exposure rate measurement at, say, 1 foot and relate this to the activity. There are several ways that might be done. If, the radionuclides are pure beta or alpha emitters, you could make several measurements on contact with a beta or alpha probe. This would require allowing the radon daughters to decay and the geometry problem would have to be accounted for. I doubt anyone would take sample of the filter for analysis but you never know. Gamma spectral analysis might also be performed if the radionuclide mix were uncertain.

Paul Frame, PhD, CHP