Respirable crystalline silica (RCS), which is listed by the International Agency for Research on Cancer (IARC) as a Group 1 Carcinogen, is a known industrial and occupational hazard. But why? Crystalline silica (of which the most common form is quartz, the second most common terrestrial mineral) is all around us, in playground sand and rocks, and as an integral part of concrete and mortar. Yet we don’t see people dropping dead from silicosis every few meters, and most people have only been injured by quartz if they dropped a big rock on their foot or got sand in their eye at the beach.

That brings us to size. Quartz (and cristobalite and tridymite, the other two primary polymorphs of crystalline silica) is hazardous if it’s small enough to get deep into your lungs and get trapped. Silicosis is a devastating lung disease, which can take years to develop, caused by fine silica particles embedding themselves in the alveolar sacs and ducts where oxygen and carbon dioxide gases are exchanged. When our lung’s cleaning macrophages try to dispose of the nuisance dust, the toxicity of the crystalline silica causes an immune response that releases a bunch of nasty stuff, and this results in characteristic fibrous nodules and scarring (plaques) in the lungs.

So – how small is small enough to cause lung damage? The usual nomenclature for airborne or potentially airborne particulate size is the Particulate Matter (PMx) parameter. The PMx values of interest are the ‘respirables’ PM_2.5 and PM₄ (2.5 is used in the USA and 4 is used in Australia – this size gets all the way down into your lungs), PM₁₀ (Thoracic – it’ll get caught deep in your throat or in the top of your lungs and will probably be coughed out in mucous), and PM₁₀₀ (inhalable – will just stick in your mouth or the top of your throat and will likely be coughed or spat out, or swallowed). The value after PM is referred to as the ‘Equivalent Aerodynamic Diameter’ or EAD and shouldn’t be confused with physical diameters.

·         Average diameter is measured by laser diffraction;
·         The second highest diameter is measured by sieving; and
·         The greatest diameter is measured by laser extinction.

All of these measured diameters cover the physical dimension of a particle, which is only one aspect that dictates how a particle will fly through the air and find its way into your lungs. The other factor that controls how long a particle floats around in the air is the density of the particle – the denser the particle, the heavier a particle of the same size will be, and the more likely it is for a particle to settle straight to the ground. So the higher the density of a material, the smaller the particle needs to be to get into the lungs. The EAD takes into account the density of the particle to model an equivalent particle with a density of 1 g/cc. For quartz, with a density of 2.65 g/cc, this is approximately 2.5 µm which is the equivalent of a water droplet with a diameter of 4 µm.

When predicting whether a material is likely to release hazardous levels of RCS, it is important to determine both the size distribution and the proportion of the PM₄ dust that is crystalline silica. This method is commonly called the Size Weighted Respirable Fraction (SWeRF) method. Another factor to take into account is the friability of the material, and whether it is likely to become finer during routine handling. If the material is likely to produce more fines due to its physical properties or the use for which it is intended then more RCS may be produced.

Using XRD, Microanalysis Australia can quantify the respirable fraction of crystalline silica (RF_cs) to a detection limit of better than 0.0001 wt % in bulk materials and below 10 µg on an air monitoring filter membrane.

If you’d like to ask about respirable crystalline silica determination on air monitoring filters or bulk samples, feel free to give us a call!

Nimue Pendragon
Lead Consulting Scientist

Epidemiologically, it soon* became very clear that known asbestos fibres like crocidolite (fibrous riebeckite) and amosite (fibrous grunerite) caused lung cancers including mesothelioma. Humans began to move away from using asbestos as a cure-all for material shortfalls.

Humans began to manufacture substitutes – slag wool, glass wool and refractory ceramic fibre were some of the first. Unfortunately, the closer to the other qualities of asbestos they came, the more likely that they were found to be hazardous in the same ways. So what properties of asbestos are so harmful?

As asbestos enters the lung, like any other dust, the depth to which it penetrates is dependent on the equivalent aerodynamic diameter. If the fibre is fine enough to get deep into the lung, the body can’t cough it out and has to use more complex disposal methods. The lungs have a self-cleaning mechanism called ‘alveolar macrophages’ which act like little mouths and enclose nuisance dusts that manage to make their way that far into the lung. The macrophages then immerse the dust in acidic fluid to dissolve it and carry the dissolved material out of the lung to be dealt with elsewhere.

With asbestos fibres, the diameter is small enough to enter the macrophage but the fibre is long enough that it can’t close its ‘mouth’. Because the fibre is not dissolved by the surrounding fluids (it is bioinsoluble), this prevents the macrophage from doing its job and removing the fibre and kills the macrophage in the process. This results in scarring and can eventually lead to mesothelioma and other lung cancers.

This relationship between the diameter:length aspect ratio and the ability of the fibre to kill macrophages is the reason that the fibre definitions for different industries are based on aspect ratio. The three main definitions of a countable asbestos fibre are:

The other property of the fibres that is significant is the mineralogy, as some minerals are biosoluble (the surrounding lung fluid will dissolve the fibre anyway, even if it isn’t disposed of by the macrophage). Only the bioinsoluble minerals pose a health risk.

Only six different minerals are classified as asbestos. Chrysotile, Crocidolite (asbestiform Riebeckite), Amosite (asbestiform Grunerite), Actinolite, Tremolite and Anthophyllite (asbestiform Cummingtonite). All of these minerals were once sold commercially as asbestos, but they aren’t the only minerals with these properties. Later, it was discovered that other minerals with the same high aspect ratio and bioinsolubility caused the same kind of diseases. Some other hazardous fibres include Winchite, Richterite, and Erionite.

There are also a significant number of minerals that have the same morphology (high aspect ratio fibres) but don’t pose the same risk as the fibres simply dissolve. Some examples are Epsomite, Ettringite, Halotrichite and sometimes simple Halite, or table salt, can form similar fibres.

So how do you tell them apart? That’s where we come in.

Optical microscopy has historically been used for identifying fibrous materials. It’s quick, cheap and doesn’t require much sample preparation so it’s ideal for a quick check for fibres. However, it has some notable limitations: