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Silica dust: the invisible threat

Clay Technology magazine
11 May 2016

Following David Fishwick’s invaluable talk at ClayTech UK 2015, Simon Frost looks at the dangers of respirable silica.

Speaking at ClayTech UK 2015, Professor David Fishwick, Consultant Respiratory Physician and the Health & Safety Laboratory’s Chief Medical Officer asked the audience, ‘How much silica do you think it takes for you, or one of your workers, to damage their lungs – particularly if they are working in an industry sector exposed to respirable silica for many years?’

Fishwick used a picture of a one penny coin to illustrate just how little silica inhaled over each shift would endanger a worker’s health. But regularly inhaling even a penny’s worth of dust each day would far exceed safe levels – the answer to the question was an amount smaller than the crown upon the portcullis on the traditional ‘tails’ side.

‘That smudge is the total amount that you should be breathing in a working day. Divide that by eight and you get the amount you can breathe in an hour. My point is that it’s completely invisible. That amount of silica dust – you just can’t see it.’ You should still be protected when working with materials that contain silica, even if you can’t see dust.

The Health & Safety Executive estimates that the crystalline silica content of the average brick is up to 30% of the total volume, while tiles can contain up to 45% silica. In order of concentration, sandstone, concrete, mortar, granite, slate, limestone and marble all include silica, too. But why can’t you breathe it in?  

The alveoli – the thin ends of the tree-like network of respiratory tubes within the human lung, are each lined with a blood-filled membrane just a micron thick. As Fishwick put it, ‘You are one micron away from bleeding to death, always.’ When freshly cut, crystalline silica is ‘jagged, sharp and needle-like. You can imagine that the structure I just defined to you does not like that type of material.’ 

High levels of short-term exposure (rare in modern practices in the UK) can cause acute silicosis, responsible for recent deaths in unregulated denim sandblasting operations in Turkey and Bangladesh, and the infamous Hawks Nest Tunnel Disaster of the early 1930s in West Virginia, USA. As Fishwick explained, ‘3,000 men dug the tunnel, and they came across a part of rock that was exceedingly rich in silica. They were asked to separately mine it.’ Of those 3,000 workers, it is estimated that between 100 and 1,000 died from silicosis. ‘Around 100 of those died very rapidly of the acute form of silicosis – those jagged little needles lodging in their lungs.’

Today, the greater threat in UK industrial workplaces is from the incurable, gradually contracted form of the disease – simple or chronic silicosis – caused by repeated, low exposure to silica dust. Deposits of silica accumulate in the lungs, causing shortness of breath, coughing, weakness and weight loss, inevitably shortening the sufferer’s life. 

Bad practices – rare but real

‘This is not just a disease of old people,’ Fishwick said. ‘There are still young people in the UK who are working in poor practices – and I know you are all committed to tackling this – but there are young people, today, getting this horrific condition.’ He showed CT scans and X-rays of several pairs of lungs with variously severe levels of silicosis – the youngest, a 24-year-old exposed to respirable silica dust in the workplace for just seven years. 

‘I still, in clinic, see people whose lungs have been irrevocably, permanently damaged by silica,’ Fishwick said. ‘There’s a lot of talk about macho culture in industrial sectors. Well, a recent patient of mine broke down in tears, in front of his wife and I, about the fact that his silicosis is not going to get any better. He has had one go at this, and that exposure will lead him to die early or need a lung transplant, it is as simple as that. There’s nothing macho about spending the last 10 years of your life in an oxygen mask.’

Prevention and detection

While there is no cure, silicosis can be easily prevented by ensuring that safe working practices are followed. Employers are required to provide respirators specifically designed to protect against crystalline silica, and working environments must use industrial ventilation systems. The official maximum exposure limit, set by the Occupational Safety and Health Administration UK, is 0.1mg of silica per m3, although, as Fishwick noted, ‘If you have been exposed at this level daily for 30 years, you have a 5–10% chance of having background silicosis.’ 

Early detection, he said, is essential. ‘Chest X-rays are a fantastic screening test, but they get a really bad press […] they are the only way to make a diagnosis for silicosis, full stop.  They are relatively insensitive if you have very minor little dots, they may be missed on a chest X-ray, but, otherwise, it is a fantastic test.’ 

Fishwick noted the exposure to radiation from X-rays is commonly put forward as a criticism, preventing people from asking for one. He explained that the amount of radiation received in a chest X-ray is equivalent to that of two days’ normal exposure, or eating a 100g bag of Brazil nuts. 

‘You get two lungs and you should look after them. Occupational exposure to silica is a really important part of that equation.’ 

HSE guidelines for employees 

Ask if the material you are using, or dust from the work you are doing, contains silica

Ask how the job should be done safely, without creating risks to your health

Follow all safe working procedures, including cleaning procedure

Use controls such as dust extraction as you were trained to do

Wear protective clothing properly

If you have to wear a respirator, make sure that:

You are wearing the right type of respirator for the job 

You have a face-fit test for a tight-fitting respirator 

You need to be clean shaven for this tight-fit type of respirator to work effectively 

You have been trained to use, check and clean the respirator

The filters or disposable respirators are changed regularly

The equipment is stored in a clean, dust-free place

You tell your supervisor or employer if you find any defects, or your respirator does not fit, is dirty or its filter is old – your employer must put it right

For more information on respirable silica dust in the workplace, visit