WHAT MATERIALS ARE LIKELY TO HAVE MORE THAN 1% SILICA?
Bulk materials containing Crystalline Silica (CS)
Background
Model Work Health and Safety Regulations (Crystalline Silica Substances) Amendment 2024 [1] 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:-
- Quartz
- Cristobalite
- Tridymite and - Tripoli.
Quartz is the second most abundant mineral in the Earth’s crust [2] after the feldspars, being a common constituent in many igneous, sedimentary, and metamorphic rocks. Quartz also occurs extensively as a secondary mineral; for example as a cementing material in sediments, in hydrothermal veins, in gangue (a hazardous unwanted material or impurity present in ore ) and in fossils as ‘tripoli’.
‘Tripoli’ is microcrystalline quartz formed from the siliceous frustules of diatoms and used industrially as a polishing abrasive. Other forms of microcrystalline quartz occur commonly in nature; for example, as chert, or as chalcedony.
Both tridymite and cristobalite form at high temperatures and remain metastable when cooled. Geologically, they typically occur in acid volcanic rocks and are relatively rare minerals. Industrially, tridymite and cristobalite polymorphs can be generated from quartz subjected to high temperatures.
Identifying a CSS from a Safety Data Sheets (SDS)
Commercially branded bulk materials containing crystalline silica substances (CSS) are used extensively in industry and so SDS should be readily available for these products. Initial identification of the CSS content in a commercial material or product can be obtained by reviewing its SDS. Section 3 SDS Composition Information section should list CS as quartz, tripoli, cristobalite, or tridymite alongside the unique Chemical Abstract System (CAS) number for the polymorph. If their combined quantity contains at least 1% w/w concentration, the material is considered a CSS according to Work Health and Safety (WHS) Regulations [1].
SDS Section 2 Hazards Identification section may also contain the health warnings [3] associated with the RCS content associated with the CSS as below:-
• If RCS >/= 0.1% Carcinogenicity – Category 1A and Hazard Statement H350i
• If RCS >/= 1% and <10% STOT-Re – Category 2 and Hazard Statement H373
• If RCS >/= 10% STOT-Re – Category 1 and Hazard Statement H372
Section 8 may also indicate if a Workplace Exposure Standard for a CS if it applies.
Determining CS and Respirable CS in a bulk material
Not all industrial and geological materials may have a Safety Data Sheet, but other information such as lithology and petrographic analysis of geological material can be useful. Solid materials may not have a SDS if they are not classified as hazardous substances in their current form. In such cases, bulk material testing may be necessary to determine if any crystalline silica substances are present.
There are two main reasons for determining the CS content in a bulk material:
1. To determine if the bulk material is a regulated CSS [1].
2. To determine the RCS fraction in the Bulk Material.
The health risk from a CSS is directly related to the inhalation of RCS ‘because crystalline silica particles that are less than 10 µm in diameter known as RCS , when breathed in, can travel deep into the lungs [3].
Laboratory methods [4,5] for estimating the respirable fraction of CS that might be released from a bulk material are available and detailed in Identification of CSS and measurement of RCS.
The known concentrations of CS and RCS is critical information when assessing whether the processing of a CSS is high risk (or not) in accordance with WHS Regulation (R529A) which requires consideration of the following:
(a) the specific processing that will be undertaken;
(b) the form or forms of crystalline silica present in the CSS;
(c) the proportion of crystalline silica contained in the CSS, determined as a weight/weight (w/w) concentration;
(d) the hazards associated with the work, including the likely frequency and duration that a person will be exposed to respirable crystalline silica;
(e) whether the airborne concentration of respirable crystalline silica that is present at the workplace is reasonably likely to exceed half the workplace exposure standard (R529A)
Likely content of CS in various bulk material
As an aid to the assessment of crystalline silica risk, the AIOH has compiled 2 lists of materials and their likely content of CS. These are:-
1. Industrial materials including commercial products
2. Geological materials
The Industrial materials are listed in alphabetical order. The geological materials are in order of igneous, metamorphic, sedimentary, secondary materials and ore concentrates.
Users of these lists should understand their limitations including, but not limited to:-
• They are not to be considered inclusive of all bulk materials in existence.
• Likely CS content values are subject to revision as new analytical data or material science becomes available .
• The CS content in a specific geological material can vary significantly from region to region, even site to site.
• Tridymite and cristobalite polymorphs can be generated from quartz subjected to high temperatures.
• Percentage of CS in a respirable dust sample will vary from that of the bulk material and workplace experience is that it is generally lower
• Neither the determination of CS concentration nor estimation of RCS content will replace the requirement to undertake air monitoring should the CS process be determined to be high risk (R529A)
A: Likely content and type of CS in various industrial bulk material
Foundry dust - non-ferrous
Stone - natural See List B
*In WHS Regulations, engineered stone is a Crystalline Silica Substance as it contains 1% or more crystalline silica (as a weight/weight concentration). Engineered stone is defined as an artificial product created by combining natural stone materials with other chemical constituents such as water, resins or pigments; and becomes hardened; but does not include porcelain products or sintered stone
From July 1st 2024 the manufacture, supply, processing and installation of engineered stone benchtops, panels and slabs is banned in Australia. This ban does not apply to sampling and analysis for the purposes of identifying a material as engineered stone (R529D).
AIOH warns that engineered stone containing < 1% w/w of crystalline silica polymorphs may still contain amorphous silica and other hazardous substances that pose a risk to health in respirable form.
List B: Likely content and type of CS in various geological bulk material
Other ore concentrates q √
References
[1] Model Work Health and Safety Regulations (Crystalline Silica Substances) Amendment 2024, Regulation 529A.
[2] W.A. Deer, R.A. Howie & J. Zussman, An Introduction to the Rock Forming Minerals, Longman, 1966.
[3] Safe Work Australia Hazardous Chemical Information System (HCIS)
[4] I. Pensis, F. Luetzenkirchen, & B. Friede, SWeRF - A method for estimating the relevant fine particle fraction in bulk materials for classification and labelling purposes. Ann. Occup. Hyg. 2014, 1 – 11; first published on line 3 Jan 2014; doi:10.1093/annhyg/met076
[5] EN 17289 - 2020 Characterization of bulk materials – Determination of a size-weighted fine fraction and crystalline silica content
WHAT MATERIALS ARE LIKELY TO HAVE MORE THAN 1% SILICA?
AIOH SILICA HUB