Silicosis - What it is and how to avoid it
Silicosis is a lung disease caused by inhalation of fine silica dust. Pottery is one of the industries that has a high risk of silicosis, as silica is in our clays and glazes. It’s a very difficult thing to know how much of a risk it is, with debates online often split between people who aren’t concerned at all and others whose enjoyment of the process is marred by the fear of silicosis.
What is it?
Wikipedia describes it as
“a form of occupational lung disease caused by inhalation of crystalline silica dust. It is marked by inflammation and scarring in the form of nodular lesions in the upper lobes of the lungs. It is a type of pneumoconiosis. Silicosis (particularly the acute form) is characterized by shortness of breath, cough, fever, and cyanosis (bluish skin). It may often be misdiagnosed as pulmonary edema (fluid in the lungs), pneumonia, or tuberculosis.”
Essentially, it’s where silica dust is inhaled deep into the lungs. The lungs are unable to remove the dust and the crystalline structure means there is damage to the tissue, which results in the scarring and inflammation. It is progressive and incurable (so it never gets better, it only gets worse over time).
None of these are reassuring words. It’s easy to read the description of silicosis and consider giving up ceramics altogether, but there are ways to keep exposure to a very low level.
The first thing to consider is that it’s dose dependant, with an element of luck. A comparison could be made to exposure to UV rays, where chronic exposure increases the rate of things like skin cancer, but with a degree of variation. So some people will be unlucky and suffer from a relatively small dose and others will avoid serious consequences from a much larger dose.
When looking at industrial rates of silicosis it’s worth bearing in mind just how little effort can go into dust reduction, either historically or in countries with more relaxed health and safety standards. Often they would be working in settings with dust piled up on the floor and a haze in the air. The finest silica particles can stay airborne for a very long time before settling, and are easily disturbed after they’ve settled.
It’s really hard to know what that means for smaller studios though. Some people take that as a sign that silicosis isn’t worth worrying about, given that a well maintained home/community studio should have a tiny fraction of the dust seen in those videos. Others aren’t interested in the comparison, only what it means for their future health, and want a more absolute answer.
Ways to Reduce Silica Dust
We’re quite fortunate in that the material we work with is very safe in its usual form (plastic clay and liquid glaze). It’s once the materials are dry that the risk of airborne dust increases.
Good studio practices can greatly reduce the amount of dust produced. These include:
Cleaning equipment, tools, and surfaces before the clay or glaze dries. Glaze spills can be wiped with a wet sponge immediately and won’t produce dust. Clay trimmings will (usually) be leather hard or softer, meaning they can be gathered up and placed into a container without creating dust, which is not the case if you let them fully dry first.
Never sand pieces dry, always add water. The water catches the fine particles and you get a paste instead of dust. As per the previous point, this should be cleaned up before it dries.
If something will produce unavoidable dust, do it outside whenever possible. This includes mixing glazes or handling/decanting powdered ingredients.
Those are some ways to limit dust production in a studio (although by no means a complete list), there are also ways to limit the impact of any dust that is produced.
Clean surfaces regularly with water. Use a damp sponge for surfaces and a mop for the floor, rather than sweeping or vacuuming (unless you have an appropriate vacuum)
Clean fabrics regularly. This could be your work clothes/apron, towels, or mats on the floor.
Be mindful of air movement. Air movement can be a good thing if it’s a gentle flow fresh air that’s replacing the studio air, but a bad thing if it’s a powerful fan kicking up dust.
Where possible, get wire shelving on wheels. These are so much easier to keep clean than immobile solid shelves as the dust build up will mainly be on the floor underneath and that can be accessed easily.
And for the situations where some dust is unavoidable, you should be wearing a tight fitting filter mask. The codes are different in the US and UK, but you want a P3 (UK) or N100/P100 (USA) rated mask. I personally find the GVS masks very comfortable (UK link / US link), but any similarly rated mask that fits you well will do. A cheap fabric mask would be better than nothing, but won’t filter out the finest particles of silica, and those are the most problematic.
Air Quality Monitor
OSHA set the limit for silica content in the workplace at 0.025 milligrams of silica per cubic metre of air (mg/m3), averaged over an 8-hour day (25µg/M3). The maximum acceptable limit in the UK is 4x higher, and Australia is 5x lower (just 5µg/M3).
Air quality can be measured more broadly by looking at the PM1.0, PM2.5, and PM10 numbers. These are the super fine particles measuring 1 micrometre, 2.5 micrometre, and 10 micrometres respectively, and they’re a problem because they can reach deep into the lungs. The UK has a target PM10 of 40µg/M3 and PM2.5 of 25µg/M3, averaged over a year.
There’s a slight disconnect between these two sets of numbers, as one talks about the composition of the particles and the other talks about the size. I can’t find any specific references that suggest how much of the silica dust we’ll produce that is the super fine PM1.0 and PM2.5 particles, but for reference the 200 mesh silica used in glazes has a maximum particle size of 75µm (link) compared to 2.5µm for the PM2.5. Some of the silica will be much larger than PM2.5, but presumably there is a range of sizes in the material and some will be as small as the PM1.0 particles.
I wanted to try and put a number on the air in my studio, so I bought an air quality monitor (this one) for £100 (similar looking one for the USA), which can measure PM1.0. PM2.5, and PM10. It has a high degree of precision (in that it can measure with 1µg/M3 resolution), but it’s not calibrated or expensive so I’m not expecting it to be that accurate at lower readings. I’ve seen a chart of how various meters (unfortunately, not the one I own) compare to a calibrated meter and they range from around 99% accurate (this one only cost $250) to basically no correlation at all (one got a score of 2%, and also cost $250). Most were 80-90% though, and price didn’t seem to be a reliable indicator, so hopefully mine is at least in that area.
I’ve been using mine to monitor my studio for a little while, and the baseline is quite sensitive to the air quality outside. It generally seems to sit at a base level of around 15µg/M3 PM2.5 but when someone nearby had a bonfire the air in the studio had 40µg. So I don’t necessarily think that the absolute number is that meaningful, I think the important thing is how much the air changes. Going from a baseline of 15µg to 40µg because you created a cloud of dust is going to be far worse than doing nothing in a studio that started at 40µg.
Silica Dust In Numbers
Mixing Glazes
Of all the common processes in the studio, this is likely to be the worst for silica dust. I found that having the meter on the bench next to where I was weighing out ingredients (inside, no airflow, wearing a mask) would show the PM2.5 go from a base of 15µg up to around 40µg, then back down to 15-20µg. This is as a tiny cloud of dust is kicked up by each action, then disperses. I weigh my ingredients out using scoops rather than tipping, as this reduces the visible dust. I’ll do further tests to put a numerical value on this.
I put my dry ingredients into a container, then close it and give it a shake to disperse the bentonite evenly throughout. I would normally go outside and pour the dry ingredients into another container which already has the water in, which prevents little pockets of dry ingredients getting stuck in the corners. For the purposes of this test, I did those stages inside.
Opening the shaken dry ingredients, even after leaving for a few minutes, cause the meter to spike into the hundreds. This is likely because the fine particles won’t settle quickly and the air at the top of the container was a cloud of dust. It dispersed quickly, but it’s worth remembering that just because the meter reading has dropped back to near baseline doesn’t mean there’s much less dust in the air, just that it’s more spread out.
Pouring the dry ingredients into the water also caused a spike, as you’d expect given how much the powders are moving. There was an unexpected finding here though, that the bubbling of the mix as the powders sink into the water causes a HUGE spike in the reading. Holding the monitor above the mix, I got it to max out at 999µg.
Conclusion - It’s best to mix glazes outdoors if you have the option, but you can keep dust to a minimum with careful movements. If you mix the dry ingredients and add them to water like I do, this is the most important part to do outside. Finally, put a lid on the mix immediately after combining the dry ingredients and water.
Cleaning
I’ve had the monitor nearby when wiping surfaces with a wet sponge and mopping the floor. There was little or no increase in the reading when wiping wet clay with a sponge, and a small increase when wiping dry clay with a wet sponge. I did a small amount of dry sweeping (probably about a square metre of floor) and the reading shot up to 300µg. I definitely wouldn’t recommend sweeping unless you’re wearing a proper filter mask and vent the entire studio afterwards. Wet cleaning really does limit how much of the dust makes it into the air.
I shook out the fragments of clay that get stuck in the Giffin Grip, once when they were freshly trimmed (+5µg) and once when they were fully dry (+30µg). I expect the minor increase on the first test was from residual fully dry trimmings rather than the fresh ones, as handling leather hard trimmings doesn’t seem to register on the meter.
Cleaning out the trimmings from the wheel when they are fresh has a similarly small impact on the air quality, whereas there will be an increase if you let them dry fully first. I got a wide range of readings for this as it depends how gently you transport them, you can move fully dry clay without producing dust but it’s hard to do with finer trimmings.
Conclusion - There is a tiny amount of dust produced from working with clay anywhere from slip to leather hard, and wet cleaning before the clay dries beyond that can keep dust to a very low level
Trimming and Sanding
There isn’t a huge amount to say with trimming, other than trimming bone dry pieces produces a lot of dust and some of that dust is the really fine PM2.5 and PM1.0 particles. Same with sanding greenware. If it’s producing dust, it’s going to be bad for the air quality.
Trimming leather hard clay, when it comes off in soft ribbons, doesn’t change the reading at all.
Sanding bisqueware and fully fired pieces can be done wet or dry (unlike greenware). I did a test with a single piece, and got a reasonable increase for sanding dry (+30µg) and a small increase with sanding wet (+3µg). I’ll have to retest when I have a larger batch of work to sand, to see if that wet sanding increase was an anomaly or a sign that even wet sanding does kick up some dust.
Conclusion - Trim before the piece is dry and wet sand if possible, wear a mask if not.
Fabrics
I shook the clothes I had been wearing in the studio all day with no meaningful increase in the reading. I stomped on the doormat at the entrance to the studio a few times and could see a cloud of dust visibly rise, with an increase on the monitor to around 400µg. As this is also used to wipe mud off shoes, not all of this dust will be silica, but it’s still not ideal.
I shook the towel I have next to the studio sink, with an increase of around 30µg. I have no idea how much of this is silica.
I don’t have any canvas in the studio, but I know from other people who have tested with theirs that if your canvas is well used and kicks up a noticeable amount of dust then it will register dramatically on the monitor too.
Conclusion - It’s unlikely that you’re bringing much silica back from the studio in your clothes, but anything fabric that spends a long time in a studio will probably pick up silica dust and can put this back into the air when used.
HEPA Filter
I was given this HEPA filter for free, to test and review. I found that running it overnight on the lowest setting would roughly halve the PM2.5 in the studio when starting from a low level (so from a background of around 16µg to 8µg), and I’m sure it would be even more successful if the air started at a higher level.
I think it’s definitely better to keep levels low in the first place, but a HEPA filter should be a great way to get levels down without letting outside air in.
Should You Worry About Silicosis?
There are a load of variables and ultimately it’s a personal risk calculation, but from what I’ve seen the best practices are most likely sufficient to avoid silicosis. A lot of them are highly effective at reducing the amount of silica in the air, and a well fitting filter masks would be enough for the times where it’s unavoidable. No amount of silica dust is good, but low enough levels should at least allow you to die of something else first.
If you like this sort of content and want to support the creation of more, I now have a Patreon specifically for it.
Additional Reading
A Brief Review of Silicosis in the United States - Authors: Thomas, Carson R., and Kelley, Timothy R.
This is a paper looking into the history of silicosis in the USA.
The abstract states ‘Respirable particles are in the size range of less than one micrometer to as large as 30 micrometers’ (1-30µm or PM1.0-PM30) and it goes on to say ‘Silicosis is usually caused by exposure to silica particles smaller than 10 micrometers’. Our glaze ingredients will be as large as 75-100µm depending on mesh size and the air quality monitors only go up to 10µm, it’s good to know that what we can detect is more important than what we can’t.
Actual numbers of silicosis cases are unknown. ‘“Our research showed that silicosis deaths represent 4 to 8 percent of the silicosis cases per year”. Based upon these estimates, some 2,500 to 5,000 cases could be occurring each year.’
Silicosis became well known about in the 1930’s - ‘The event that introduced the nation to the dangers of silica dust, considered one of the U.S.’s worst industrial disasters, took place in Hawk’s Nest, West Virginia.’. Mining through silica rich ground with lax safety measures meant almost all of the 2500 workers went on to develop silicosis, with 764 acute cases. This lead to laws being passed and a greater worker awareness of the risk.
Silicosis can take decades to develop. With public awareness lower due to reduced prevalence and a run of high profile frivolous lawsuits in the early 2000s, there’s a risk of workers in high risk industries taking it less serious than they should and not realising until it’s too late.
My thoughts - This was an interesting history of silicosis in the US and a great reminder of the possibility of complacency after H&S laws have decreased (but not eliminated) the risk.
Comparison of Risk of Silicosis in Metal Mines and Pottery Factories - Authors: Dongming Wang, MD; Min Zhou, MD; Yuewei Liu, MD; Jixuan Ma, MD; Meng Yang, MD; Tingming Shi, MD; and Weihong Chen, MD
A follow up of almost 40k people over 44 years to see if the risks of silicosis were different with exposure to silica in different industries. Study looked at 1960 to 2003, in China, and compared pottery to metal mining.
There are many industries that have some exposure to airborne crystalline silica. This paper looks at the differing rates of silicosis between industries even with the same exposure level. They found that pottery is actually significantly safer than mining - “Based on data from 9,377 cases of silicosis, we found that the risk of silicosis was higher for workers in metal mines than for those in pottery factories when silica exposure levels were similar.”
The reason isn’t clear, but they suggest it’s because of the different sources of silica dust - “The mechanism for the difference in the risk of silicosis between metal mines and pottery factories is unknown. Experimental studies have suggested that the toxic and fibrogenic potentials of silica dusts differ depending on the innate characteristics of the silica dust and the surface properties of the silica dust particles. The study conducted by Harrison et al indicated that the average sample percentage of aluminum oxide or aluminosilicate occlusion silica particles for pottery factories was much higher than that for metal mines. Meanwhile, studies also showed that aluminum oxide or aluminosilicate surface coatings of silica particles could alter the cytotoxic and fibrogenic activities of the silica.”
They also looked at smoking and how it affected silicosis. Unsurprisingly, it wasn’t great news for smokers - “Our study also indicated that smoking cessation could help reduce silicosis risk for silica-exposed workers. Future studies are needed to clarify the potential mechanism.”
My thoughts - This doesn’t change a huge amount for potters. Knowing that other industries are worse doesn’t mean anything for the risk we’re exposed to. It does mean that risks and safety measures that look at the aggregate will be overestimating the risk to potters, as it’s at the lower end, but that shouldn’t mean that we’re less careful as a result. And smoking is bad for your lungs, who knew?
If you like this sort of content and want to support the creation of more, I now have a Patreon specifically for it or a page on my website if you just want to make a single donation.
