If the fumes/vapors of the MC are reaching other rooms, the ventilation system is not working correctly. The general rule is that the lab is to be under negative pressure to the hallways and other exits with the exhaust - which includes hoods, snorkels, or other exhaust means - taking all the air and exhausting it to the outside with no subsequent entrainment back into the building. The lab I was associated with, that meant that some rooms had as much as 15 to 21 air exchanges per hour. This is far above the air cleaning necessary to vent all the vapors and keep the air clean, but may be a necessary evil.
Remember that some of the GC's also use MC as a carrier, so it has to be added to the equation which can get very expensive for the school or industry.
Ken Smith, ex CIH now retired
Thanks for the reply and I am sorry for the cryptic comment! I meant that according to the OChem professors there aren't enough fume hoods (5) for the number of students (up to 24) to perform the extraction in a timely fashion. So to prevent students from lining up and waiting for a fume hood, they are doing it at the bench. Sigh.....
How good is the ventilation system in the lab and fume hoods? Our fume hoods seem to be doing their job and containing fumes, but the general lab ventilation is possibly off as fumes from the OChem lab spread into adjacent rooms and labs. Whether this spreading of fumes is due to poor handling practices or poor lab ventilation, I don't know. We don't have an EH&S dept but our facilities people claim that the general lab ventilation is working up to specs. But I'm not sure if they are correct.
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------ Original Message ------
From: Ralph Stuart
Sent: November 18, 2014 at 3:48 AM
Subject: Re: [DCHAS-L] Benchtop Methylene Chloride Use in Undergraduate OChem labs
>As many OChem labs lack adequate fume hoods, many colleges do these extractions at the bench,
My first response doesn't answer the overall question directly, but is about a question that I've been wondering about. What do chemists mean when they say "adequate fume hoods". Is it a question of the number available for the number of work stations desired? Or is it a question of fume hood performance?
One reason I ask is that I've seen peer reviewed papers that direct someone trying to replicate the procedure in an "efficient fume hood" for safety reasons. I'm not clear what that means; from a safety point of view, to fulfill their safety function, fume hoods should capture fugitive chemical vapors released and efficiency isn't part of the safety picture. I'd like to understand the chemists' perspective on fume hoods better.
To more directly answer the question, the industrial hygiene approach to the overall question is that measuring the concentration of methylene chloride in the air during the work and comparing it to an appropriate exposure limit (take your pick from
https://www.osha.gov/dts/chemicalsampling/data/CH_253450.html ) should help determine whether the lab facility is up to work being conducted in it. When you look at that link, you'll see why that is a trick answer...
Ralph Stuart, CIH, CCHO
Chemical Hygiene Officer
Keene State College
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