INTRODUCTION TO OPERATOR AIR SAMPLING PROGRAMS
by Doris A. Cash
Exposure to respirable silica-bearing dust (silica dust) can put miners at grave risk. Silicosis, a potentially fatal lung disease associated with overexposure to silica dust, is entirely preventable if mine operators and miners act in concert to prevent exposures to hazardous levels of silica dust. As part of the Mine Safety and Health Administration's (MSHA) national effort to eliminate silicosis in the mining industry, MSHA reminded operators of their obligation under 30 CFR 56.5002 and 57.5002 (exposure monitoring) to conduct sampling as frequently as necessary. This sampling is conducted to determine the adequacy of measures that have been implemented to control the levels of employee exposure to silica dust as well as other airborne contaminants.
Engineering controls are the most effective and reliable method for assuring that individuals are not exposed to unhealthful levels of silica dust. The only sure way to determine the effectiveness of these engineering controls is to conduct airborne contaminant sampling on a regular basis. To assist operators in determining the adequacy of their control measures and in planning for effective monitoring of their employees' exposure, MSHA is preparing general guidance materials. The following is the first in the series and is intended to provide an introduction to the subject of operator air sampling programs.
The following information has been prepared as basic guidance material to assist mine operators in planning for effective monitoring of their employees' exposures to silica dust and other airborne contaminants, and in determining the various needs for and adequacy of control measures required by 30 CFR 56/57.5002. This information may also assist affected miners and their representatives in understanding or otherwise questioning various monitoring and control measures taken in regard to hazardous airborne contaminants.
The ultimate goal of mine operator sampling activities is the prevention of occupational disease. Occupational disease may be prevented by limiting miner exposure to physical and chemical hazards. A successful program involves the recognition, evaluation and control of health hazards in the work environment. This can be done by:
The following gives an overview of the basic elements of a mine operator sampling program.
The collection of valid exposure measurements is necessary for measuring occupational health hazards and determining the need for environmental controls. There is not one best sampling strategy or plan for all work and exposure situations, and the choice of a strategy should be based on site-specific conditions. The following basic considerations can be used to help develop a sampling strategy:
The objective of all sampling strategies is that the collected samples be representative of a miner's normal, typical work activity and exposure so that overexposures can be prevented. Conditions within the work environment on the day or week of sampling must be similar to those likely to be experienced by the miner when sampling is not being done. It may be necessary to observe the miner's activities during the work shift. Notes should be taken on the miner's time spent in each work area, on the work routines, on the duration of work activities, and on the location and condition of engineering controls such as ventilation and baghouses. Converse with miners to obtain information about work that has not been observed and to determine whether their activities and exposures are representative of a "normal" work day or week.
A normal work day or week in some operations may include large variations in work activity as determined from observations and conversations. Indications of representative activity may include the number of:
Samples are representative when collected on days which are within the range of these variations. However, it is important to remember that preventing overexposures is the objective.
Personal samples should not be collected when conditions of exposure are not representative of typical mining conditions. However, if miners are repeatedly exposed to situations, such as engineering control breakdowns or rainy days, those situations may be representative, and it would be proper to continue sampling even if such situations occurred during the sampling period. Re-sample miners as often as necessary in order to collect representative samples.
Whom to Sample
Preliminary walk-around observations and direct-read area sampling data can be used to select miners for personal monitoring. In an ideal situation, each potentially exposed miner would be individually sampled. It is not necessary, however, to sample all miners on a mine property in order to evaluate occupational exposures.
Maximum Risk Miner
There is no single method for selecting the maximum risk miner in all mining operations and processes. A miner experiences high risk because of his or her work area (location) or work procedures (tasks). The following considerations may be used to help identify a maximum risk miner:
The mine operator may find it useful in determining sampling priority to compare the inherent toxicity of the contaminant to the possibility or likelihood of exposure. The prioritization process then becomes:
Where to Sample
The identification of a high risk area may be more straightforward than the selection of a maximum risk miner. The following considerations should help determine sampling areas:
How to Sample
Contaminant-specific sampling procedures may be obtained from the sources referenced in the appendix. Health specialists and industrial hygienists in your local Metal and Nonmetal Mine Safety and Health (MNMSH) District office are also available to assist you.
Personal Exposure vs. Area Samples
Number and Duration of Samples
What to Sample For and Why
The following sections describe the most common types of contaminants sampled, the usual sources of these contaminants, some of the health effects encountered with each, and give suggestions for evaluating the hazard and designing a sampling strategy.
Figure 1. Human Respiratory System
Respirable dust is also defined as the fraction of the dust which passes a size selector (e.g., cyclone). MSHA's Metal and Nonmetal division defines respirable dust as having the following characteristics:
It is important to remember that, at less than 10 µm in diameter, respirable particles are too small to be seen with the naked eye. For comparison, a human hair is about 50 µm in diameter. Visible dust may indicate the presence of respirable dust, but a lack of visible dust does not mean that respirable dust is not present.
Total dust is a term which refers to airborne particles that are not selectively collected with regard to their size. Large particles that make up the total dust cloud may overcome the body's natural clearance mechanisms simply by overwhelming those defenses. The smaller particles can be drawn deeply into the respiratory system and retained.
The fraction of the total dust which is respirable varies widely depending upon the nature of the operation and the composition of the ore. In turn, the concentration of any specific contaminant also may vary in the different size fractions. Respirable-size dust particles do not readily settle out of the air, and can remain suspended for long periods of time. Once settled, however, accumulations can be re-suspended by vehicles, by persons walking through the accumulations, or by the wind, and continue to present an inhalation hazard.
Silicosis, a form of pneumoconiosis, is a condition of the lungs caused by the inhalation of silica dust and marked by nodular fibrosis (scarring of the lung tissue) resulting in shortness of breath. Silicosis, in its advanced stages, is progressive even if the individual is removed from exposure. Silicosis is an irreversible disease. As the disease progresses, the body's ability to fight infections may decrease and susceptibility to diseases such as tuberculosis and pneumonia may increase.
Chronic silicosis is the most common form of the disease. It is caused by long-term exposure to crystalline silica at relatively low levels and may not show up for 10 years or more. Accelerated silicosis results from exposure to high concentrations of crystalline silica and develops five to 10 years after the initial exposure. Acute silicosis can occur in miners exposed to very high dust levels. Unlike chronic silicosis, acute silicosis develops rapidly, usually after months, not years, of exposure.
Cigarette smoking increases the incidence of respiratory cancers and other diseases caused by the inhalation of toxic substances, such as silica dust. In addition, other contaminants in the workplace may have synergistic effects with the mineral dust, that is, the combined effect is greater than separate exposures to each of the individual contaminants.
Evaluating the Hazard:
Cristobalite and tridymite are thermally altered forms of quartz. They may be present in areas where substances containing quartz are heated, such as in refractories, sintering, calcining, or heat expansion. Because these two substances are not detected in routine quartz analysis, mine operators should request a special analysis for cristobalite or tridymite if either of these silica materials are suspected to be present in the dust.
The Threshold Limit Value™ (TLV™) for silica-bearing dust is dependent upon the amount (percentage) of free silica present in the dust. The TLV™ for silica-bearing dust decreases as the percentage of free silica increases. Whenever the dust composition includes two or more hazardous substances which affect the same organ or body system, the combined effect (additive) must be given primary consideration. If the dust composition includes two or more hazardous substances that do not have the same biological effect, the TLV™ for that dust will be the most restrictive TLV™ of the constituents.
MISTS AND ELEMENTAL DUSTS
Dust is a term used to describe airborne solid particles, ranging in size from 0.1 to 25 micrometers, created by the crushing, grinding, breaking, drilling, or the general abrasive handling of a solid material.
Elemental is the term used when sampling for, or requesting an analysis for, any of the substances in the "Periodic Table of Elements" without consideration of the compounds or mixtures in which the substance may be present.
Sources of Mists and Elemental Dusts:
The sources of elemental dust in the mining industry are the same as those for mineral dusts in general. Certain elemental dusts, however, are associated with specific ores. Vanadium and uranium, for example, are elements commonly found in uranium mine dust; arsenic may be encountered in salt mine dust; antimony is found associated with lead and arsenic, as well as copper and silver.
Systemic reactions are caused by the absorption of a toxic agent into the body. The reactions may occur in other parts of the body in addition to the original site of absorption. Inhalation is the most common route of entry. However, systemic poisons may also be ingested or absorbed through the skin. The effects of acute inhalation poisoning may appear quickly because of the rapidity with which the toxic agent can be absorbed in the lungs and pass into the bloodstream. Acute symptoms of systemic poisoning such as blurred vision, headache, nausea, fatigue, weakness, dizziness, delirium, breathing difficulties, and diarrhea, may be non-specific as to cause. Some chronic symptoms, such as a blue line appearing in the gingival tissue at the margin of a tooth (lead), or a green tongue (vanadium), are known to be caused by a specific contaminant.
Sensitization is considered to be a systemic allergic-like reaction because absorption of a small amount of the contaminant at one point can cause a reaction in all areas of the body. A sensitization reaction is the abnormal response of an individual to a contaminant due to previous exposure. A sensitization reaction may be triggered by increasingly lower concentrations of the sensitizing contaminant.
Dermatitis is a condition of the skin caused by chemical reactions, physical irritation, or systemic poisoning.
Evaluating the Hazard:
Personal exposure sampling for mists requires a series of partial-shift consecutive samples or random short-term samples. Personal exposure sampling for elemental dusts requires full-shift sampling.
Pneumoconiosis caused by the inhalation of metal fumes can range from simple dust accumulation to nodular or interstitial fibrosis (lung scarring).
Sources of Fumes:
Any welding operation should be checked for hazardous concentrations of contaminants. The base material in most cases will be iron or steel, resulting in airborne concentrations of iron oxide (Fe2O3), nickel compounds (Ni), and chromic oxide (Cr2O3). Hard facing on stainless steel can result in hazardous concentrations of manganese (Mn) and other highly toxic compounds. Welding of nonferrous metals, such as brass and copper alloys, may produce copper oxide fume (CuO), zinc oxide fume (ZnO), lead fume (Pb), or tin oxide fume (SnO2). Aluminum welding can produce an aluminum oxide fume (Al2O3).
Coatings on base metals will be vaporized. Galvanized metal, when welded, will release cadmium fume (Cd) or zinc oxide fume (ZnO). Metals with lead based paint will release lead fume (Pb) when welded. Some latex paints contain mercury that will be vaporized when heated. It is also possible that the base metal being welded has surface deposits of the ore being mined or milled or has residue of cleaning solvents or other chemicals that could become a toxic air contaminant. This would be true, for example, if the ore contains lead, arsenic, beryllium, barium, cadmium, mercury, platinum, selenium, or other metal compounds.
During arc-welding, part of the welding rod will be vaporized and will release metal fumes into the atmosphere. Fluorides will be produced in gaseous or particulate forms from welding rods containing fluorides. The filler material on most welding rod coatings is a silica or calcium compound, and, therefore, silicon dioxide (SiO2) or calcium oxide (CaO) can also be expected as air contaminants. These contaminants should be counted as part of the total dust concentration.
Consult the welding rod manufacturer's literature or a local supplier to determine the major components of the rods in use at the site to be sampled. Any of the components could be released by vaporization and many could become a health hazard.
Gases Associated with Fume Generation:
Evaluating the Hazard:
All personal samples should be taken in the miner's breathing zone. The breathing zone of a welder wearing a welding hood is considered to be under the hood. The cassette must be placed under the hood whenever possible.
Full-shift sampling should be done when miners will be welding or exposed to other fume sources for all or most of their work shift. Short-term samples should be taken along with any full-shift samples to determine if short-term or ceiling limits for any contaminants are exceeded. If full-shift sampling is interrupted to take short-term samples, the contaminant amounts determined by analysis of the full-shift sample and each short-term sample must be added to determine the full-shift exposure for each contaminant.
Additional short-term samples should be taken when the welding rod, material being welded, or welding flux is varied during the shift. The welding samples will, therefore, have to be monitored closely. Short-term samples should cover at least the maximum time period allowed by the short-term exposure limit that has been set for the specific contaminant(s) sampled.
Sampling performed in areas where molten metals may be generated by a furnace will have to address the known or suspected elements present. These operations are usually performed at a permanent site and on a more regular basis than welding.
In all cases, whether or not short-term sampling interrupts the full-shift sampling, each short-term sample should be analyzed and evaluated to determine if exposure exceeds a ceiling limit or short-term limit for specific contaminants.
The interrupted and simultaneous sampling strategies for collecting full-shift and short-term samples are illustrated below.
Short-term samples S2<--> S3<-->
Full-shift exposure = S1 + S2 + S3
Short-term samples S2<--> S3<--->
Full-shift exposure = S1
Controlling the Hazard:
Permanent indoor facilities can be equipped with local exhaust ventilation to minimize the exposures. In general, the capture hood should be placed no further from the source of the contaminant than a distance equal to the size of the capture hood opening. Care must be taken to place the capture hood where it will not draw the fumes across the miner's breathing zone en route to the exhaust duct. A hood placed above the miner's head, for example, may have ample capture velocity to remove the contaminant, but may do so by drawing the fumes across the miner's face. The hood should be located behind the fume generation source so that it will remove the contaminant without exposing the miner.
Simple smoke tube checks may help determine the adequacy of the capture hood and the air currents in the miner's breathing zone. Inoperable fans, open doors or windows, holes in duct work or hoods, faulty bag houses, and improperly adjusted blast gates may defeat the ventilation system. The effectiveness of well-designed systems is sometimes negated at a later date by the addition of collection points along a circuit.
ASBESTOS AND MINERAL FIBERS
The term mineral fiber refers to particles greater than five µm in length which have a length at least three times greater than its width.
Fibrous talc is a magnesium silicate (Mg3Si4O10(OH)2) which is greater than five µm in length with a length three times greater than its width. Fibrous talc has the same fiber limit as asbestos. This is due to the similarity of the reaction in the lungs produced by fibrous talc and asbestos fibers.
Sources of Asbestos and Mineral Fibers:
Fibrous dust particles do not readily settle out of the air, but can remain suspended for long periods of time. As a result, accumulations of fibrous dust can continue to present an inhalation hazard when they are stirred up by vehicular traffic, by persons walking through them, or by the wind.
Asbestos exposure most often occurs from products brought on to the mine property, such as brake linings, asbestos welding blankets, and pipe insulation, or products that are used in building construction, such as transite panels. Replacement of these items with asbestos-free materials is encouraged, but should be done only by miners trained in asbestos removal and abatement methods.
Mesothelioma, another cancer associated with asbestos exposure, is a tumor made up of cells from the pleura (chest lining) or peritoneum (abdominal lining). Cancer of the gastrointestinal tract and of the larynx have also been associated with exposure to asbestos fibers.
Full-shift Limit: No miner shall be exposed to an 8-hour, time-weighted average airborne concentration of asbestos dust which exceeds 2 fibers, greater than 5 µm in length, per milliliter of air, as determined by the membrane filter method at 400-450 magnification, 4 millimeter objective, phase contrast illumination.
Short-term Limit: No miner shall be exposed at any time to airborne concentrations of asbestos fibers in excess of 10 fibers, longer than 5 µm , per milliliter of air, as determined by the membrane filter method over a minimum sampling time of 15 minutes.
MISCELLANEOUS GASES AND VAPORS
The gas sampling methods used for personal exposure or area monitoring are quite diverse. Mine operators may already be familiar with the vacuum bottles, Bistables, detector tubes, charcoal tubes, and direct-read instruments. However, certain gaseous contaminants may require the use of other less familiar sampling methods, such as gas bubblers with various collection fluids, adsorbent tubes other than charcoal, or the great variety of passive dosimeters on the market.
Gases are formless fluids at room temperature and pressure, retaining no specific volume or shape. Gases diffuse, completely filling any space into which they are introduced. They can be expanded or compressed between wide limits.
A vapor is the gaseous form of a substance which is a liquid or solid at room temperature and pressure. Substances with a low boiling point will usually have a high vapor pressure at ambient temperatures.
Health Effects from Overexposure:
Asphyxiation is the result of a reduction of the oxygen and an increase of carbon dioxide in the body tissues and fluids, resulting in suffocation. Simple asphyxiants, such as methane, hydrogen, and acetylene, act by replacing and lowering the percentage of oxygen in the inhaled air.
Other asphyxiants, called chemical asphyxiants, such as carbon monoxide and nitric oxide, act on the body tissues to alter their ability to pick up oxygen. Carbon monoxide, for example, binds the hemoglobin so that it cannot pick up oxygen.
The chemical action of certain gases or vapors causes inflammation at the area of their contact with or absorption by body tissue. Some results of this irritation may be eye burns, fluid in the lungs, hoarseness, holes in the tissue between the two nostrils, and various types of dermatitis.
Most irritant reactions are acute, that is, they are noticed almost immediately. However, there are some substances which cause a delayed reaction, working slowly to cause severe damage. The most notable example of this is NO2, which causes a fluid build-up in the lungs which may not occur for several hours and which acts as a barrier, preventing inhaled oxygen from reaching the blood. Because the irritation is not felt immediately, the miner tends to remain for longer periods in hazardous concentrations.
Gases and vapors can cause the same systemic reactions as particulate contaminants. Gases and vapors are usually absorbed at a much faster rate than particles and, therefore, the symptoms are likely to appear faster. Almost all toxic gases and vapors cause one or more types of systemic reactions. Sensitization, which also may occur, is considered to be a systemic reaction because absorption of a small amount of the contaminant at one point can cause a reaction in all areas of the body. Systemic reactions may involve liver and kidney failure, central nervous system disorders, nausea, headache, or weakness.
Sources of Exposures:
Toxic gases and vapors are routinely produced by many processes in the mining industry. Toxic and asphyxiant gases can be created by welding operations, combustion, and blasting. Exhausts from fuel-burning engines contain large amounts of toxic gases. Toxic vapors are usually found in the mineral process areas where liquid reagents are used. Vapors may be encountered around solvent cleaning operations, painting, fuel storage areas, maintenance shops, chemical storage areas, and certain reagent mixing areas.
The danger from toxic gases and vapors can be encountered anywhere that they can settle or collect. In addition, toxic gases can be formed accidentally when contaminants react with the ore, with moisture, or with each other. Often the accidental production of a toxic gas can be anticipated and precautions taken to minimize adverse effects.
Control of Gas and Vapor Exposures:
Other methods of controlling miner exposure to a toxic gas are those which isolate the miner, such as in an environmentally controlled booth or cab, and those which prevent the contaminant from being released into the environment. These are also considered engineering controls. The latter method is preferred over ventilation controls. Some examples of controlling toxic gases or vapors prior to release into the environment are the catalytic converters on diesel equipment, the scrubbing systems on coal-fired furnaces, and activated charcoal filters for removing organic contaminants.
Administrative controls are those which involve changes in miner or production schedules or procedures to reduce miner exposure to a contaminant. Administrative controls may or may not change the contaminant level in the ambient environment. Some examples of administrative controls would be rotating miners so that each spends some time in the control booth and requiring that all blasting be done at the end of the workday.
Personal protective equipment (PPE), primarily respiratory protection, is appropriate while engineering controls are being installed, if the hazard can't be controlled any other way, or additional protection is required. Like administrative controls, PPE does not remove the hazard from the work area. Furthermore, respirators are often uncomfortable and may make it difficult for miners to breathe or communicate.
General Sources of Publications
General Reference Materials