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Author Topic: Respirable silica dust when grinding wet  (Read 249 times)

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R.U. Sirius

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Respirable silica dust when grinding wet
« on: April 10, 2022, 09:06:09 AM »

We are hopefully all well aware of dangers of silicosis. When it comes to mitigating the risk, I found many examples of people claiming with confidence that "if you grind wet, there is no danger". Few people also wear a respirator, and almost nobody adds a proper ventilation/filtration system.

There are several ways for you to check whether your lapidary setup generates invisible, respirable dust of silica particles. Most crudely, you could try wiping a smooth surface some distance away from your machine with a wet, clean tissue and see what you picked up. This barely tells you anything useful, though, as actually dangerous particles are so small that they do not settle down like larger, visible particles do. They float in and with the air. They travel deep into the lungs, and stay there. But if there is a coating of dry dust on your work area floor and bench tops, it's a reasonable assumption that smaller particles are there, too.

A bit more involved is the use of a "dust lamp" - a light source producing a tightly focused beam of light, positioned in an otherwise dark room so the light beam goes through the expected cloud of otherwise invisible particles while someone is actually grinding. If you (or a camera) then observe the beam just slightly off its direction (i.e., almost having it shine into your eyes, or camera, but not quite), you will be able to observe otherwise invisible fine dust and mist. This is due to the Tyndall effect - light tends to scatter off of small particles at shallow angles.

I finally decided to buy a quality particle counter from Sensirion, as I am in the process of setting up my lapidary space at home, and want to see how well my risk mitigation measures work. Yesterday, I took the sensor to my local club shop, and am sharing the results here hoping that this will raise the awareness and bust some dangerous myths.

There were two or three people grinding or trimming rocks, with plenty of fresh water. There is no active ventilation in the room, beyond natural draft through the door and small openings in the ceiling of this commercial building space.

The PM10 and PM2.5 (particles smaller than 10 and 2.5 micrometers, respectively) counts started at ~10 μg/m3, and promptly elevated into the 50-100 μg/m3 region as we started working. For reference, my living room reads usually less than 1 μg/m3. Canadian occupational safety limits for respirable silica as an 8-hour average are presently set at 25 μg/m3. The U.S. is stuck at 50, I believe.

The club shop actually looks clean. We thoroughly clean the machines after each session, and there is very little dry crud in the trays, on the wheels, and on bench tops.

This clearly shows that "if you grind wet, you've got nothing to worry about" is a dangerous myth. The mechanics of dust generation and inhalation, and the biological effects, are complex and nuanced, and there is no silver bullet that will make our lapidary activities perfectly safe. There is a lot you can do, however, to know and to minimize the risk.

For those willing to listen and learn, there are plenty of reputable sources available online - studies showing how the dust spreads, how mouth breathing makes it more dangerous than nose breathing, how humidity and temperature affect the behavior of mist droplets carrying silica, how various mitigation methods work or don't work - from water to ventilation to respirators to cleaning up. I am sure many here will also appreciate the fact that wheel speed and the design of arbor hood and water delivery system all play a role, as do our habits of positioning our faces this or that way when grinding and polishing.

Please don't trust blindly that whatever measures you took to make yourself and those around you safe actually work as intended (or, even worse, as merely hoped).
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irockhound

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Re: Respirable silica dust when grinding wet
« Reply #1 on: April 10, 2022, 01:02:20 PM »

Thanks for the eyes wide open to hidden dangers in our hobby.
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Ryaly2dogs

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Re: Respirable silica dust when grinding wet
« Reply #2 on: April 13, 2022, 10:33:39 AM »

I echo that note of appreciation.  I think I might now be moving my wetted grinding wheels into a covered outdoor environment.  Probably like most garages, ventilation in my garage is limited to opening the garage door, and I would imagine respiratory dust size particles are a thing to contend with in that scenario such as you have laid out for us.
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R.U. Sirius

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Re: Respirable silica dust when grinding wet
« Reply #3 on: April 13, 2022, 11:02:34 PM »

Just to be clear - when it comes to my home setup, I don't have any mixed feelings at this point. I am putting some effort into really understanding the risk, and verifying that whatever measures I take actually work.

I will share updates as I finish refurbishing the equipment and start grinding and polishing, with the particle counter running!
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AgateLicker

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Re: Respirable silica dust when grinding wet
« Reply #4 on: April 16, 2022, 10:29:59 AM »

As a budding stone worker with a child in the home any foundation for health based on critical analysis is overly welcomed. Thank you for shining light (har har) on this lurking hazard.
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peruano

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Re: Respirable silica dust when grinding wet
« Reply #5 on: April 17, 2022, 05:36:28 AM »

A few months before the covid pandemic started, I bought a case of N-95 masks at a bargain rate at the local Habitat for Humanity Restore   It was like $20 for a couple of hundred masks that were past their expiration date, but hey, my wife had been bugging me to wear a mask in the shop when grinding and slabbing and these look)ed like more substantial masks than what most folks use in the wood shops or work locations.  The mask number was TC 84A -4276.  Only once I arrived home did I do my research and find that this mask was not approved for silica or oil mists.

Fast forward a few months and our local hospitals were running short of N95 masks and actually re-sanitizing masks for their staff.  A friends SIL was a supervisory nurse lamenting the shortage, and consequently received nearly 200 or so of my masks for the hospital front liners.  They were grateful. 

So the message is: make sure you buy the correct protective gear if you are going to use it.  Me I still don't wear a mask except when cutting really nasty stuff (anything I can taste or smell when I'm cutting it).  Some flints, lapis, and other common stones fit my definition of those warranting special masking. 

Contrast that with the man who came to my house to cut a driveway entrance in an existing curb.  He was on a big tractor with a 30" or so blade on a lateral hydraulic arm which produced a plume of rock dust that engulfed the entire tractor and surrounding 6'.  He pulled his tee shirt over his mouth and nose as his protection.  He does this everyday for a living.  I should have given him one of my masks (not the right one, but far better than what he was using). 
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