A Treatise on Silica

Associate Professor Dr David McBride, University of Otago Department of Preventive and Social Medicine.talks about his reflections on Silica.


You have probably heard about the recent events with silicosis and engineered stone benchtops, however earlier in the year I had been thinking about silica in general.

During my trainee days with British Coal I had dealt with coal workers pneumoconiosis in the ‘Wales and Western’ collieries, silicosis in the ‘pot banks’ (or potteries) of Stoke-on-Trent and asbestos in the Ferodo brake factory at Chapel-en-le-Frith. I therefore knew some of the complexities inherent in silicon chemistry. It is the most common of all chemical compounds, making up 21% of the earth’s crust. Much of this is in the form of silicates (metals which have reacted with silicic acid), but free silicon dioxide is the hazardous stuff, occurring in either the crystalline or amorphous form. Of these, Of the crystalline forms, Quartz is the most common, Tridymite and Cristobalite being rarer but more hazardous. Diatomaceous earth or Kieselguhr is a form of amorphous silica used in NZ for water filtering, in gardens, has been used in livestock nutrition research and as a pharmaceutical product.

It’s OK if you ingest it, but don’t inhale it, because ‘as a natural product’ it might contain a proportion of respirable crystalline silica (RCS), and if calcined (heated to high temperatures), it can be quite hazardous.

RCS in the form of Quartz or Cristobalite with the most potential for harm. Dust particle size is critical because RCS has a particle size which, for aerodynamic reasons, reaches deep into the lungs.

Chronic silicosis is a pneumoconiosis (dust disease) disease which causes extensive fibrosis, or scarring, of the small airways and the part of the lung where gas exchange takes place, the alveoli. In some cases the lung destruction is severe, causing ‘progressive massive fibrosis’, PMF. This is quite rare, but heavy exposure to coal dust causes a similar disease, coal workers pneumoconiosis or ‘Black Lung.’  Chronic silicosis develops over a time-frame 10-30 years after exposure starts, the symptoms being similar in all cases, T a cough, shortness of breath and wheezing.

Acute silicosis is caused by massive overexposure, and may still be a risk if someone uses beach sand to carry out sand blasting- a thing that should never be considered, garnet or other ‘non-toxic’ abrasives should be used. It can cause immediate and serious breathing problems and can be fatal, or the symptoms develop over a 1-3 year period and can be fatal within 3-5 years. 

 In accelerated silicosis, which has occurred in Australian kitchen fabricators, the onset can be within 10 years of first exposure, and PMF is more likely.

The strange thing about silica is that it also causes chronic bronchitis,  tuberculosis, lung cancer and ‘autoimmune disease’ including arthritis and kidney disease. This is due to the peculiar inflammation that it causes.

The really high-risk groups lie in Mining, quarrying and tunneling; sand and abrasive blasting; foundry work; construction (including concrete cutters and labourer's in civil construction) and now, of course, the manufacture of engineered stone benchtops. Building work in general has a risk of silica exposure:  Gib stopping was something I hadn’t thought about, but the Gib board itself and the stopping compound both contain RCS at varying percentages.

As regards primary prevention, the ‘Workplace Exposure Standards (WESs) set by WorkSafe set minimum standards for exposure, RCS has a WES of 0.1 mg.m-3  over 8 hours, but is on the list for review this year, the proposal being a WES of 0.05 mg.m-3. As a primary control, ‘wet cutting’ is a powerful dust suppressant, if you see a concrete cutter using a saw without a water hose attached, tell WorkSafe or email me. Extract ventilation works well and personal protective equipment is a last resort.

 What do you need to do?

 Find out if you have ‘at risk’ employees or exposures, and if they haven’t been measured up against the WES, call a hygienist. If you suspect that exposure might have taken place, send the employees to an occupational physician. WorkSafe will definitely want to know, and X-rays and lung function tests must be carried out, but bear in mind that (to the best of my knowledge) we have no one In New Zealand qualified to ‘grade’ pneumoconiosis X-rays according to the International Labour Organisation ‘B reader’ (Black Lung) standard.

All in all quartz chemistry is complex, as is silicosis: we have much to learn, and lets hope its not the hard way!