Breathing in respirable crystalline silica (RCS) at high concentrations causes silicosis, a disease which results in the formation of scar tissue in the lung resulting in loss of lung function, and may eventually cause lung cancer.
New cases of silicosis occurring in Australian industries appear to be either due to historic long-term high exposures or to contemporary acute uncontrolled exposure situations above the current exposure limit.
The AIOH believes that exposure may be adequately controlled by the use of wet methods to suppress dusts, local exhaust ventilation, use of positive pressure cabins and segregation of workers from areas of high concentration.
AIOH recommends limiting worker exposure to RCS to as low as reasonably practicable (ALARP) below an 8-hour time weighted average (TWA) of no more than 0.1 milligram (mg) respirable fraction in each cubic metre (m3) of air. In addition, a TWA value of 0.05 mg/m3 should be applied as an action level which triggers investigation of the sources of exposure and implementation of suitable control strategies as well as health surveillance.
There are limitations in measurement technology which do not allow the accurate measurement of very low-level exposure below 0.05 mg/m3, and the AIOH recommend near full-shift monitoring and the use of a NATA accredited laboratory applying standardised analysis and reporting methods.
Further, exposed worker respiratory function testing and radiological assessment should be regularly performed according to evidence-based standards.
What is respirable crystalline silica?
Crystalline silica, a form of silicon dioxide, is one of the most abundant minerals in the earth’s crust, with quartz being the most common form. It is present as part of a mixture of minerals in almost all types of rock, sands, clays, shales and gravel. It is also a major constituent of construction materials such as bricks, tiles and concrete, and in artificial stone used to fabricate kitchen and bathroom benchtops (i.e. engineered stone). Hence, crystalline silica is of great economic importance and there is widespread potential for exposure.
How do we measure it?
In order for RCS to present a risk to health it must be inhaled deeply into the lungs. Exposure is therefore assessed by measuring the air-borne concentration dust which is able to penetrate to the alveolar region of the lungs. This is called respirable dust and is measured according to AS 2985 (2009) Workplace atmospheres – Method for sampling and gravimetric determination of respirable dust
Hazards associated with respirable crystalline silica
RCS particles induce lung (bronchogenic) inflammation that persists even after cessation of exposure. It impairs clearance of dust from the lungs and allows dust particles to accumulate and persist in the lungs.
Silicosis is an irreversible and progressive condition in which healthy lung becomes replaced with areas of fibrosis which results in shortness of breath which may progress into ultimately disabling and potentially fatal cardio respiratory failure.
Silicosis may develop into progressive massive fibrosis (PMF). When progressive massive fibrosis occurs, the patient develops progressive respiratory symptoms from reduction in lung volume, distortion of bronchi, and bullous emphysema.
Very high concentrations of RCS may result in accelerated or acute silicosis where a person may be disabled or die in a few years or a few months respectively.
Exposure to RCS also renders a person susceptible to developing pulmonary tuberculosis. Silico-tuberculosis was a common disease on the Australian goldfields in the 19th and well into the 20th century.
RCS is also implicated in the development of lung cancer and has been assessed category 1, Known human carcinogen, by the International Agency for Research on Cancer. Not all studies find the link and RCS appears to be a low potency carcinogen.
There is an increasing weight of evidence that exposure to RCS can cause Chronic obstructive pulmonary disease. COPD is known by a number of other names including chronic obstructive airway disease (COAD), chronic airflow obstruction (CAO) and chronic airway limitation (CAL). It is also referred to as chronic bronchitis and emphysema.
What are the major uses / potential for exposure in Australia?
Workplaces where RCS is known to be present include:
- Mining including Quarrying and Exploration
- Brick and refractories manufacture and heavy clay
- Industrial minerals and the production and use of silica sand and flour
- Construction and Tunnelling
- Stonemasonry (either using natural stone, i.e. sandstone, or engineered stone, i.e. kitchen bench tops)
How is exposure to RCS controlled?
Design and operate processes and activities to minimise generation, release and spread of dust;
- Position people so they are out of the dust either in enclosed and filtered cabins with positive pressure (at least 50 Pa pressure differential) or so they are working upwind of dust emission;
- Use sharp cutting tools that minimise the generation of fine dust;
- Use wet processes to prevent dust generation and use water (or water with additive) suppression to prevent dust spread;
- Use ventilation, either dilution or preferably extraction, to control dust spread and dust release;
- Ensure suppressed dust is captured by scrubbing or filtering so it cannot be re-entrained in workplace air;
- Apply good house-keeping practices to prevent dust build-up (especially important inside vehicle dust-proof cabins);
- Provide training in the health effects of dust and its control;
- Where adequate control of exposure cannot be achieved by other means, provide, in combination with other control measures, suitable PPE. For most exposures to RCS this will be a P2 efficiency half face respirator. Training in the use and limitations of respiratory protective equipment, a clean shaven policy and face fit testing are essential.
Want to know more?
The AIOH has a full position paper on Respirable Crystalline Silica which may be accessed through the website https://www.aioh.org.au/ or by asking a Certified Occupational Hygienist.