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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