Mineral dusts and prevention of silicosis, vol 4; No.2, September 1997Prevention of silica dust hazards,by Erkki Kahkonen, Finland Nancy Beaudet, USA |
Introduction
Silica is the term chemists have given to minerals that contain silicon dioxide (SiO2). Quartz, abundant in the Earth's crust, is the most abundant natural form of silica. The most common crystalline forms of silica encountered in industrial processes include quartz, tridymite and cristobalite.
Exposure to silica is associated with the development of silicosis in workers. Many epidemiological studies have addressed the relationship between occupational exposure to crystalline silica and silicosis, but few have provided information on the prevention and control of silica dust exposure.
Exposure to silica
Identification of exposure to silica involves recognition of industries where significant silica exposure can occur, and a process to characterize workers' exposure, such as qualitative exposure assessment. Occupational exposure to silica dust occurs in many industries or industrial processes, including the following; mining of rock containing silica; quarrying of granite (cutting, crushing, loading and transporting); highway construction (tunnelling, cement mixing); flint work (breaking, crushing or grinding flint); sand blasting; foundry work; the manufacture of china or earthenware; and glass-making industries (unloading, storage and mixing of silica sand). In addition, many other industries and occupations may involve exposure to silica.
The percentage of crystalline silica in a mineral dust mixture is an important factor in evaluating the respiratory hazard associated with breathing the mixture. Generally, the silica dose to the lungs is highest with exposure to fine dust mixtures containing high percentages of silica. The percentage of crystalline silica found in various materials used during different processes can vary greatly. Here are some examples of the silica content found in various materials: foundry moulding sand, 50 90% silica; pottery, 15 25%; bricks and tiles, 10 35%; buffing wheel dressing, 0 60%; road rock, 0 80%, limestone, 0 3%; feldspar, 12 25%; clay, 0 40%; mica, 0 10%; talc, 0 5%; slate and shale, 5 15%.
In the European Union and in the USA, an employer is required to have a material safety data sheet (MSDS) an excellent source of information developed by the product manufacturer which identifies the hazardous chemicals present in a product or material and which summarizes the potential health hazards and safe handling of the product in question. The percentage of silica in a material is usually reported on the MSDS.
Identification and location of significant sources of exposure also require an understanding of the manufacturing process involved. Written manufacturing information as well as discussions with company representatives familiar with the process are very useful in identifying specific areas at a facility where workers may be subjected to exposures of concern. In addition, interviews with potentially exposed workers are useful for gathering information about the nature of the work, about intermittent exposures of importance, and about the existence and use of engineering controls and personal protective equipment. Perhaps most importantly, a visual evaluation of the specific work process made by an occupational hygienist or by a trained worker can be invaluable in identifying exposures of concern.
Monitoring of the ambient air
In order to determine workers' occupational exposure to silica, the respirable crystalline silica content of the ambient air at the workplace must be quantified. These measurement results can be compared against internationally recognized levels of respirable silica which are considered safe for the average healthy worker. Quantitative exposure assessment information is also needed to select appropriate engineering controls and respiratory protection. Monitoring of the ambient air at the workplace can be used to evaluate the effectiveness of control measures.
Before field sampling is done, it is important to develop a sampling strategy designed to address specific questions. Examples of sampling strategy factors to consider include: characterization of the worst exposure or of average exposures; knowledge of average or high work production rates; and knowledge of optimal (recently cleaned) or substandard (bag-house dirty) ventilation system. For field sampling of silica, sampling is usually conducted throughout the full shift, although when characterizing the exposure that occurs during a short interval, activities involving high exposure levels can also be important. Personal sampling methods or breathing zone samples are preferred for characterizing workers' exposures. Area samples typically are collected to describe background contaminant levels or to determine the effectiveness of engineering controls.
In the determination of respirable silica, a cyclone sample collection device is used to separate respirable particles from larger particles. It is important to remember that filters should be weighed before sampling. The amount of respirable dust is then equal to the silica mass collected on the filter divided by the total volume of air which passed through the filter. The mass of silica can be quantified by means of infrared spectrophotometry (IR), X-ray diffraction, and visible absorption spectrophotometry. IR or X-ray diffraction techniques are necessary in order to distinguish between the different types of silica, such as quartz and cristobalite. The laboratory may also require collection of bulk samples for analysis.
Methods of controlling exposure to silica
Elimination of silica or substitution of a less toxic material
Sand blasting is very common the world over, and so substitutions available for the silica sand used in sand blasting are discussed below. Ordinary sand may contain significant amounts of quartz, and many countries have restricted or banned the use of quartz sand. A wide range of materials can be used as substitutes for hazardous quartz sand; examples include glass beads, steel grit, steel or iron shot, plastic blast materials, aluminium oxide and zirconium oxide. These materials are more expensive than quartz sand, but they are recyclable and they have other benefits, such as improved quality of the finished product.
Glass beads, for instance, do not remove material or metal and the cleaned surface is smooth and dull, which reduces glare. Studies show that stainless steel surfaces finished with glass beads inhibit the growth of bacteria. This is particularly important for components used in the medical or food industry, where hygiene and cleanliness are of paramount importance. In addition, glass bead blasting is quicker and less labour-intensive than conventional methods.
The ferrous abrasives that are available include steel grit, which is a sharp-edged cutting material, and steel or iron shot having spherical particles. Steel and iron abrasives are not inherently hazardous. Aluminium oxide is a hard, sharp-edged, and effective cutting and cleaning material. Surface preparation with aluminium oxide abrasives is excellent for items requiring painting.
Few substitutions are available for the silica sand used in foundries. Olivine sand is perhaps the only economical alternative. Zircon or chromite sands are expensive; they are useful in special cases.
Isolation or enclosure of dirty operations
Isolation is the process of enclosing a process in order to eliminate or significantly reduce workers' exposures. In general, isolation involves building an enclosure around a dusty work process.
The enclosure must be built as air-tight as possible. It should be properly ventilated to eliminate the build-up of contaminants inside the enclosure. Natural ventilation is sufficient occasionally, but a more complex installation may require forced-air ventilation. One form of enclosure is a cab equipped with efficient supply-air filtration and air conditioning.
Since mechanical processes create heat, an enclosure may result in ambient temperature increases that can accelerate machine failure. This is especially true in hot environments.
Sand blasting cabinets and machines are examples of closed processes. Enclosures usually require small vision ports, to allow inspection of certain aspects of the process.
Enclosures, like most engineering controls, frequent attention to maintenance and a dedicated ventilation system in order to ensure continued effectiveness in controlling exposure to silica. Malfunctioning or poorly maintained equipment is less effective in capturing dust and other emissions than equipment which is maintained properly.
Modification of work practices can reduce workers' exposure
Modification of a manufacturing process is one way of reducing workers' exposure. A good example of this is the introduction of water to control dust levels.
Wetting down is one of the simplest methods for dust control; its effectiveness depends on the use of proper techniques to wet down the dust. Tremendous reductions in dust concentrations have been achieved by forcing water through the drill bits used in rock drilling operations and by wet sawing of materials containing silica. These methods control the hazards presented by exposure to silica at the source, protecting adjacent workers from exposure. When possible, castings can be cleaned with water rather than with sand blasting, and the addition of moisture to moulding sand decreases airborne silica exposures.
Local exhaust ventilation
Ventilation is one of the most effective methods available for preventing hazardous substances from entering the workroom atmosphere. Systems are often used ineffectively, however; either they do not provide the required degree of control, or their function and use are not properly understood.
A local exhaust ventilation system consists of a hood to capture the contaminants, ducts to transport the trapped contaminants outside the building, an exhaust fan to circulate the air, and air cleaners for particulate removal to ensure protection of the environment. With respect to local exhaust ventilation systems, guidelines to follow include: place local exhaust hoods as close as possible to the contaminant source; direct the flow of air so that clean air passes the worker's face (breathing zone) before becoming contaminated; enclose the process as much as possible; provide adequate make-up air, which is a problem especially in tight buildings; locate local exhaust fans outside so all ducts are under negative pressure; avoid recirculation of contaminated air; and maintain the system.
Personal protective equipment
Personal protective devices must be used when neither engineering methods nor work practices are feasible to control or avoid exposure. However, respirators are the least desirable method for controlling workers' exposure. The principles guiding proper selection and use of personal protective equipment at work are embodied in national and international standards. Individual anthropometric and physiological characteristics, as well as respirator comfort, are of vital importance.
In general, particulate cartridges are changed when particulate loading on the filter material makes the resistance excessive and uncomfortable for the user. For chemical cartridges, odours inside the mask indicate that the cartridges must be changed. High temperature and high relative humidity can reduce the service life of filters.
The purity of the air must be ensured when compressed or supplied air systems are used. In hot and humid countries, sweating and discomfort make a full-face or half-face mask respirator inconvenient. A face shield or hood with a battery-operated blower and filter unit is cooler and easier to use, and should be considered when possible.
The wide variety of devices and the complexity of some devices require that users receive proper education and training. The people responsible for maintaining and cleaning respirators also require training. A high-efficiency particulate air-purifying (HEPA) filter is necessary for protection against silica dust.
Housekeeping and hygiene practices
Order and cleanliness, or industrial housekeeping, are associated with many production aspects, such as safety, exposure, quality, and company image. Development of a housekeeping programme is the best way of improving order and cleanliness.
Successful programme development utilizes a cooperative worker and management team approach to identify programme goals. In addition, incorporation of worker and management perspectives as to the best method of reaching and maintaining these goals is critical to the success of the programme. Cornerstones of effective programmes include worker participation, modern motivation principles, team observations, quantified measurement, evaluation, and positive feedback.
Good housekeeping includes adequate facilities for washing the hands and face, and clean eating facilities. Workers' hands and face should be washed before eating, and eating in dusty or contaminated areas should be avoided.
Dusty clothes can easily contaminate clean areas both at work and at home. Workers should thus shower and change into clean clothes before leaving the work site. Dirty clothes should be left at work, and the employer should arrange for laundering of work clothes.
Conclusion
A number of technical methods are available for controlling and reducing workers' exposure to dust. In a foundry, for example, the risk of silicosis can be substantially reduced or eliminated by combining several of the following technical measures: substitution of olivine sand for silica sand; use of closed instead of open conveyor systems for dry sand; installation of general, or preferably local, exhaust ventilation systems; utilization of totally enclosed systems for sand preparation and blasting work; and central vacuum cleaners instead of compressed air for cleaning. Finally, personal protective equipment must be used when no other methods are feasible to avoid or eliminate hazardous exposures at work.
This article is based on the authors' lectures given during the course entitled Primary and Secondary Prevention of Silicosis in Vietnam, held on 5 10 May 1997 in Hanoi, Vietnam.
Erkki Kahkonen
Chief of Laboratory
Finnish Institute of Occupational
Health
Topeliuksenkatu 41 a A
00250 Helsinki, Finland
E-mail: Erkki.Kahkonen@occuphealth.fi
Nancy Beaudet
Industrial Hygienist
University of Washington
Occupational and Environmental
Medicine Program
Seattle, Washington, USA