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Workers in surface coal mining, rock drillers, driller helpers, employers of drillers, and drill rig manufacturers are at risk for silicosis.
Silicosis has long been recognized in rock drillers employed in metal mining, slate quarries, caisson construction, tunnel construction, highway and dam construction, and rock quarries.
Recent studies have concluded that surface coal mine drilling presents a serious risk of silicosis to drillers and driller helpers, indeed most of the recent case reports on silicosis in rock drillers involve surface coal mine drillers. Traditionally rock drillers in underground coal mines were thought to be at risk of silicosis.
Surface Mining Process
An early step in the surface or strip mining process is the removal of topsoil and other overburden materials, including sandstones, shales, limestones, and unconsolidated soils. Removal of these materials may require drilling holes into the rock formation to accept explosive charges for blasting. After blasting, the debris is cleared with earth?moving equipment such as drag line cranes, end loaders, or power shovels.
Silica Exposure During Surface Mine Drilling
When the drilled rock has significant crystalline silica content, the drill operators and helpers may be exposed to large amounts of respirable crystalline silica. Such exposure places these workers at high risk of developing silicosis. Rock drillers operate large, mobile rotary rigs that drill holes in the rock.
Rock drillers working at surface and underground mines are covered by Mine Safety and Health Administration regulations. Nonmining hard rock drillers are covered by Occupational Safety and Health Administration regulations.
The National Institute for Occupational Safety and Health described 23 cases of silicosis from exposure to crystalline silica during rock drilling. Of the 23 workers reported, 2 workers have already died from the disease, and the remaining 21 may die eventually from silicosis or its complications.
Case No. 1 Acute Silicosis.
A 33 year old male quarry rock driller developed respiratory symptoms in August 1986 after working in Pennsylvania surface coal mines since 1975. A chest X ray in August 1986 showed relatively minor abnormalities, but the worker's symptoms improved and he continued to work.
In May 1987, the driller's respiratory symptoms returned along with severe weight loss, fevers, night sweats, joint pain, reduced lung function, swollen lymph nodes, and progressive worsening of his chest X?ray. Antibiotics did not improve his condition, and open lung biopsies confirmed acute silicosis.
After treatment with high dose corticosteroids and antituberculosis drugs, the driller stabilized and improved slightly until February 1990, when his condition deteriorated. He died of progressive respiratory failure from silicosis in September 1991.
The driller reported that most of the drilling he did before 1983 involved wet techniques to suppress dust. Thereafter he drilled without dust suppression measures and without a respirator, even though the dust was sometimes so thick that he "couldn't see anything."
Case No. 2 -Acute and Accelerated Silicosis
In April 1979, a 34 year old male rotary driller reported a 6-month history of shortness of breath, dry cough, weight loss, and pale skin color in body extremities. For the preceding 5 years, this worker had operated a rotary drill at a surface coal mine using dry drilling methods.
Examination revealed increased heart and breathing rates, abnormal lung sounds, healed ulcers on the fingertips, reduced lung function, and abnormal chest X rays. Open lung biopsy confirmed acute silicosis. Although the patient was treated with aggressive therapy for 8 months, lung function continued to deteriorate and the patient died of respiratory failure in June 1981.
Discovery of this case of acute silicosis led to a medical survey of nine other drillers who worked for the same company. Two of the workers, aged 28 and 31, had accelerated silicosis and had been drillers for fewer than 6 years. Exposure concentrations are unknown because the crystalline silica content was not measured in the coal dust samples collected during the years 1972. However, much of the rock encountered by the drillers was sandstone, which is likely to have had a high crystalline silica content.
Case No. 3 Accelerated and Chronic Silicosis
Seven cases of silicosis in surface miners were reported from a single hospital in West Virginia during the period 1978-88. All cases were in men aged 25 to 51 who were involved with surface drilling for 3 to 19 years.
Five of the drillers suffered from accelerated silicosis: one was infected with Mycobacterium tuberculosis, one had a positive tuberculin skin test without proven active infection, and one had progressive massive fibrosis (a condition in which large areas of lung tissue become scarred and collapse). The other two drillers suffered from chronic silicosis. Both were in advanced stages of the disease, including progressive massive fibrosis.
Case No. 4--Chronic Silicosis
A survey of 18 caisson drilling sites in Hong Kong identified 12 cases of chronic silicosis among 118 workers exposed to high concentrations of silica. Six of these workers also had active cases of tuberculosis. All 12 workers had 10 to 20 years of occupational exposure.
Site visits showed little evidence of attempts to control dust at the source. For example, dust suppression measures were not used with pneumatic tools, and exhaust ventilation was not supplied in the shafts. In deeper caissons, fresh air was usually supplied by a compressor through a makeshift duct, but the blowing increased airborne dust concentrations. Some dust was suppressed by the natural seepage of water at the bottom of the excavation, and workers at some sites routinely wet the work face to suppress dust.
In the past, reusable cloth masks were commonly used at caisson drilling sites. More recently, however, workers have provided their own particulate filter masks.
The cases of silicosis described in the case studies illustrate the risk of serious or fatal illness in rock drillers and the conditions that favor the development of silicosis. These conditions are as follows:
The presence of respirable crystalline silica dust
Inadequate dust control measures
Inadequate respiratory protection
The absence of adequate medical screening and monitoring programs
The absence of adequate air monitoring programs for respirable dust
The National Institute for Occupational Safety and Health recommends the following measures to reduce crystalline silica exposures in the workplace and prevent silicosis and silicosis related deaths:
1.Before mining begins, assess the potential for exposing workers to crystalline silica during removal of the overburden.
2.Conduct air monitoring to measure worker exposures.
3.Use control measures such as wet drilling and exhaust ventilation to minimize exposures.
4.Practice good personal hygiene to avoid unnecessary exposure to silica dust.
5.Wear washable or disposable protective clothes at the work site; shower and change into clean clothes before leaving the work site to prevent contamination of cars, homes, and other work areas.
6.Use respiratory protection when source controls cannot keep silica exposures below the safety levels.
7.Provide periodic medical examinations for all workers who may be exposed to crystalline silica.
8.Post signs to warn workers about the hazard and to inform them about required protective equipment.
9.Provide workers with training that includes information about health effects, work practices, and protective equipment for crystalline silica.
10.Report all cases of silicosis to State health departments and to OSHA or MSHA.
Effective source control measures and good work practices should be implemented to minimize worker exposure to crystalline silica. Examples of control measures include dust suppression skirts, wet drilling, local exhaust ventilation, and isolation of the worker in enclosed drilling rig cabs with positive pressure air conditioning.
The following personal hygiene practices are important elements of any program for protecting workers from exposure to crystalline silica:
All drillers should wash their hands and faces before eating, drinking, or smoking.
Workers should not eat, drink, or use tobacco products in the drilling area.
Workers should shower before leaving the work site.
Workers should park their cars where they will not be contaminated with silica.
The following measures should be taken to assure that the drillers' dusty clothes do not contaminate cars, homes, or work sites other than the drilling area:
Workers should change into disposable or washable work clothes at the work site.
Workers should change into clean clothes before leaving the work site.
Respirators should not be used as the only means of preventing or minimizing exposures to airborne contaminants. Effective source controls such as dust?suppression skirts, wet drilling, enclosed cabs, local exhaust ventilation, and good work practices should be implemented to minimize worker exposure to silica dust. The National Institute for Occupational Safety and Health prefers such measures as the primary means of protecting workers. However, when silica source controls cannot keep exposures below the National Institute for Occupational Safety and Health safety standards, controls should be supplemented with the use of respiratory protection during rock drilling operations.
When respirators are used, the employer must establish a comprehensive respiratory protection program as outlined in the National Institute for Occupational Safety and Health Guide to Industrial Respiratory Protection and as required in the OSHA respiratory protection standard and in the MSHA standards. Important elements of these standards are an evaluation of the worker's ability to perform the work while wearing a respirator, regular training of personnel, periodic environmental monitoring, respirator fit testing, maintenance, inspection, cleaning, and storage, and selection of proper respirators.
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