HOW DO I MEASURE
SILICA IN MY MATERIALS
AIOH SILICA HUB
Measurement of the Crystalline Silica (CS) content in bulk materials and the estimation of the Respirable Crystalline Silica (RCS) fraction.
Background
Model Work Health and Safety Regulations (Crystalline Silica Substances) Amendment 2024 defines a Crystalline Silica Substance (CSS) as “material that contains at least 1% crystalline silica (CS), determined as a weight/weight (w/w) concentration”.
In these regulations crystalline silica means “crystalline polymorphs of silica” which are:-
o Quartz
o Cristobalite
o Tridymite, and
o Tripoli.
Each polymorph or type of CS has a unique Chemical Abstract System (CAS) number and Workplace Exposure Standard (WES) so it is important to know what type of silica may be present and measure its concentration with an established analytical technique.
Source : Safe Work Australia Workplace Exposure Standards
WHS Regulations R529CB requires a silica risk control plan when the risk is high and is reasonably likely to result in a risk to the health of a person at the workplace’
The health risk from a CSS and its silica content is not a one-for-one relationship, there are several factors involved, including the;
• concentration of a silica polymorph in the bulk material of a CSS
• concentration of respirable crystalline silica (RCS) that can be released from the CSS
• concentration of RCS dust that a person can be exposed to in the air
Regulations require that all forms of CS will require laboratory analysis in order to accurately quantify concentration. However, currently there is no analytical method to definitively distinguish tripoli from α-quartz. AIOH considers that products containing tripoli are infrequently found in workplace settings although tripoli may be found as an abrasive in polishing compounds. Any tripoli present in a CSS is likely to present as αquartz.
Purpose and Scope
This paper provides technical guidance on quantifying CS composition within a bulk material or product. The purpose of this guidance is to support those with duties to prepare Safety Data Sheets (SDS), assess CS exposure risk or develop silica risk control plans. It describes the methods available to measure % CS within a bulk material and in the dust that could be released during processing or handling of that CSS This includes a description of a method to determine the estimated respirable dust from a CSS.
Why do CS analysis?
Under WHS Regulations, controls to protect worker health are triggered at 1% CS as a weight/weight (w/w) concentration. In these Regulations, CS means any crystalline polymorphs of silica (R529A). Quartz is the most abundant polymorph in natural and man-made CSS; however this should not be assumed.
Why do RCS analysis?
The Globally Harmonised System (GHS) of Classification and Labelling of Chemicals is used in Australia as a systematic way of classifying workplace hazardous chemicals and communicating their hazards through labelling and SDS.
Whilst Safe Work Australia Hazardous Chemical Information System (HCIS) has no listing for RCS it designates the respirable fractions of quartz and cristobalite as a: -
• Category 1 Carcinogen
• Category 1Specific Target Organ Systemic Toxicity (STOT) Repeated Exposure (Re).
Under this system, the concentration of RCS (quartz or cristobalite) requires these hazard warnings and classifications to be declared in SDS and on labelling as follows: -
• If RCS >/= 0.1% Carcinogenicity – Category 1A and Hazard Statement H350i applies
• If RCS >/= 1% and <10% STOT-Re – Category 2 and Hazard Statement H373 applies
• If RCS >/= 10% STOT-Re – Category 1 and Hazard Statement H372 applies
There is AIOH guidance available (hyperlink Task 8) on various common CSS and what silica polymorph they are likely to contain, in what proportion. However, laboratory analysis is necessary to confirm this for the specific product or material, if not already stated in the applicable SDS
Who can do CS analysis?
Selection of a competent and experienced laboratory to undertake the analysis is an essential element of assessing risk from processing of CSS AIOH recommends selection of a facility that is accredited under AS ISO/IEC 17025 or by the National Association of Testing Authorities (NATA) for analysis of respirable dust and respirable crystalline silica (RCS). These can be found at www.nata.com.au.
Where to start?
The process of determining whether a product or material is a CSS should start with a conversation with the analytical laboratory about the purpose of the analysis and appropriate methods. The purpose may be;
• determining whether a material meets the definition of CSS
• quantifying one or more polymorphs in a CSS
• producing a SDS with accurate GHS classification based on RCS.
Utilizing an inappropriate technique may lead to erroneous results and mischaracterisation of the hazard.
How to take a sample suitable for analysis?
The sample for analysis should be representative of the entire material. Typically, 50g is suitable for bulk materials, while obtaining representative particulate samples may be more complex. International Standards Organization standard ISO 14488 provides guidelines for sampling particulate materials. In cases involving multiple materials within a product unit, consultation with the lab is recommended, and provision of the entire unit may be necessary. Confirm with the analytical laboratory the required sample amount as it may vary with analytical method. The sample should be stored in a bag suitable for its size and physical state or container with an identification label for tracking. Where known, provide detailed information about the sample’s origin, such as source, collection method, likely silica polymorphs present, formation conditions, and any processing history. This data can help identify possible phases present in the sample, possible interferences, and use of multiple techniques as required for complex samples. Products such as engineered stone*, porcelain, artificial wood and bricks may require preparation before analysis.
* From July 1st2024, the manufacture, supply, processing and installation of engineered stone was banned in Australia. This ban does not apply to sampling and analysis for the purposes of identifying a material as engineered stone (R529D).
What are the analytical methods available to measure CS %?
Laboratories will use established analytical methods for measuring the common crystalline silica polymorphs in bulk materials and there is a new International Standards Organization standard ISO/DIS 6868 - Workplace air Quantitative determination of quartz and cristobalite in bulk materials by X-ray powder diffraction methods in development.
For specific advice on whether X-Ray Diffraction (XRD) or Fourier Transform Infrared (FTIR) spectrometry is suitable for a CSS, consult a NATA accredited facility. An important consideration in selection of either method will be the presence of Interfering substances which include, but are not limited to, kaolinite, feldspars, muscovite, and some other silicate minerals (including amorphous silica). If these substances are present in significant amounts, the laboratory can advise which analysis should be used to avoid over or under estimation of the CS present.
Both FTIR and XRD instrumentation must be calibrated against the specific crystalline silica polymorph standard Typically XRD allows speciation of all minerals in a sample with a Limit of Detection of about 0.5% with normal scanning times.
What is the method to estimate the RCS fraction of dust?
The RCS concentration can be measured using the Size Weighted Respirable Fraction (SWeRF) Method published by Pensis et al .The SWeRF Method uses Stokes Law of sedimentation to allow the separation of the respirable size fraction of particles from particles larger than the respirable size. The sedimentation time is calculated by Stokes Law using the density of quartz (about 2.65 g/cm3). The sedimentation (elutriation) time varies but it must allow for the respirable fraction of particles to be collected to a depth of about 5cm. The respirable fraction is filtered and then analysed to determine the RCS in the <75µm fraction. The mass >75µm is taken into account in the calculation of the % Crystalline silica SWeRF. Should the material have a SWeRF value > 0.1% w/w then it must be declared as carcinogenic in an SDS, according to Safe Work Australia
The AIOH recommends the analysis of CS and calculation of RCS is done following consultation with experts from NATA accredited facilities and Certified Occupational Hygienists (COH)® .