September 17, 2001
This article examines the hazards associated with metalworking fluids. Health effects, including skin and respiratory disorders and cancer, are addressed. A look at engineering and administrative controls that can be implemented to ensure the safe use of metalworking fluids is included.
Metalworking fluids (MWFs) are widely used in many manufacturing industries. These fluids are used in the cutting, machining, and grinding of metal parts to cool and lubricate tools and workpieces, wash away the removed metal, prolong tool life, and prevent corrosion of the workpiece.
MWFs often are complex mixtures of several hazardous ingredients, including oils and other lubricants, detergents, surfactants, biocides, anticorrosive agents, and other additives. Additional hazards may be created during normal use if fluids become contaminated with tramp oils, degreasers, or other chemical products used at the facility. Frictional heat and pressure at the point of operation can partially convert certain additives to nitrosamines, a potent human carcinogen.
Additionally, the water-based MWFs, in particular, provide a good growth medium for mold and bacteria. Though the risk of direct infection is slight, exposure to these organisms or the endotoxins they produce may cause illness or allergic sensitivity.
Millions of workers employed in manufacturing industries are exposed to these fluids. Occupational exposure to MWFs has been associated with certain respiratory diseases, skin disorders, and various types of cancer. This article discusses these hazards and suggests ways in which workers can be protected.
First, however, is a brief introduction to MWFs and their use.
MWFs are grouped into one of four classifications based on their composition:
1. Straight oils. Straight oil MWFs are made of highly refined lubricant-base oils. Animal, vegetable, or synthetic oils may be used instead of, or in addition to, petroleum-base oils. Straight oils may or may not contain performance-enhancing additives. These fluids are not diluted with water.
2. Soluble oils. These fluids are lubricant-base oils (30 to 85 percent) with emulsifiers (which allow the oil to mix with water) and other additives. For use, soluble oils typically are diluted with five to 40 parts water.
3. Semisynthetics. These fluids contain less lubricant-base oils than soluble oils (5 to 30 percent) and have additional emulsifiers and performance-enhancing additives. Semisynthetic concentrates usually are diluted with 10 to 40 parts water for use.
4. Synthetics. Synthetic MWFs contain no lubricant-base oils. Simple synthetics contain additives that provide for corrosion resistance and heat removal. They may also contain synthetic lubricants. Synthetic concentrates typically are diluted with 10 to 40 parts water for use.All four classes of MWFs may contain a variety of additives to enhance performance, including biocides; emulsifiers; detergents; surfactants; antimist, antifoam, and antiweld agents; and additives to control pH.
MWFs are delivered to the point of operation by pump and are sprayed, dripped, or flooded over the tool and workpiece. In small operations, the fluid usually flows by gravity to an individual sump located near each machine, where it is filtered and pumped to the point of operation. Larger operations often use a central system that provides fluid via piping to each machine, and troughs in the floor return used fluid to a central reservoir.
The application of MWFs can create airborne mists. Mists can be produced mechanically by spraying or by contact with rapidly spinning tools or parts. High-speed sawing and machining may also can create a thermal vapor of MWFs at the point of operation, which then condenses to form a mist in the cooler surrounding air.
Workers are exposed to MWFs by inhaling mists or vapors or by direct contact with the skin. The potential for airborne exposure depends on the quantity of mist generated, how the operation is enclosed, and the local exhaust ventilation. Skin contact with MWFs can occur when an operator is setting up the machine, filling reservoirs, mounting or removing parts, changing cutting tools, or maintaining the machine.
Many of the ingredients in MWFs are skin and/or respiratory irritants. Workers chronically exposed to MWFs are susceptible to skin disorders, asthma and other respiratory diseases, and possibly certain cancers. Adverse irritant effects to the eyes and mucous membranes also have been reported.
Skin Disorders. Workers exposed to MWFs suffer a high incidence of skin disorders. Generally, MWFs comprised of straight oils cause inflammation of the hair follicles on the hands and arms (folliculitis), oil acne, and keratoses. Semisynthetic, synthetic, and soluble MWFs are associated with irritant-contact dermatitis and/ or allergic-contact dermatitis.
The high incidence of skin disorders in workers using these MWFs indicates that many workers are susceptible to the irritating and sensitizing properties of the water-based MWFs. Though these skin disorders are seldom initially serious, many workers continue to work with MWFs until the situation becomes so severe that they are disabled or the disorder causes infection.
Respiratory Disorders. Worker exposure to MWF aerosols causes numerous respiratory effects, including hypersensitivity pneumonitis (HP), asthma, airway irritation, chronic bronchitis, and, occasionally, impaired lung function. MWF-induced asthma usually involves exposure to MWF chemicals that are known to be allergic sensitizers. However, even if no known sensitizing agents are present, MWF-induced asthma can occur in workers because of the irritant nature of the MWF.
Most cases of HP are reported by workers exposed to aerosols of MWFs that contain large amounts of water. Microbial agents or their toxins are suspected to be the cause of most reported cases of HP.
Cancer. Evidence suggests that chronic exposure to some MWFs is associated with an increased risk of cancer of the larynx, pancreas, skin, rectum, scrotum, and bladder. The onset of a cancer following exposure to the cancer-causing agent may take 20 years or more. Therefore, it is difficult to establish a specific cause-effect relationship between a particular MWF ingredient and a resulting cancer.
Recently, efforts have been made to eliminate from MWFs chemicals that are known to cause cancer. Examples are the refining of straight oils to eliminate polynuclear aromatic hydrocarbons (PAHs) and reducing the use of chemicals that contribute to the formation of nitrosamines. Chemical changes to MWF formulations likely will reduce future incidents of cancer in workers exposed to MWFs. However, new chemicals used in the manufacture of MWFs could pose new cancer hazards decades from now. Therefore, it is important to minimize, as much as possible, worker exposure to MWF aerosols.
Currently, the Occupational Safety and Health Administration (OSHA) does not have a substance-specific regulation for MWFs. Worker exposures to fluids containing petroleum-base oils are evaluated using the permissible exposure limit (PEL) for mineral oil mist of 5 milligrams per cubic meter of air, as an eight-hour time-weighted average exposure. For synthetic MWFs, the nuisance dust PEL of 15 milligrams per cubic meter of air applies.
Exposures can be evaluated by sampling full-shift oil mist or dust concentrations in the personal breathing zones of machine operators and setup and maintenance employees. Samples are collected on a filter using a battery-operated personal sampling pump. Area sampling also may be useful in evaluating the effectiveness of ventilation and other engineering controls.
OSHA recognizes that impaired health is possible at exposure levels well below the currently enforced PELs. Accordingly, OSHA has designated metalworking/machining fluid as a priority area for new comprehensive rulemaking.
Because the use of MWFs is essential to parts manufacturing, their harmful effects on workers must be controlled.
Some exposure conditions are better today than ever before because of improvements in MWF chemistries. For example, petroleum-base MWFs are more highly refined than they were decades ago, thereby reducing or eliminating polycyclic aromatic hydrocarbons and other aromatics, many of which are suspected carcinogens.
However, the long-term chronic health effects of new and different chemical formulations may not be fully understood. A fairly common component of synthetic and other water-based MWFs is a chemical group called ethanolamines. These chemicals are irritating to the skin and may contribute to dermatitis in many workers. Some ethanolamines are themselves suspected carcinogens; under certain operating conditions, they may convert to nitrosamines, which are known carcinogens.
There has been a general move from using straight oils to using synthetic or semisynthetic MWFs over the past several decades as tool speeds have increased. Concurrently, technological advances in industrial engineering and safety practices have provided for enclosures and better ventilation for metalworking processes. As a result, studies of automobile workers indicate that exposures to airborne MWFs have steadily declined since the mid-1950s.
More recently, however, there may be a reversal in these trends. As energy costs increase, plants strive to become more energy-efficient. As a result, there is an economic incentive to reduce general ventilation.
Furthermore, some cost-conscious companies have reduced their maintenance staffs. Without regular preventive maintenance, some local exhaust ventilation systems may no longer function as designed. The grouping of manufacturing functions into cells, in which one worker operates several machining or sawing machines, for example, may overextend local exhaust systems. Besides the negative impact on the ventilation systems, cost cutting efforts may adversely impact the maintenance of machine enclosures, splash guarding, and even the maintenance of coolant quality.
The most effective means to control worker exposures to MWFs is through the use of engineering controls. The use of less hazardous MWFs is an obvious example. Selection of MWFs should be based both on performance of the MWF and the associated hazards. The proper MWF must be selected for each machine, and each application method should be chosen to minimize mist production.
Employee exposures also can be controlled with local exhaust ventilation. A properly designed ventilation system can nearly eliminate hazards posed by airborne MWF mists. Local exhaust ventilation systems should be tested periodically to ensure that the systems are operating as designed. Splash guards or enclosures should be installed around high-speed cutting machines to protect the operators.
Administrative controls are changes in work procedures that reduce or eliminate workplace hazards. Administrative controls include improvements in MWF maintenance and handling, as well as employee training.
Synthetic, semisynthetic, and water-soluble MWF concentrations should be maintained at the dilution level recommended by the coolant manufacturer or supplier. Concentration usually is measured with a refractometer. Coolants that are too "rich" are more irritating to the skin. Conversely, coolants that are too "lean," besides not offering protection to the part and machine, may be more susceptible to microbial growth.
As much as possible, the coolant system should be kept free of tramp oils. Sources of tramp oils include antirust oils applied to parts, leaking hydraulic oils from the machines, gearbox oils, bearing grease, etc. Tramp oils can clog the coolant pumps and filters, affecting efficient delivery of the coolant to the point of operation. Additionally, tramp oils on the surface of coolant tanks prevent air from reaching the coolant, thereby promoting the growth of some bacteria.
Many companies manufacture equipment designed to recycle MWFs while removing oils. These machines usually incorporate a continuously running belt or wheel that skims oil from the tops of coolant sump tanks. Biocides can be used to prevent bacteria growth, but they cannot kill existing bacteria. Coolants with heavy biological contamination should be discarded and replaced.
Central coolant systems in which one large volume of MWF is distributed by pumps and piping to several machines may be susceptible to spills or leaks from, for example, degreasing operations. Extra care should be taken to ensure that any spills or oil leaks from machinery are not allowed to reach the central system. The use of oil-absorbent pads around machinery can help control this problem.
Many floor cleaning chemicals contain solvents or pine extracts that can adversely affect the MWF if they are permitted to contaminate central system troughs or pits. Maintenance and janitorial employees should be trained to prevent this.
Employers must establish safety and health training programs to train both machine operators and maintenance personnel on the proper handling and hazards of the various MWFs. Training on MWF hazards should be incorporated into the company's comprehensive hazard communication program.
Worker training should include all the elements of an OSHA-acceptable hazard communication program, as well as the following elements:
1. Methods and observations that may be used to detect the presence or release of a hazardous chemical in the work area.
2. The physical and health hazards of the chemicals (both MWFs and metals).
3. Measures workers can take to protect themselves from these hazards, including proper work practices, proper personal protective equipment (PPE), and good personal hygiene.
4. The details of the employer's hazard communication program, including the location of the program and Material Safety Data Sheets (MSDSs), and an explanation of hazard warning labeling.
PPE is necessary when engineering and administrative controls do not totally eliminate workplace hazards. PPE should be used to reduce or eliminate skin contact as much as possible. Machine operator PPE should include face shields or splash goggles, protective gloves, sleeves, and splash aprons as appropriate to protect skin from contact with MWFs. Maintenance personnel who handle concentrated coolants and/ or biocides also should wear this equipment when handling these chemicals.
The MSDS and warning labels for the MWF provide information on the proper PPE to use when handling or using the product. The MSDS guidelines should be followed, and the safety supplier can be contacted for advice on which PPE material offers the best chemical resistance to the MWFs in use.
Glove wearing may not be suitable for some operations, particularly if rotating machinery can snag an operator's gloves and cause serious injury. In these cases, some employers prefer to use barrier creams. The effectiveness of these creams is limited, however, and the manufacturer's instructions should be closely followed.
Respiratory protection should not be relied upon to protect machine operators unless all other efforts to control exposures have failed or if engineering controls, such as ventilation, must be temporarily interrupted. Maintenance personnel may be required to wear respiratory protection when mixing MWFs.
Employees wearing respirators are required to participate in a respiratory protection program, including medical clearance and training.
Workers should be encouraged to clean contaminated skin regularly with mild soap, clean water, and soft towels. Scouring and harsh soaps should be avoided because they can scratch the skin and make it more susceptible to absorption of the MWF. Heavily contaminated clothing should be removed before the employee leaves work.
In some instances, it may be necessary to institute an active medical surveillance program for persons exposed to MWFs. The objective of a medical surveillance program is to identify, as early as possible, persons who develop symptoms of conditions associated with MWF exposure, including asthma, HP, and dermatitis.
Early identification of MWF-related conditions will allow workers to control the exposures. This minimizes their risk of developing recurring problems such as acute asthma and irritant-contact dermatitis or chronic conditions such as permanent lung function impairment and allergic-contact dermatitis.
Incidents of dermatitis and adverse respiratory symptoms must be reported by all workers exposed to MWFs, and accurate, detailed records must be kept.
Metalworking fluids are a necessity for many parts manufacturing operations. Careful selection, use, and maintenance of fluids and fluid-handling systems, along with proper application of engineering controls, administrative controls, and personal protective equipment, will minimize worker exposure to these potentially hazardous materials.
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