From: Monona Rossol <actsnyc**At_Symbol_Here**>
Subject: Re: [DCHAS-L] Gas Leak Training Input
Date: Sat, 26 Dec 2015 09:50:15 -0500
Reply-To: DCHAS-L <DCHAS-L**At_Symbol_Here**MED.CORNELL.EDU>
Message-ID: 151dec4c17b-5259-7836**At_Symbol_Here**
In-Reply-To <34163510-0A62-45CF-808B-7C1438EBE4CA**At_Symbol_Here**>

This is the most difficult thing I deal with in building planning.  Both chemists and HVAC engineers have this mistaken concept.  And I stress it in my classes as well. 

Unless we are talking radon, which is so heavy and charged that it attaches to dust particles and settles, vapors in the air behave as if they were weightless.  Once they have been mixed with air, they are molecules bumping into other molecules and being dispersed in all directions from their source.  The larger vapor molecules may move more slowly than smaller gas molecules, but they will all eventually disperse evenly throughout the room just as any gas will.

And most important:  If after dispersal the air currents are eliminated, those suspended vapor molecules will NOT resettle.  

It is only when the air is absolutely still, such in that store room used in those fire department videos, will the gasoline vapors slide down the sides of the container and along the floor to the pilot light.  That just doesn't happen if there are people in the room or there is a ventilation system. For an explosion to occur with people in the room, the concentration near the pilot light must reach the LEL instead.  And that actually, makes a bigger boom than the fire started by the vapor trail.

I have e-mailed the person responsible for that mercury vapor video and am hoping to add it to my course materials.   It will save me a million words.

Monona Rossol, M.S., M.F.A., Industrial Hygienist
President:  Arts, Crafts & Theater Safety, Inc.
Safety Officer: Local USA829, IATSE
181 Thompson St., #23
New York, NY 10012     212-777-0062


-----Original Message-----
From: Ralph Stuart <rstuartcih**At_Symbol_Here**ME.COM>
Sent: Sat, Dec 26, 2015 8:42 am
Subject: Re: [DCHAS-L] Gas Leak Training Input

> It also makes the point I have trouble getting people to understand--that heavier than air does not mean put the ventilation exhaust at the floor! Once airborne and mixed well with air, vapors will disperse and will not resettle quickly. It's why heavy chlorinated solvents like the Freons end up in the stratosphere.
This is an important point. I heard a recent discussion with a PhD chemist who intuitively assumed that acetone and chloroform would move differently in the lab atmosphere. After all, he said, acetone is lighter than air (it isn't - it's vapor density is around 2) and chloroform isn't. My guess is that this sort of ad hoc reasoning is common among chemists who haven't had the occasion to think through macro properties of the chemicals they work with.

- Ralph

Ralph Stuart, CIH

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