BRAWM Team Questions Regarding Sample Collection Differences

Submitted by Anonymous on Mon, 2011-05-30 11:23

Berkeley Radiological Air and Water Monitoring Forum

This is the exact field in which the BRAWM team has expertice. So, I
hope they can give definative answers.

With different amounts of contamination being reported by different
teams in the same area on the same day, and different devices (air filter vs charcoal filter) giving different results, I'm curious how we can
consider any of the reported amounts as accurate.

1. Which type of air filter is accurate?

2. If true, why would a charcoal filter give a result that's 10x higher
than simple air filter?

3. Why is it acceptable for radiation measurement devices to give
significanly different results for the same volume of air? And why
is it acceptable to report those measurements?

I wouldn't accept a measurement device that tells me there's 10 gallons
of gas in my tank when another tells me there's 100 gallons.

4. How would the BRAWM team classify the 1.92 pCi/M3 level reported in
Anaheim, CA or the 2.42 pCi/M3 level reported in Dutch Harbor, AK? Do you
still consider those levels as trace/miniscule/tiny/insignificant?

With the BRAWM team reporting air results for I-131 at least 10x lower
than the CDPH (which is also using air filters), I don't know which to
believe or if the 10x difference
really matters.

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Accuracy of air filters

Submitted by bandstra on Fri, 2011-06-03 18:56.

I haven't addressed this yet because I think most of this was covered in the other thread: Cross-calibration is difficult. But I'll give this a shot:

(1) Both types of filters can be accurate. It depends on how the measurement is made, how the detector is calibrated, and other things. But all of these factors should go into how the final result is calculated, if it is to be accurate. This is what scientists spend all their time doing, basically...

We here in BRAWM have tried to understand our air filters, and we don't see a 10 times increase — maybe 80% with activated charcoal. But, as discussed in that other thread, we also need to perform a more thorough calibration of the system before we revise our numbers.

(2) Most air filters (such as HEPA) work by having a thick, microscopic mesh of fibers that small particles in the air get stuck in. Charcoal filters work differently. They rely on activated charcoal", which works in two ways. First, it is porous and thus can trap particles like a filter that uses fibers. Second, the carbon will chemically bind to some of the particles, allowing for the trapping of smaller particles. So there is the potential to trap more particles than with a fiber-based filter.

(3) Why can results be different? First, the actual activity in the air can be different in different places and at different times. Second, a particular instrument will have its own peculiarities, and it is our job to know all of those peculiarities as best we can. But sometimes there are systematic effects that we don't understand.

Also, if the idea is to know whether the air is safe or not, a factor of 10 difference doesn't really matter. This leads into the last question:

(4) Those measurements are still very low, for two reasons. First, using our dose conversions, I estimate that the 2.42 pCi/m^3 of I-131 (9.0E-5 Bq/L) would require 8 years of continuous breathing to equal the dose from a cross-country plane trip. This is a very low dose. Second, those are peak values, not long-term sustained numbers. So the very low dose we calculate would never be reached, not by a long shot.

Did I hit on the points you wanted addressed?

Mark [BRAWM Team Member]

Thanks, Mark!!!

Submitted by VB (not verified) on Fri, 2011-06-03 20:24.

You pretty much answered my questions.

The main thing I was curious about was the filter results accuracy. "all of these factors should go into how the final result is calculated, if it is to be accurate" is what I was looking for. So, a statement of "all charcoal filter results will be higher" is not a correct statement if the final result takes the filter differences into account.

As for the dose, I was just curious if the 10x diffence was significant enough for it to be deemed other than very or super low dose as the BRAWM team has called their measurements.

Thanks for getting back to me. I really appreciate it and all the BRAWM team is doing.