Date: Mon, 9 Jul 2007 12:29:57 -0700
Reply-To: Gordon Miller <miller22**At_Symbol_Here**LLNL.GOV>
Sender: DCHAS-L Discussion List <DCHAS-L**At_Symbol_Here**LIST.UVM.EDU>
From: Gordon Miller <miller22**At_Symbol_Here**LLNL.GOV>
Subject: Re: Ductless Fume Hood - need information

I would be very uncomfortable with recirculating hoods in academic 
(vice research) labs, particularly those with sorbent beds rather 
than aerosol removal filters.

All of the issues that attend gas/vapor sorbent cartridges for 
respirators apply here. I think these issues all come from answering 
the question, "When do I replace the sorbent bed?"

With respirators, NIOSH said replace it when you can smell/sense the 
contaminant coming through.  Well, what if the stuff is toxic at 
levels below sensation threshold levels. NIOSH said use an 
air-supplying respirator.

There are materials of sufficient volatility that you can't use a 
sorbent respirator for them (e.g., methylene chloride and 
chlorofluorocarbons). A recirculating hood is just a big respirator 
that constantly exhales so a recirculating hood most likely won't 
work with methylene chloride either.

OSHA now says run a breakthrough time calculation. Gary Wood, 
formerly at Los Alamos, developed a a basic single-contaminant model 
and is working on a multi-contaminant math model. I haven't seen the 
multi-contaminant model, but it likely will not be simple. The 
single-contaminant model is on the OSHA Web-site. The models have to 
consider quantity of sorbent, the properties of the contaminant(s). 
relative humidity, the concentration of contaminant entering the 
sorbent bed, the maximum allowable concentration of contaminant 
leaving the sorbent bed, and work rate (source term).

For you to be able to use these big respirators called recirculating 
hoods you'll need known and controlled chemicals that the sorbent is 
known to work against and either be able to ensure exposure levels 
will be unsensible while safe or be able to do the math modeling.

Another related issue is, "What happens to the sorbed contaminants?" 
Some stick to the sorbent, some don't. Those that sorb poorly can 
desorb and migrate through the bed and finally emerge.

Are you ready to do this for the big respirator that Mr. and Mrs. 
Smith count on you to use to protect their Bobby or Bobbie while 
doing some experiment while taking Chem 101?

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