Anyone who has worked in a shop where machining of metal parts is the primary business knows that these operations tend to be noisy and "smoky." During my 21 years of industrial hygiene consulting practice, I've been in many machine shops where a visible, smoky haze is present (usually it can be seen by looking at lighting fixtures near the ceiling). This smoky haze generally consists of metalworking fluid mists and vapors.

Metalworking fluids (MWFs) are used during machining and grinding of metal parts to prolong the life of the tool, carry away metal chips and other machining debris and to protect the surface of the work piece. These fluids reduce friction between the cutting tool and the work piece, reduce wear and dissipate heat generated by the machining process.

The first generation of MWFs included sperm whale oil, mineral oils and various types of petroleum oils. These products were in use from the beginning of the Industrial Revolution up until the early 1940s.

With the changes in manufacturing brought about by World War II (and the declining availability of sperm whales), the second generation of MWFs began to appear, including the first water-soluble and semi-synthetic MWFs. By the 1970s, water-soluble and semi-synthetic MWFs were in common use throughout the metalworking industry.

The advent of high-speed machining operations, new metal alloys and closer tolerances for machined parts brought about the current generation of MWFs, which began about 1985 with the introduction of amine-based, reduced-hydrocarbon, synthetic MWFs.

Today, health and safety professionals may encounter all three generations of MWFs, even within a single shop. MWFs in common use today are classified into four general types:

1. Straight Oils/Neat Oils. These MWFs are severely solvent refined petroleum (lubricant-based) oils, or other animal, marine, vegetable or synthetic oils, used alone or in combination, and with or without additives. These MWFs are not designed to be used with water.

2. Soluble Oils. These MWFs are combinations of severely refined straight oils and emulsifiers. Other additives may be included. Soluble oils are intended to be mixed with water before use.

3. Semi-Synthetics. These MWFs contain from 30 percent to 50 percent water (they also have lower amounts of straight oils and higher amounts of emulsifiers). Semi-synthetic oils may also contain other organic compounds, such as amines (i.e., triethanolamine).

4. Synthetics. These MWFs contains no petroleum oils (they are generally designed to be mixed with water before use).

Occupational exposure to MWFs occurs from the inhalation of the fine mists/aerosols that often are generated in machining processes and from direct skin contact. It is estimated that more than 1.2 million workers are currently exposed to MWFs in the United States.

Health Concerns

There are several heath concerns that have been associated with exposure to MWFs, including:

  • Dermatological (skin) disorders, including contact dermatitis.
  • Carcinogenisis (of various types, including cancer of the larynx, rectum, pancreas, skin, scrotum and bladder).
  • Pulmonary (lung) disorders, including hypersensitivity pneumonitis, and MWF-induced asthma.

It is not unusual to see dermatitis in workers exposed to MWFs. Workers who handle metal parts after machining often experience skin irritation and/or dermatitis on their forearms, chests and abdomens (when metal parts are in direct contact with the body during handling/loading). The cause of the irritation/dermatitis may be chemical, biological or mechanical (used MWFs also contain very fine metal particles).

Other factors that play a role in the development of contact dermatitis and other skin diseases include (1) the type of MWF used, (2) the amount of skin contact (including the use/reuse of MWF-soaked clothing), (3) individual susceptibility to irritants or allergens present in MWFs, (4) personal hygiene/inadequate cleansing of the skin after contact, and (5) the use of impervious personal protective equipment.

Cancers generally are associated with the second generation of MWFs. Two factors associated with these MWFs seem to be in play. Nitrating compounds (used for microbial control) can form carcinogenic nitrosamines when combined with amines, which are used in MWFs. Even though nitrating compounds currently are not recommended for use in amine-containing MWFs, it is prudent to check the material safety data sheet (MSDS) for the MWF (and all additives) to ensure that they are not present.

The other factor that affected the carcinogenicity of second generation MWFs was the refinement of the straight oils and other oils that were used. Oils that were not "severely hydrotreated" (as are most current oils) sometimes resulted in polynuclear aromatic hydrocarbons (PAHs) being present in the MWFs. Many PAHs are carcinogens.

Pulmonary Disorders

Pulmonary disorders are perhaps the most interesting (and least understood) health concerns associated with MWFs. A variety of lung diseases have been associated with MWFs, including occupational asthma and hypersensitivity pneumonitis (HP).

Occupational asthma is triggered in affected individuals by something in the workplace. Like non-occupational asthma, individuals with this condition often experience asthma-like symptoms following workplace exposures at concentrations that do not cause similar symptoms in other workers.

HP usually is associated with the toxins produced by the growth of certain bacteria in MWFs. Symptoms of HP include inflammation of the lung tissues, shortness of breath, dry cough and weight loss. Outbreaks of this disease can occur when the delicate balance of microbial growth and control in MWFs is altered.

Evidence linking MWFs to pulmonary diseases and cancers still is evolving. Additional details concerning potential health effects associated with MWFs are outlined in the Final Report of the OSHA Metalworking Fluids Standards Advisory Committee (7/15/99). This reference is available at

The National Institute for Occupational Health and Safety (NIOSH) has conducted Health Hazard Evaluations following outbreaks of MWF-associated disease. Additional MWF information can be accessed on NIOSH's Web site at

Exposure Controls

MWFs should be thought of as a tool, similar to the machine tools in which they are used. Like any tool, proper care and maintenance will help ensure trouble-free use. Also, like other tools, neglect of MWFs can result in adverse consequences, which can result in occupational diseases in exposed employees.

Maintaining MWFs is as much an art as it is a science. Too much microbial control (such as adjusting the pH too high/low or adding too much biocide) can result in dermatitis, or eye and/or pulmonary irritation. In particular, too much biocide added into MWFs can result in a variety of overexposure symptoms. Many biocides are regulated as pesticides by the Environmental Protection Agency (EPA). A "pesticide applicator" license may be required by various states for individuals applying the biocides.

Of course, insufficient microbial control, including the use of the wrong type of biocides or the application of biocides at the wrong time, can result in outbreaks of dermatitis and HP.

Many companies assign one or more staff members to maintain MWFs. Maintenance of MWFs includes regular microbial, pH and biocide level testing; a "sniff" test to detect rancidity; visual observations to detect color changes and the presence of "tramp" oils floating on the surface of the MWF; filter tests to detect fine metal particulate; and comprehensive visual inspections of machine sumps, MWF galleries, catch pans and other MWF system components to detect irregularities. These services may be contracted to other firms.

The most common method of controlling airborne exposures to MWFs is enclosing machining operations to prevent MWF aerosols from entering the workplace environment. Enclosures combined with local exhaust ventilation provide the greatest control of MWF aerosols. Another control method includes the use of selected additives to reduce the potential for MWFs to be dispersed as airborne mists (by increasing the droplet size of generated mists).

Skin contact is controlled via the use of impervious gloves, "splash" aprons and other personal protective equipment when handling MWFs and coated parts.

An integrated written MWF management plan incorporating these types of exposure controls is strongly recommended.

Health and safety professionals responsible for facilities with MWFs need to keep a close watch on how MWFs are selected, used and maintained. OSHA, NIOSH and other national and international groups have published "best practice" guidelines for MWFs. MSDSs for MWFs (and their additives) should be reviewed regularly to familiarize yourself with the specific compounds that are used. Be sure to ask questions about how MWFs are maintained in your facility and, in particular, find out how microbial control and monitoring are performed.

Thomas J. Martin, CIH, CSP, is manager, occupational health and safety, for the Detroit and Cleveland regional offices of Clayton Group Services ( He has 20 years of experience as an occupational health and safety consultant. He specializes in safety and industrial hygiene, with emphasis on industrial operations. His primary focus at Clayton is OSHA compliance audits, safety-related evaluations, health and safety training course development and delivery, emergency response planning, accident investigations and industrial hygiene studies. Martin has taught many health and safety-related courses, including HAZWOPER, Incident Command and machine guarding.