What are Metalworking Fluids?

Metalworking fluids (MWFs) refer to fluids used during grinding and machining and include four classes of MWFs: straight oil, soluble oil, semisynthetic, and synthetic MWFs. MWFs serve to prolong the life of the tool, carry away metal debris that results from cutting and grinding, protect the part being produced, and carry heat away from the ground or cut surface.

Occupational exposures to MWFs are a concern and need to be controlled. Repeated inhalation of MWF mists can cause respiratory illnesses and asthma, leading to non-reversible lung damage. Additionally, contact with skin can cause various skin diseases, most notably allergic dermatitis.

Types of MWFs

MWFs are grouped into four categories. The first category is “straight” oil (neat oil) MWFs. These oils are solvent-refined petroleum oils, animal, marine, vegetable oils, or synthetic oils that are not diluted with water. The straight oils may or may not contain additives that may include corrosion inhibitors, emulsifiers, buffers, and extreme pressure additives.

The next class are the soluble oils (emulsifiable oils), which are between 30 to 85 percent severely refined lubricant base oil with emulsifiers and may also contain additives. Soluble oils also contain a small percentage of water.

The semisynthetic class contains less of a percentage of the severely refined lubricant base oil (5-30%) and contains a higher percentage of emulsifiers and up to 50 percent water.

The final category is synthetic MWFs, which contain no petroleum oils and are diluted with 10-40 parts of water.

Occupational Exposure

Occupational exposures to MWFs occur either by inhaling mists and aerosols or from direct skin contact. Besides the MWF aerosols and the added additives or biocides, exposure risk can include metals and alloys from the parts ground or machined. Additionally, background contaminants in the work area and bacterial or fungal contamination in the water component of MWFs pose a risk.  

The National Institute of Occupational Safety and Health (NIOSH) has established a Recommended Exposure Limit (REL) of 0.5 milligrams per cubic meter (mg/m3) for an eight-hour time-weighted average exposure. The REL applies to all categories of MWF. In addition to NIOSH, the Occupational Safety and Health Administration has established a Permissible Exposure Limit (PEL) of 5 mg/m3, which only applies to mineral oil-based MWFs. The NIOSH REL is based on regular and recent reviews of health hazard data and is the occupational exposure limit referred to for best practices.

During machining operations, the risk of Inhalation exposures is greater depending upon how close the worker is to the machine, whether there is an enclosure or splash guard, and whether the machine is operating at a high speed and making deep cuts. Other factors contributing to exposures during machining or grinding are whether there is exhaust ventilation installed to capture mists and aerosols near the point of generation and whether the exhaust system has been poorly designed or maintained. Additionally, improper machine maintenance can result in excessive fluid application or contamination of the oil with machine tramp oils.

In addition to inhalation, skin contact is a major concern with respect to exposure. Exposures occur when there are not enclosures or guards to protect from splashes during machine operation, and from handling parts, tools, or equipment without personal protective equipment (gloves and aprons), or prolonged contact with clothing contaminated with oil. 

Health Effects

The NIOSH REL was established to address non-malignant respiratory disease. MWF concentrations above the REL in nine out of ten studies have been shown to cause respiratory conditions, including hypersensitivity pneumonitis (HP) which produces flu-like symptoms, acute airway irritation, chronic bronchitis, impaired lung function, and asthma. Cases of HP have been linked to water-based or diluted oils, and microbial contamination is believed to be the most likely cause. Three recent studies have shown that the risk of developing asthma is elevated and up to three times greater for workers exposed to synthetic MWFs than for workers who are not exposed.

A variety of skin diseases can result from direct skin contact with MWFs. The following factors contribute to the development of disease: 

  • The MWF category and additives used
  • The duration of the skin contact
  • An existing skin abrasion or cut
  • Individual susceptibility
  • Inadequate or infrequent skin cleansing following contact
  • The use of irritating soaps or detergents
  • High or low humidity, hot or cold temperatures
  • Wearing MWF-soaked clothing or handling soaked rags.
  • The general cleanliness of the surrounding work area
  • Lack of controls i.e., machine enclosures, exhaust ventilation, personal protective equipment (gloves, sleeves, aprons)

The most reported skin diseases are irritant contact dermatitis or allergic contact dermatitis. The latter type of dermatitis is caused by an immune response to a substance that the body has become sensitized to. Dermatitis causes skin lesions and discomfort from burning and itching. Dermatitis is common with contact with soluble, semisynthetic, and synthetic MWFs.

Other skin diseases, including folliculitis (hair follicle infection), oil acne, and keratosis (rough, scaly skin patches), are linked to contact with straight oil MWFs.

Controlling Exposures to MWFs

The machine tool industry has undergone major changes in recent decades, leading to significant exposure reductions. The changes made have included the increased use of synthetic MWFs, which have increased tool and cut speeds which reduce machining time. Technological advances have also been made, leading to the development of machines with partial enclosures and the installation of local exhaust ventilation. During the 1970s and 80s, industries began installing air cleaners (mist collectors) and improving the recirculation of air and filtration.

Recommendations for Controlling Exposures

NIOSH recommends developing and implementing a comprehensive health and safety program to control exposures to MWFs. For programs to succeed, management must have a strong commitment and include worker involvement. The four main components recommended are safety and health training, worksite analysis, hazard prevention and control, and medical monitoring of exposed workers.

Worker training programs should teach workers to identify potential exposure hazards in their work areas and the adverse health effects of MWF exposure.

Worksite analysis refers to monitoring work practices and assessing personal exposures (air sampling) to assess the effectiveness of controls.

Hazard reduction can be achieved by the proper selection of MWF, i.e., using the most non-irritating, non-sensitizing fluids, regular fluid maintenance, isolation of the operation, and installation of exhaust ventilation.

With respect to PPE, workers should be trained in the proper use and care of protective equipment. If personal exposure assessment indicates that respiratory protection is needed, a respiratory protection program must be established in accordance with the OSHA Respiratory Protection standard (29 CFR 1910.134). 

For more in-depth information about MWFs and controlling exposures, consult NIOSH publication 98-116, Occupational Exposure to Metalworking Fluids  

Mary Dunlap is a Certified Industrial Hygienist and has been with Cornerstone Environmental, Health and Safety since 2016. When she is not working, you can find Mary enjoying the outdoors.

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