Skip to content

Industrial ventilation: Silica

Presented by Dr David Bromwich

Industrial ventilation is one of the major engineering interventions to minimise toxic exposures to dusts and ranks part the way down the hierarchy of controls but must be considered before other approaches like dust masks are contemplated.

In this presentation, recorded at the Healthy Lungs Forum in November 2019, David explores industrial ventilation approaches specific to silica dust.

Note: From 1 July 2020 in Queensland the new national workplace exposure standard for respirable crystalline silica was revised from a time weighted average of 0.1 milligrams per cubic metre (mg/m3) down to 0.05mg/m3. This means that, from 1 July 2020, the reference to the workplace exposure standard for respirable crystalline silica in this presentation should be understood as a reference to the new standard of 0.05mg/m3.

About David

Dr David Bromwich is an occupational hygiene consultant with 40 years’ experience.

He is a Certified Occupational Hygienist (COH) and Fellow of the Australian Institute of Occupational Hygienists (FAIOH). He has an MSc from the London School of Hygiene and Tropical Medicine where he specialised in industrial ventilation, and an MSc in Medical Physics from QIT.

As Senior Health Physicist and Mines Inspector in the Mines Department in the Northern Territory, David was heavily involved with radiation protection in the uranium mines. He moved to occupational hygiene where silica exposure was a major issue in hard rock mining and quarries. His interest was in industrial ventilation and he has evaluated and designed many industrial ventilation systems.

David recently rewrote the chapter on industrial ventilation in an occupational hygiene text produced by the AIOH and is currently an adjunct Associate Professor in a research group attached to the School of Medicine at Griffith University. During his time with Griffith University, he has set up a comprehensive Industrial Ventilation Laboratory for teaching and research for undergraduate and graduate engineers.

Run time: 30 minutes 56 seconds

View presentation slides (PDF, 1.21 MB)

Download a copy of this film (ZIP/MP4, 1GB)

Dr David Bromwich: Got some notes, but I'd rather not use them. Okay. I've been messing with industrial ventilation for over 30 years. I really started in the mining industry, as a mines inspector and I found that a lot of mines inspectors and mining engineers knew very little about local exhaust ventilation to be able to suck away dust before people breathed it in.

Okay, this. Go on. All right. Some pictures there. Now, that's my world of industrial ventilation. Well, the center top one is my laboratory at Griffith University and at the back of the laboratory, there's a glass wind tunnel. So, it's a bit hard to see because it's made of glass, but the bottom left hand corner, you can see a student using my favorite tool, which is a smoke tube to visualize air flows. Middle bottom, you can see all the students with chalk gathered around, and they're actually looking at the pattern of airflow into a slot hood and working out how fast the air flows, the pattern of airflow and where the capture zone is. So, that blue line that's in there would be the outside of it.

I would point out a book that's being launched next week in Perth at the Australian Institute of Occupational Hygiene Conference, Principles of Occupational Hygiene, third edition. I did the chapter on industrial ventilation.

Okay. We've looked at various industries. I'm going to be mainly concentrating on stone countertop fabrication. This graph here gives you a bit of an idea of the risk. One of the first things you have to do when you're designing industrial ventilation systems is to look at what the risk is, what the materials are, how people are doing the work so that you can design a system that won't just be thrown off into the long grass. On the left hand axis, the vertical axis, that's the risk as a percentage, the lifetime risk of silicosis. The horizontal axis is the respirable crystalline silica concentration. We can see here, that's the Australian limit at the moment. It looks like it's going to be coming down and Australia 1993 was up here, but here, we are present limit. It's about two and a half percent chance of getting silicosis if you're exposed to that sort of level, day in, day out for working lifetime.

Right. The topics I'm going to look at is the historical context. I'll do a bit more on the toxicology, particularly the dust size, how industrial ventilation fits into the hierarchy of control, and this is the important thing. The industrial ventilation concepts, what I want to do is empower you by giving you the concepts that you can understand, take it back to the workplace and then apply it, right? So, I won't be giving so much in a way specific here's how to do it, but to give you the overall information. So if you're a mechanical engineer, once you've got that, you can do the work properly. I look at also at the effectiveness of dust suppression with water and also applying industrial ventilation principles to respiratory protection.

Right. Go back to the 1930s, right? Silicosis was a big deal then. It was a much bigger thing than asbestos ever was, right? One of the earliest speakers said was silica is the new asbestos. No, no, no. It's the other way around. Silicosis was a massive thing in the 1930s. People were getting silicosis and it was only called silicosis when they got too ill to work.

Hawks Nest tunnel 1927, 3,000 workers and 1,000 of them died, right? That was Union Carbide, the company that bought you Bhopal in India. Sydney Harbor Bridge, there's a plaque commemorating the 16 people who died in the building of the bridge. Not mentioned was a vastly greater number of people who were stonemasons who got silicosis chiseling the big granite blocks. A good read is this one here called Deadly Dust. That will give you a lot of information of how things were and how workers' compensation in America was there to hit off the big claims against the foundries, because they're going to go bust. So, it wasn't a pro worker thing, it was really to limit what workers got paid out.

Right. This document's freely available on the internet. It's a 1930 conference in Johannesburg. Australia was well represented. There were four representatives from Australia at this conference, 1930. Here's a bit what they said about the relative value of the use of water and ventilation in the prevention of silicosis. They said the finest dust would pass through any form of water. In other words, just spraying lots of water at the problem was not sufficient. Ventilation is never 100% efficient nor are water sprays in controlling the very fine toxic particles.

Right. Let's look at the hierarchy of control for silica. The ideal thing is elimination. Get rid of the silica. We've done that largely with sandblasting, right? We got rid of sand for sandblasting. We use garnet. You might call this substitution, isolation, control cabins. We had a good presentation on that. Engineering controls and within that, I put the new technology like water jet cutting. Now, I haven't been to find any good papers to saying what sort of levels of silica dust that you're getting with the water jet cutting, but it can't be done pretty well and totally underwater, right? But because you got the jet going through the air, it's not going to be a, you're still going to have problems with air and the dust will escape into the air, but I would expect it to be a small amount. Water sprays and we'll be coming back to that in some detail, industrial ventilation, which is the main part of this topic.

Administrative controls. I've done quite a bit looking with I think [inaudible 00:07:54]. So, I did quite a bit of work in Sweden, and that's where you take video of the person, you monitor the levels and you look at how people performing a task or fix their exposure. Lastly, I'm going to be looking at personal protective equipment and applying industrial ventilation principles to better understand the level of protection you'll get.

Right. Yet another picture here, but this is the magic number you must remember. 10 microns, 10 microns is basic, the limits. Stuff smaller than that, you'll breathe deep into your lungs. It'll be called respirable dust.

Silica dust is shown here, but it actually goes a bit finer and a bit coarser, but the 10 microns is also the limit for what you get. You can actually see with the naked eye. So, see the stuff that which is toxic, the stuff that will get deep into lung is the stuff you can't see. Also, it's not going to be the stuff that lands in your nose and throat. So, it's not going to be unpleasant to breathe it in.

Here's a picture of a human hair and these little dots, the stuff about 10 microns, and the red ones across the little blue dots are the 0.25 micron. Right? So, it's very, very small. If you can focus on a human hair, you just imagine a whole dots across it. If it was asbestos fibers, you get about 10 asbestos fibers or more across the width of a human hair. The stuff that's going to kill you is the stuff you can't see.

Right. Particle size. There's your nose. There's your mouth. That's the head and the stuff that's going to get deep into your lungs is going to be the stuff 10 microns or below. The very, very small stuff will actually deposit by process called diffusion. So, this thing here is going to be the stuff where it gets past your nose and mouth. The second one here is the upper airways in orange, and then the very, very fine stuff, which is going to be causing the silicosis and get into the alveoli. Right. That's all going to be less than 10 microns or go down to about a thousandth of that.

Now, one of the things about the dust is the freshly fractured silica is quite toxic. Freshly fractured silica has free radicals on the surface. After about 30 days in air or 20 days in water, you'll get about half that level and it continues to decrease. That's why you don't get silicosis going to the beach.

Right. Let's look at industrial ventilation concepts. The most important one is the difference between sucking and blowing air. Now, I'd like you to all do an experiment with me. Hold up your finger as though it's a birthday candle, right? Now, I want you to blow it out. How far away from your mouth does it have to be? Can you blow it out at an arm's length? Try blowing. You might feel a breeze on your finger. Okay. Now, we'll turn you into industrial ventilation engineer. How close does it have to be to suck it out? Bit closer? How close? Okay.

One of my students went home and their kid tried it with the birthday candle and they burned their nose. It's got to be millimeters away from your lips, right? It has to be really, really close. There's a huge difference between sucking and blowing.

Right. I was really lucky. I was in UK with one of my sabbaticals and I went to health and safety executive in ... they were in Sheffield at the time. They gave me the keys to the archives. I spent three days photographing pages and looking through their archives from 110 years ago and more, 120 in some case. They really knew industrial ventilation. It was a big thing there. Their design was good. You'll see similar pictures to these in their archives.

When you blow air about 30 diameters away, it'll be down to about 10%. But when you suck air in one diameter away, it will be down to 10%. So, if it's not incredibly close, suction doesn't work, right? A lot of engineers, mechanical engineers designing ventilation systems come from air conditioning background. Guess what? They're blowing air. Now, they're into this thing trying to suck air and they get it completely wrong, right? But simple concepts like this, the difference between sucking and blowing, once you understand that, then you can apply your mechanical engineering degree and you can start to do good design.

Right. Let's imagine a black hole, right? There it is. It's sucking air from all directions. Right. Let's enlarge the hole. Let's join a pipe up to it, right? This looks like industrial ventilation system. The capture zone, it'll be from all directions from behind the pipe and in front of it, right, is not directional. Unfortunately, often the contaminant escapes because it's not close enough to the suction. You can shape the air flow. Let's go with problems. Often it gets in the way, if you start putting flanges on, but here's some hoods without flanges and here's hoods with flanges. So, you put a flange on, you stop all that air coming behind. If it's not coming from behind, it'll actually come from further out in front and you're going to be capturing more of the contaminated air.

Right. Here's some cartoons I gathered. Sucking, right? Looks nothing like that. There's nothing covering behind. This person here being sucked in the vacuum cleaner. You could do some things on the tensile strength of attachment of the things and work out what a suction would be needed. Here's another one here. Even on new scientist, they're showing us directional suction. That doesn't happen in the real world.

Right. The second concept is that of the air flowing from the face, right? Imagine that's the contaminated air. Here's your face. The obvious direction is from your face through the middle of that contaminated air and that way, but a lot of ventilation systems try and make the air flow in almost the opposite direction, right? So in green, that's what you like and red, that's what the design's going to do. Well, I said, "Go that way and you went that way." You say you're not listening to me, but this is typical of the designs you get in the workplace, right? Simple concept, but these simple concepts build to appear to look at industrial ventilation systems and see bad design.

Right. Extraction close to the source. Those pictures, the cartoons showed someone's flesh being ripped off by a vacuum cleaner. You had to be very close to the source. If you're vacuuming your car, you know that it's really difficult to get all that dust and dirt out of the carpets. You have to be right up close with the nozzle. Holding a nozzle half an inch away, nothing much happens.

Right. This is a person doing wet grinding of a stone bench top. They're using oodles of water, right? You can see the water everywhere, but behind him, they've got this Nederman type hood, trying to suck the air away. Now, that hood really needs to be right up close here because a lot of the really fine dust is not going to be stopped by that water spray. Instead, it's over here, it's doing nothing.

When you're designing industrial ventilation systems, 90% of the design is with the hood. Getting the hood right so you're actually capturing the dust. If the capture is ineffective, then the person's going to be breathing more toxic air, you're going to need a much larger vacuum, a much bigger fan, much bigger initial cost and a much bigger running cost. It's also going to be very noisy. It's not uncommon to see a system turned off because it's too noisy. Many systems are very badly designed even on tools.

Right. There's some research done by the health and safety executive in England. They looked at on tool controls. Their estimate was it reduces exposure by 90%. Now, is 90% enough? Because the tools will never completely eliminate exposures.

Right. One of the things you can do is change the particle size. This is where we're talking about the water sprays, but go back to what we saw from the 1930s. It didn't matter how much water you use, you never going to get complete capture of the very fine toxic silica particles.

Right. This person here, doing it dry, doing it wet. I think I can see a little bit of a haze of the very fine stuff, but you're not going to be seeing that very fine toxic stuff because it's not visible to the human eye.

Traditional water suppression is a bit like gluing peas together with basketball sized lumps of water. Right? Now, as Martin said in the last talk, the physics is totally different. These particles of silica can move between the water. Huge amount of water there and it's just going to make the whole process wet. Right? Good dust suppression, it should be fairly dry. Right?

Now, that doesn't look particularly optimistic, right? Here, you got your little blue water droplets. Now, the problem with very fine droplets is they tend to evaporate quickly and the nozzles will tend to clog. You see your silica particles. Right. Down here, you've got two silica particles being glued together by a tiny droplet. Right? Any more than that is you feel like a waste, but that will actually be a fairly dry operation. It'll be a lot more effective than if you can put the same amount of water or even a tiny fraction of that water into very fine droplets, you'll be able to glue a lot of the small particles together and make them non-respirable or even drop out of the air.

Okay. This is from a paper published in the Annals of Occupational Hygiene. Now the Annals of Occupational Hygiene is the best source of published information on dust with occupational hygiene. Here we're looking at respirable silica dust suppression during artificial stone countertop cutting. Sounds good, doesn't it? But do it dry, we're getting about 44.4 mg per cubic meter, which is a lot higher than a 0.1 mg per cubic meter. We do it wet, we're down to 4.9, still too high. Using a water curtain as well, right? There's the machine they're using, a saw, and they're supplying water and that's the extraction ventilation, right? It got wheels and this is doing stone benchtops.

Now, just get the right button. That's where the extra water as a curtain, but if you actually went to wet plus the local exhaust ventilation, you're down to 0.6. now, the duty cycle of the cutting and not cutting probably would be so that you're actually down below the exposure limit, but if it was like that all day, you're going to use wet cutting local exhaust ventilation, LEV, plus respiratory protection to give you adequate protection. Right? That can actually be freely downloaded. The slides, will they be made available? Yeah?

Right. Had to mention tunnelling. Right? We've had all these tunnels built under Brisbane. Probably not as bad as Sydney where it's pure sandstone. This is taken from the tunnelling company's website for Legacy Way. There's the huge machine. They're really impressive bits of engineering, about 100 meters long and these huge cutters.

What's really weird is the ventilation is actually backed up front. They supply fresh air from outside, and then the tunnel itself is used. They filter the air, but the filtration is not complete. You can tell that when you're in the tunnel, when the machine's going, because there's a haze along the length of the tunnel. That haze are those invisible respirable silica dust particles. Along the tunnel, you're getting people, this isn't a Legacy Way one, but they're digging tunnel between the two tunnels. Right. There's the north tunnel and the south tunnel and between the two, in case of emergency, they have all these things. Plus they've got people going up and down the tunnel all the time in vehicles.

Another really weird thing is that we have an exposure standard for silica. We have an exposure standard for diesel particulates. They both cause lung cancer, but you could be 90% of the exposure limit for both of them and that's okay. I feel really uneasy about that. I haven't been able to find any research on any synergism between the two, but if asked to put money on it, I would think there would be some synergism.

Enough on that. I thought I had to mention it. Back to front. It should be sucking air from the workplace and the tunnel should have then the fresh air coming in, so that all the people in the tunnel are protected and you'd filter the air before releasing it to the environment.

Right. Applying it to respiratory protection. Right. A bit hard to understand this picture, but that's the person's eyes and there's their nose and there's their mouth. When you breathe in, the air moves over the surface of your face. When you breathe out, it goes in a nice plume from your nose and mouth, which is really good because it means you're not sucking in the same air, right? Breathe out, it goes here. Breathe in, it moves over the surface of your face. Same concepts as we had at the beginning with a difference between sucking and blowing applies to breathing.

Right. If you got a face mask on, there's going to be resistance to air flow going in. So, guess what? More air is going to try and flow over the surface of the face. You're trying to protect yourself by having a filter that's going to stop the dust and having a good seal to the face. It's extraordinarily difficult to get a good seal. I've tested thousands of people and where to get a protection factor anywhere near 10, right? It tends not to happen [inaudible 00:27:14] under laboratory conditions. If you got a beard, the level of protection is going to be very low.

Right. Let's do some calculations. Facial hair grows at something like about half an inch or about 1.3 centimeters a month. At the end of an eight hour shift, it's got about 150 microns. Respirable dust is 10 microns. So, the picture of a power line used as a fence to keep mice in to me is fairly appealing, right? It's very difficult to get a good seal after a day's facial growth.

This person's appeared on the ABC news site. Here's what's wrong. Okay. Start off. Hearing protection is sitting on the outside. Got a noisy party next door or in the next room, the instant you crack the door open a tiny amount, you're going to hear the noise. Close the door, that few millimeters, you don't hear the noise or not as much. The hearing protection sitting on top, it almost is totally ineffective. Now, this person is a stone bench top worker and you can see the dust, right? He obviously doing dry cutting. You can also see a small amount of dust around here, and it looks fairly clear, but because of this facial hair, you're not getting a good seal and that will be fairly ineffective. So, none of the protective equipment he would have been wearing would have done much of a good job.

Now, these powered respirators, they seem to have various names. When they first bought them out, they didn't have this skirt. When they did put the skirt on, that made them work really well, because what happens is the air moves up and down in this zone here. That reduces the effectiveness by probably a factor of 10, but this is uncomfortable. If you remove it, you'll greatly reduce the effectiveness just about there, but I like those a lot better than the half face respirators, because you can still have facial growth. You can have a bit of a bed and they'll give a higher level of protection than a half face respirator.

Right. So, the overall thing is try to agglomerate the dust with a fine water spray, remove it with a good extraction system. If you do use respiratory protection, you really need to go to quantitative fit testing and a professional respiratory protection program. Right? If you just give a person a respirator, it's unlikely that they'll get very good protection. Thank you.