re: Hoods for Iodination

["rad tech" <Rad=Tech%SS%DFCI@EYE.DFCI.HARVARD.EDU>]

Based upon my experience as RSO and also because my M.S. thesis was 
entitled "The Behavior of Iodine-125 in a Fume Hood", I'd like to offer some 
advice on iodination hoods.

Thanks to the availability of pharmaceutical products already iodinated by 
the manufacturer, and thanks to alternative, non-radioactive methods, the use 
of volatile iodine compounds for research is going down markedly each year
You should take the hoods' usage requirements into consideration before you 
go all-out on an expensive system for filtering iodine.  Here's what I 
recommend:  

First and foremost, I recommend a stainless steel hood.  Many of them come 
equipped with alarms when there is fan failure.  Stainless steel is 
about the best practical material for resisting plateout of the radioiodine.  
If you look in any reputable catalog for fume hoods, you will find the 
stainless steel hoods.  I can find the names of a few manufacturers if you 
ask.  The stainless hoods are more money, but well worth it because 
decontaminating is a breeze!

The ductwork does not need to be stainless, but keep in mind that while 
iodine fumes or gases do not readily adhere to stainless, it will gladly 
plate on galvanized steel.  That could cause problems years from now when you 
have to decommission the building and you've got hot ductwork.  With that in 
mind, I'd still opt for the much cheaper galvanized steel ductwork.

As for filters, they are recommended for keeping releases ALARA.  I recommend 
them only if you are expecting to approach any regulatory levels for effluent 
releases.  The filter should be TEDA (tri-ethylene diamine) impregnated 
activated charcoal, 3 inches thick or more.  With a TEDA charcoal filter, 
your releases to the atmosphere should be very low.

In my opinion, a HEPA filter is unnecessary for collecting radioiodine.  
We've never used them in our iodination hoods, and never had any problems 
with effluent releases as a result.

The nice thing about TEDA charcoal is that it loves to collect iodine.  If 
you are giving the filter radioactive iodine, eventually the iodine decays 
into another element (such as Tellurium-125 in the case of I-125) and because 
TEDA doesn't want Te-125, it lets go of it.  With that space now available 
for more iodine, in a way, the filter regenerates itself and can last years.  

The problem with TEDA, however, is that it also loves organic compounds and  
in numerous cases, not only is a reasercher releasing iodine, but also 
releasing those organics.  High humidity levels can influence this as well. 
Eventually, the organics and the humidity will use all the available spots 
meant for iodine.  And that is when you will begin to release iodine to the 
atmosphere, and that is when you will have to replace the charcoal.  

The only testing we do relies on monitoring the effluent to determine when 
our charcoal filters need to be replaced.  We conduct continuous air sampling 
for each month, and our monthly releases are routinely very low.  As soon as 
we detect any unusual amount of iodine in our effluent, we immediately 
suspect it may be time to change the filter.  As long as the increase does 
not put us near any regulatory levels, we continue using the filter for 
another month to verify if indeed the filter is beginning to lose 
its effectiveness.  Repeatedly higher monitoring results indicate it is time 
to change the filter.  We try to keep a supply of TEDA charcoal on hand for 
those changes.

Our methods are far from complicated, and in practice, the system seems to 
work well.  If you need more information, call or email.
 
*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
*  Steven J. Alford, M.S.       |  Dana-Farber Cancer Institute *
*  Radiation Safety Officer     |  44 Binney St.                *
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