CRYSTALLINE SILICA

Andrew Stewart, Kilpatrick & Associates

WHAT IS CRYSTALLINE SILICA?
Silica (silicon dioxide or SiO2) is one of the most common oxides in the earth’s crust and occurs in both crystalline and noncrystalline (amorphous) forms.

Crystalline silica, also known as free silica, is a component of most soil and rock types. Quartz is the most common form of crystalline silica. Analysis of various samples from CMPA sites throughout Victoria indicate the following approximate percentages of crystalline silica:

• Basalt <1.0%
• Scoria 0.5—2.0%
• Clay 20—30%
• Granite 25—40%
• Hornfels 35—60%
• Siltstone 45%
• Sandstone >80%
• Sand >90%

Note: variations from the above percentages will occur depending upon the source of the material.

HEALTH EFFECTS
Silicosis
The main disease associated with occupational exposure to crystalline silica is silicosis. Silicosis is a scarring of the lung tissue from prolonged excessive exposure to airborne respirable crystalline silica. Respirable dust is the portion of dust which, when inhaled, is capable of entering the lower region of the lungs.

A worker may develop either chronic or acute silicosis. Chronic silicosis usually occurs after ten or more years of relatively low exposure. Acute silicosis may develop within a period of only a few weeks to five years following exposure to very high concentrations of respirable crystalline silica.

Lung cancer
The carcinogenicity of crystalline silica in humans has been strongly debated in the scientific community over recent years.

In 1997, the International Agency for Research on Cancer (IARC) released a review of studies of lung cancer and occupational exposure to crystalline silica. This study concluded that there is “sufficient evidence in humans for the carcinogenicity of inhaled crystalline silica in the form of quartz or cristobalite from occupational sources.”

Also in 1997, the American Thoracic Society (ATS) found that “the available data supports the conclusion that silicosis produces increased risk from bronchial carcinoma.” It noted however, that less information was available for lung cancer risks among silicosis sufferers who had never smoked and for silica-exposed workers who did not have silicosis, and that it was “less clear” as to whether silica exposure was associated with lung cancer in the absence of silicosis.

The conclusions of the ATS also agree with the American National Institute of Occupational Safety and Health (NIOSH) which have recommended that crystalline silica be considered as only a “potential” occupational carcinogen.

Non-Occupational Exposure to Crystalline Silica
Due to its ubiquitous nature, nonoccupational exposure to respirable crystalline silica will occur from both natural and anthropogenic sources. Limited information is however available regarding silica related diseases due to nonoccupational exposure to crystalline silica.

An extensive literature search by Kilpatrick & Associates Pty Ltd has failed to identify any reported cases of silicosis or other silica related disease associated with nonoccupational exposure to crystalline silica.

Silica related diseases are generally associated with repeated and prolonged exposure to relatively high levels of respirable crystalline silica. Such conditions would not normally be experienced in a nonoccupational environment and accordingly, the risk of silicosis or silica related lung cancer in the non-occupational environment is considered to be negligible.

RISK ASSESSMENT
In order to comply with the requirements of the Victorian Occupational Health and Safety (Hazardous Substances) Regulations 1999, all CMPA sites should undertake a risk assessment in relation to possible silica exposure.

The first stage of this assessment is to analyse a sample of their product in order to confirm crystalline silica content. It is important to ensure that the analysis is performed for crystalline silica, not total silica. This analysis also provides important information for the preparation of a MSDS.

Where detectable concentrations of crystalline silica have confirmed, there is then the need to assess the actual risk to employees from exposure to airborne respirable crystalline silica. This risk assessment is performed through the assessment of the airborne concentration of respirable crystalline silica dust to which a worker is exposed and comparison of these results to the exposure standard for crystalline silica.

The assessment of worker exposures involves the use of a personal dust monitoring device which is worn by selected workers. This monitoring is performed in accordance with Australian Standard AS2985-2004 “Workplace Atmospheres—Method for Sampling and Gravimetric Determination of Respirable Dust,” with analysis of crystalline silica content of the collected samples being performed by a process known as X-Ray Diffraction.

In addition to “personal sampling”, the risk assessment may also incorporate the assessment of airborne respirable dust/crystalline silica concentrations at fixed locations (static sampling). While the results of static sampling should not be used to assess actual employee exposure, they can be useful in assessing the general quality of the work environment at specific locations throughout a site and can assist in the determination and implementation or appropriate dust control measures.

EXPOSURE STANDARDS
Following a prolonged review period, in October 2004 the National Occupation Health & Safety Commission declared a revised national exposure standard of 0.1mg/m3 for respirable crystalline silica (quartz).

This new standard was gazetted in the Commonwealth of Australia Chemical Gazette on 8th December 2004 and came into effect on 1 January 2005.

Pursuant to Regulation 107(b) of the Victorian Occupational Health and Safety (Hazardous Substances) Regulations 1999, the new occupational exposure standard for respirable crystalline silica becomes enforceable by Worksafe Victoria from 1 July 2005.

The exposure standard for respirable crystalline silica is defined as an 8-hour time-weighted average which represents an airborne concentration which, according to current knowledge, should neither impair the health of, nor cause undue discomfort to, nearly all workers. The exposure standard is based on an eight hour working day, five day working week over an entire working life.

Accordingly, by maintaining exposure below the occupational exposure standard, the health risks associated with crystalline silica are minimised.

CONTROL OF RISK
It is standard occupational hygiene practice to investigate and implement risk control options when personal exposures to a chemical substance exceeds one half of the occupational exposure standard (ie 0.05 mg/m3 for respirable crystalline silica).

Within the extraction industry, the main control options are as follows:

Engineering Controls
Modification of equipment or work process to minimise the generation and/or emission of dust (eg. water sprinklers) or to contain and segregate the dust from the worker (eg. dusty proof booths). Once installed, it is very important to ensure that regular inspections and maintenance of engineering controls is performed.

Design and installation of new processes and plant should however be undertaken with consideration to reducing dust levels that may lead to excessive exposures.

Administrative Controls
Administrative controls are designed to reduce either the duration or magnitude of exposure to dust. Examples include job rotation to reduce average personal exposures, scheduling of certain activities at times of low dust , or the removal/relocation of personnel from dusty areas.

Respiratory Protection
Prior to any engineering or administrative controls being implemented, or where engineering or administrative controls are found to not be effective or currently practicable, respiratory protection should be worn by personnel performing tasks or working within areas where respirable silica exposure has been shown to exceed or approach the exposure standard.

It is also recommended that consideration should be given to wearing respiratory protection at all times when working within areas of the site where visible airborne dust is present, or when performing tasks that generate visible airborne dust.

The use or respiratory protection should not however be relied upon for long term control of dust exposures.

As a minimum, the recommended respiratory protection for use with airborne respirable crystalline silica is a Class P2 half face particulate respirator. Higher respiratory protection (eg. P3 full face respirator, powered air purifying respiratory) may however be required for specific activities which generate significant concentrations of airborne crystalline silica.

For more info contact Andrew Stewart on (03) 9596 7655

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