Silica fume (thermally generated)
TWA: - ppm 2 mg/m3 (respirable dust) (containing no asbestos and less than 1% crystalline silica)
STEL : - ppm - mg/m3
Documentation notice: National Occupational Health and Safety Commission documentation available for these values.
No standard should be applied without reference to Guidance on the interpretation of Workplace exposure standards for airborne contaminants.
Note: This exposure standard is based on a review (1) conducted by Dr. Charles Mitchell, (Respiratory Physician, Princess Alexandra Hospital and Associate Professor of Medicine, University of Queensland ) . The review was commissioned by a joint State Government and industry working party consisting of representation from the Tasmanian and Western Australian Governments, SIMCOA ( Tasmania ) TEMCO ( Tasmania ) and Pioneer Silicon ( WA ) . Copies of the review are available from the library of the National Occupational Health and Safety Commission or the library of the Department of Mines and Energy in Tasmania . The recommendations of Dr. Mitchell are presented in full below.
1. RECOMMENDATIONS FROM DR MITCHELL'S REPORT
"On the available evidence it is very difficult to quantify the risk of developing permanent pulmonary changes and disability in workers exposed to amorphous silica fume with less than 1% crystalline silica.
In the absence of any other more recent epidemiological studies it is clear that the (threshold limit value) TLV should be less than 5 mg/m3 . There is clearly no evidence to suggest that it should be as low as 0.2 mg/m3 (or twice the value for quartz) as had been recommended by the ACGIH .
Until there is evidence for an alternative level, I would suggest that a TLV of 2.0 mg/m3 is reasonable.
Clearly where amorphous silica fume is contaminated by crystalline silica the appropriate mixture formula should be used.
There is still some doubt as to whether metal fume fever and acute pulmonary reactions occur in response to pure amorphous silica fume exposure . If these reactions do occur on exposure to amorphous silica fume the risk of such reactions at various concentrations of amorphous silica exposure is unclear.
Therefore, it is not reasonable to recommend a (Short-Term Exposure Limit) STEL .
It is essential that further studies in the ferrosilicon and silicon industries be undertaken.
The work environment should be evaluated to document:
- Total and respirable dust concentrations;
- The content of amorphous and crystalline silica; and
- The presence and concentrations of other fumes.
These measurements should be made regularly and, where possible, at times of unexpectedly high emissions.
Those exposed should be medically reviewed to document:
- Respiratory and systemic symptoms and signs;
- Smoking habits;
- Lung function data; and
- Chest X-ray findings.
These reviews should occur:
- Before employment;
- Regularly thereafter, probably once a year; and
- At any time an individual has been exposed to an unusually high concentration of emissions . In such a situation the individual should be reviewed within 24 hours and one week later.
Given that the total number of exposed subjects in Australia is small, the methods used in both environmental and biological monitoring should be standardised so that the data may be combined to add to the power of the analyses."
2. RECOMMENDATION FOR EXPOSURE STANDARD
The Exposure Standards Expert Working Group examined Dr. Mitchell's review and recommendations and is of the view that they represent both a thorough examination of the available data and the basis for setting an exposure standard for this substance . In addition, the Working Group noted that Dr Mitchell's proposed exposure level is consistent with the information provided and exposure level recommended in a review paper by Jahr (2) . The Working Group therefore recommends that an exposure standard of 2 mg/m3 (respirable dust) be set for silica, amorphous fume (thermally generated) and that no short term exposure limit be set . Further, the Working Group recommends that this standard apply to silica fume containing less than 1% quartz and that the standard be modified by application of the mixtures formula where quartz concentrations exceed 1%.
The reader is encouraged to review the appropriate sections of the Guidance Note on the Interpretation of Exposure Standards for Atmospheric Contaminants in the Occupational Environment, for guidance on the interpretation of this standard and the use of the mixtures formula . Readers are also encouraged to obtain copies of the review on which this standard is based from either the Worksafe Library or the library of the Department of Mines and Energy Tasmania .
1. Mitchell C., Literature Review to Evaluate the Health Effects of Occupational Exposure to Thermally generated Amorphous Silica Fume . Tasmania Department of Resources and Energy . Division of Mines and Mineral Resources, Tasmania , 1990.
2. Jahr . J., Possible health hazards from different types of amorphous silicas , in: Dunnom , DD (ed) . Health Effects of Synthetic Silica Particulates (ASTM STP 732) . American Society for Testing and Materials, 199-213, 1981.
Entries carrying a notice for National Occupational Health and Safety Commission documentation indicate that these substances have been reviewed in detail by the Exposure Standards Expert Working Group and that documentation supporting the adopted national values is available in the National Commission's Documentation of the Exposure Standards [NOHSC:10003(1997)].