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Previous by Date: Subject: [DCHAS-L] Guidelines for Changing Compressed Cylinders NFPA Health/Flammable Rating of 3 or 4 Date: Thursday, April 21, 2016 7:13:21 AM Author: Yamaira Gonzalez <gonzalyi**At_Symbol_Here**YAHOO.COM>

Next by Date: Subject: Re: [DCHAS-L] Standard Chemical Fume Hood Face Velocities Date: Thu, 21 Apr 2016 13:45:52 +0000 Author: Michael D. Cimis

>existing general ventilation / air exchange in the room, plus size of room

This is an important point for ventilation questions in general and relates to the question about fume hood containment that was discussed recently as well. As part of industrial hygiene student project work here, we have been looking at ventilation effectiveness in rooms driven by local exhaust ventilation that aren't fume hoods. The results reinforced the importance of room size and shape in terms of ventilation rates within the room and the impact of position of the exhaust point relative to the source in terms of capture. My experience is that often design engineers don't have enough information to design ventilation systems for emerging technologies, so this factor can be overlooked in designing MakerSpaces and other labs.

This observation is among my reasons for my reluctance to generally rely on tracer gas capture as the acceptance criteria for fume hoods, as is suggested by some lab ventilation experts. These reasons include:
1. There are situations (e.g. teaching organic labs) where general ventilation rates in the lab are close to 40 air changes per hour, so small amounts of tracer gas escape from a hood is insignificant, particularly for the most commonly used solvents in teaching labs;
2. The user behavior assumptions of tracer gas protocols are routinely violated when they are in operation;
3. Tracer gas test results for hoods that have been installed often don't provide actionable information - the failures result from placement of the hood relative to the room layout, but there are no alternative locations for the hood.

So while tracer gas tests are interesting pieces of information, they are only part of the picture in assessing whether the ventilation system is appropriate for what it's being asked to do.

- Ralph

Ralph Stuart, CIH, CCHO
Chemical Hygiene Officer
Keene State College

ralph.stuart**At_Symbol_Here**keene.edu

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