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Controlling the silica hazard
In order for silica to be a hazard, silica-containing dust particles that are small enough to be inhaled (i.e., respirable) must get into the air. The strategy for controlling the silica hazard can therefore be broken down into three basic approaches:
- prevent silica dust from getting into the workplace air
- remove silica dust present in the air
- if present, prevent workers from inhaling the dust.
To avoid the inhalation of silica, it is essential to have the following control methods in place:
- engineering controls
- work practices and hygiene practices
- respirators and personal protective equipment
However, even with appropriate measures to control silica, some workers may still be affected. For this reason, periodic medical examinations are important for determining if the control measures in place are effective and if workers are suffering from any of the effects of silica exposure. This is known as medical surveillance (see Appendix 1), and can be considered to be a method of early detection and prevention of silicosis.
4.1 Engineering Controls
Engineering controls are methods of designing or modifying equipment, ventilation systems, and processes to minimize the amount of a substance that gets into the workplace air. They include:
- process control
- enclosure and/or isolation of the emission source
Substitution can eliminate silica from certain processes by replacing it with a less toxic material. Some examples are:
- silica sand used in abrasive blasting may be replaced by metal shot and grit, alumina, garnet, cereal husks, sawdust, high pressure water, steel sand, silicon carbide or corundum (Note: When choosing non-silica containing abrasives, avoid choosing abrasives that may introduce new health hazards to the workplace. For example, abrasives containing walnut shells may cause allergic reactions in some workers.);
- the replacement of sandstone grinding wheels with ones using an abrasive like aluminum oxide; and
- the use of magnesite or aluminum oxide bricks in place of silica bricks in furnaces.
When it is not possible to use a silica substitute, changing how a process is performed can lower silica exposures. For instance, wet methods reduce dust and should be used whenever practical, particularly in cutting, grinding, and drilling operations. Another example is the modification of an abrasive operation to produce a coarser dust that is less hazardous because it settles more readily and is less likely to be trapped in the lungs if inhaled.
If a process cannot be modified to reduce exposure, it may have to be isolated or enclosed. Dusty operations can be isolated by carrying them out in areas that are physically separated from non-dusty areas and keeping workers not involved in the operation out of the area. Where isolation is not effective, the process can be completely sealed off from the rest of the workplace with an enclosure.
Ventilation refers to engineering controls that rely on the removal of contaminated air from the workplace and the replacement of exhausted air with filtered air. The most effective use of ventilation to control a silica hazard is the removal of dust at its source (local exhaust ventilation). Often dust-generating tools are equipped with dust collection systems to prevent dust from spreading or becoming airborne. An essential component of these systems are the cleaning devices, such as filters, which will effectively remove the dust.
4.2 Work Practices and Hygiene Practices
Work practices and hygiene practices are on-the-job activities that reduce the exposure potential from contaminated surfaces and work areas. Silica can also accumulate on the hands, clothing and hair. From there it can be disturbed, re-suspended in air and inhaled. Workers should therefore be able to wash and shower at the end of each shift. There should be no smoking, eating, drinking or chewing in contaminated areas and lunches should be stored in an uncontaminated area. It is therefore important to follow good work and hygiene practices whenever silica is present.
Good housekeeping is important wherever silica dust is generated. Containers of silica-containing waste should be kept tightly covered to prevent dust from becoming airborne. Surfaces should be kept clean by washing down with water or vacuuming with a vacuum equipped with a high-efficiency particulate air (HEPA) filter. Cleaning with compressed air or dry sweeping should be avoided.
4.3 Personal Protective Equipment
Personal protective equipment includes protective clothing and respirators. The purpose of protective clothing is to prevent the contamination of regular clothing and the transportation of silica-containing materials from the workplace. Clothing that is contaminated with silica dust should not therefore be worn home without cleaning.
Sometimes engineering controls and work practices cannot lower the concentration of silica to non-hazardous levels and workers must wear respirators for protection. If respirators must be used, a respirator program should be implemented. It should include written procedures for the selection, use, care and maintenance of personal respiratory protection equipment. Workers should be instructed and trained on the care and use of personal protective equipment before using it. Some workers may have a medical condition that causes them to have difficulty breathing when wearing a respirator. Such workers should not be assigned to do work that requires a respirator if they have written medical proof of their condition.
Where respirators are provided, they should be appropriate in the circumstances for the type and the concentration of airborne silica. Respirators should be selected in accordance with the U.S. National Institute for Occupational Safety and Health (NIOSH) assigned protection factors (APF).
Use, Care, and Maintenance of Respirators
The following general use, care, and maintenance procedures should be followed whenever respirators are required:
- respirators should be used and maintained in accordance with the manufacturer's specifications
- proper seal of respirators should be checked prior to each use
- storage of respirators should be in a convenient, clean and sanitary location and stored in a manner that does not subject them to damage or distortion
- respirators assigned for the exclusive use of one worker, should be cleaned, disinfected and inspected after each shift
- respirators used by more than one worker, should be cleaned, disinfected and inspected after each use
- any respirator parts that are damaged or that have deteriorated should be replaced before the respirator is used.
For additional information on the use, care, and maintenance of respirators, please refer to CSA standard Z94.4-02. (View CSA standards)
Ideally respirators should be assigned for the exclusive use of one worker. But before a decision is made for a respirator to be shared by more than one worker, the following factors should be considered:
- the fit of the equipment
- the health and safety risk to the worker that would be caused by non-exclusive use of the equipment
- any undue economic hardship to the employer that would be caused by exclusive use of the equipment.
Respirators with a tight-fitting facepiece must be fitted to the worker in such a way that there is an effective seal between the equipment and the worker's face. Each worker must be fit-tested for each type of respirator to be worn.
Training is an important component in preventing worker exposure to silica. Control methods, measures and procedures can only be as effective as the workers carrying them out. It is therefore essential for training to cover the following:
- WHMIS training
- the hazards of silica, including health effects and symptom recognition;
- the recognition of typical operations containing silica;
- personal hygiene, respirator requirements, and work measures and procedures;
- the use, care, maintenance, cleaning and disposal of personal respiratory protective equipment.
Instruction and training should be provided by a competent person. This could be the employer or someone hired by the employer. A competent person is defined under the OHSA as a person who:
- is qualified because of his/her knowledge, training and experience to organize the work and its performance;
- is familiar with the provisions of this Act and the regulations that apply to the work; and
- has knowledge of any potential health and safety hazards in the workplace.
The health and safety representative or the representative of a joint health and safety committee should be advised about when and where the training and instruction is to be carried out.
4.5 Medical Surveillance
Medical surveillance can be used as a preventive and remedial measure. By providing regular medical examinations and clinical tests on workers exposed to silica, subsequent adverse health effects can be detected. The examining physician can then alert the worker, the employer and the joint health and safety committee to exposure problems in the workplace that might otherwise go unrecognized. This should ensure that remedial steps will be taken.
Workers working with silica on a regular basis should have pre-placement medical examinations that include chest X-rays and pulmonary function tests, followed by periodic medical examinations. The frequency of the periodic examination will depend on the intensity and length of exposure to silica and shall be decided by the examining physician. It need not be the same for all workers but shall be done at least once every two years. Additional information on the medical surveillance program for silica exposed workers can be found in Appendix 1.