When The Size Makes The Poison

You’ve heard the saying ‘the dose makes the poison’, and it’s true – the concentration of a toxic substance makes the difference between being at or below the ‘no observable effect concentration/level (NOEC or NOEL) and death. But concentration isn’t the only factor when you’re looking at certain hazardous materials.

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.

There are several different types of physical diameter that can be measured:

·         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.

Apart from appropriate PPE (always wear your P2 mask!) and dust suppression measures, there are several ways that RCS can be monitored and controlled. In areas with likely exposure, personal and location air monitoring is used to monitor the levels of airborne dust. Either FTIR or XRD can be used to determine the concentration of RCS on the filters and this can be equated to a mass per unit volume, and compared to the relevant industry TWA limits.

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