In 1984, the International Agency for Research on Cancer (IARC) reviewed petroleum lubricant base oils and products derived from them. IARC noted in their review the following:

The processes used to produce lubricant base oils and, correspondingly, product formulations have changed considerably over the years. Until about 1940, processing consisted of acid refining with clay finishing and subsequent dewaxing by chilling. Solvent refining (and solvent dewaxing) was first introduced into the USA and in Europe in the 1930s. Hydrotreating, as a newer, more severe process than "hydrofinishing" was introduced in the 1960s. The trend has been to more highly refined oils with associated removal of impurities including polynuclear aromatic compounds.

Since the late 1950s, most mineral oils used in the formulation of MRFs have undergone severe solvent extraction and/or severe hydro treatment and, where properly used, these treatments essentially eliminate the carcinogenic potential of the base oil. Modern severe refining techniques of solvent extraction selectively remove PACs from base oils, and the hydrogen treatment chemically disrupts the ring structures of PACs, producing non-ring compounds.

The recycling of used industrial oils is becoming more popular as it avoids much of the cost of purchasing new oils and almost all disposal costs. However, research has demonstrated that some reprocessed oils may have unacceptably high levels of polycyclic aromatic hydrocarbons (PAH). Unless the oil reprocessing involves severe refining techniques, it is important that all reclaimed oil destined for reprocessing be tested; any showing high levels of PAH should be segregated, and not processed for reuse as a manufacturing lubricant.

It is now possible to routinely produce base oils for MRFs that have been shown, through a variety of bioassays and screening tests, to be non-carcinogenic. However, oil-based MRFs may be manufactured from a variety of sources, including reprocessed oils. Therefore, it is recommended that organizations purchasing base oils for use in MRFs request information from suppliers on the severity of treatment of those products prior to their purchase and use. Analytical methods which measure the concentrations of polycyclic aromatic compounds in base oils are the Institute of Petroleum IP 346 test, ASTM Method E-1687-95, "Determining Carcinogenic Potential of Virgin Base Oils in Metalworking Fluids," the Food and Drug Administration ultraviolet absorbency test and other tests for total polycyclic aromatic content. The March 11, 1997 issue of Lubes ’n’ Greases Magazine discusses a recent review of the scientific literature concerning the potential for used MRFs to develop and accumulate levels of PACs that might represent a hazard. Based on available data, the article concludes that there is little scientific basis for health concerns from the generation of carcinogenic PACs in straight oils used in metalworking.

Diethanolamine has been a common alkanolamine in many metal removal fluid formulations. It is produced as a primary chemical and it is a contaminant in commercial grade triethanolamine. Concern over liver and kidney target organ effects in animals has caused use of diethanolamine in metal removal formulations to decline. Recently diethanolamine has been associated by NTP with liver and kidney effects in laboratory animals. While this chemical has not been listed as a human carcinogen, most suppliers of metal removal fluid have eliminated it as a primary component of fluids and they are working to reduce its concentration as a contaminant from triethanolamine materials.

Selection of the most effective biocide for a particular operation is a key decision, and should depend on an analysis of the most important considerations involved. For instance, is microbial contamination the source of a particular problem and, if so, is a biocide the right way to control it? Knowledge about the specific microbiological population and the effect of specific biocides on each microbial species would be useful, but is usually not available. What is the cost effectiveness of other means to control biological growth compared to the use of biocides? And, finally, what are the risks presented to potentially exposed employees by various biocide products?

A thorough understanding of the system to be treated is important in determining the most effective product and the concentration that will achieve the desired results without potentially having adverse consequences among exposed workers. Good microbiological control is important, as biocides may destroy bacteria and fungi but not their associated toxins. Typically, a MRF system will require both a bactericide and a fungicide to prevent contamination problems, although some chemicals can act against both types of microorganisms. It is important to select the correct biocide and appropriate concentration for the problem at hand. The use of dipsticks can show operators which type of contamination is prevalent. Dosing with the appropriate concentration is important, because maintaining a system with low levels of biocide can promote the development of microbial resistance to that biocide. Also, if a single biocide for both bacterial and fungal control is used, too low a concentration may only knock back the faster growing bacteria, leaving the fungi unaffected and free to continue growing. A common practice, the mixing of multiple biocides in MRF systems, may complicate management of systems and should be carefully considered.

Many biocides have as a key component chemicals known as formaldehyde-condensates, which release formaldehyde in the bacterial cell in use. These products have received close attention since formaldehyde was identified as a suspect carcinogen. Formaldehyde and formaldehyde- condensates are in general quite toxic, but all biocides should be considered potentially toxic to humans at high concentrations. A comprehensive recent study of formaldehyde in the air in plants using triazine as a biocide in the MRF found that employees working in a variety of metal removal operations were not exposed to formaldehyde at or above the OSHA Action Level of 0.5 PPM or to concentrations that would exceed the Short-Term Exposure Level of 2 PPM. The exception to this general finding was for employees working for extended periods around poorly ventilated sumps.

Where employees are exposed to formaldehyde gas, its solutions, and materials that release formaldehyde, OSHA’s Formaldehyde Standard, Code of Federal Regulations Title 29, Part 1910.1048, applies. Employers are required to make Medical Surveillance (1910.1048(l)) programs available for all employees exposed to formaldehyde at concentrations at or above the Action Level of 0.5 PPM, or the Short-Term Exposure Level of 2 PPM (1910.1048(l).

Many biocides are human sensitizers and, depending on individual variation and conditions of exposure, may have the potential to cause problems even if concentrations are not excessive. It should be noted that toxicological dose responses are typically not linear. A thorough understanding of the particular conditions a biocide is intended to correct is important in determining the most effective product and concentration that will do the intended job and not have potentially adverse consequences among exposed workers. This is especially important as some biocides may destroy bacteria and fungi, but not their associated endotoxins. Since bacteria are in general more susceptible to some biocides than fungi and yeasts, it is important to be sure that both the correct biocide and appropriate concentrations are used. Too low a concentration of biocide may result in differential growth of fungi. A common practice, the mixing of multiple biocides in MRF systems, may complicate management of systems and should be avoided.

Mist suppressants are dissolved polymers that work by increasing the elongational viscosity or elasticity of the fluid, increasing the size of the mist droplets and thereby reducing the amount of suspended mist. Examples of suppressants are polyisobutylene (PIB), used with straight oil, and polyethylene oxide (PEO), polyacrylamide, and sulfonate polymers used in soluble oils and synthetics. The amount of polymer needed varies with the MRF and the polymer type, generally in the range of 100 to 1000 PPM. Mist reductions of approximately 50 percent are generally expected, with as much as a 90 percent reduction with PIB in certain applications.

Commercially available polymers used for mist suppression are broken down by shear force in the machining and pumping processes. Polymer levels must be monitored and adjusted. In some cases, a constant addition is required. The additives should not adversely effect the machining operation. No adverse effects on tool wear, part finish, filter life, etc., are acceptable. It should be noted that PEO has some potential for increased Chemical Oxygen Demand (COD), which may affect water treatment. These mist suppression polymers may also affect filter life.

The generation of oil mist levels in association with high-speed machine tool operations where straight oils are used as a major component of the MRF can be a serious health and fire safety problem. Straight oils atomize readily and produce aerosols that remain in the ambient air for long periods. Straight oils are often necessary to achieve a high-quality surface finish, and thus are an important component of modern quality machining operations. One promising approach to reducing the amount of oil mist generated by machining operations involves the use of high molecular weight polymers that are dissolved in the MRF in very low concentrations.

It should be noted that this technique has not worked equally well in all systems. Experimental work performed on dilute solutions of polyisobutylene in mineral oil demonstrated that, when atomized, they produced droplets that were on average much larger than oil without the polymer. With a larger average size of droplet, there are fewer drops generated, and their settling rate is greatly increased. In one study, where 20 parts per million of polyisobutylene were added to oil-based MRFs, oil mist concentrations were reduced by 80-90% with no adverse effects on the quality of the machining. This mist suppression can be achieved at a cost of $10 to $100 per week for a 10,000-gallon system. Work is progressing on a similar system for water-based MRF.

Useful toxicity references include ASTM Guide E 1302, which describes procedures to assess the acute toxicity of water-miscible metal removal fluids, and ASTM Method E 1687, which describes a test method to screen for the presence of potential skin carcinogens in metal removal fluids.

Metal removal fluids contain a variety of organic components of varying vapor pressure. Some of the more volatile compounds may be considered volatile organic compounds (VOC) in your state. Selecting fluids without highly volatile components may reduce regulatory permitting or reporting requirements. Most water-diluted metal removal fluids will not contain obvious VOC materials. However, when tested using methods developed for water-based coatings such as ASTM Method 2369 (Standard Test Method for Volatile Content of Coatings) or Method 24 of 40 CFR part 60, Appendix A, they often give non-zero results. Many plants, especially the auto industry, must use these results to calculate their VOC emissions under the Clean Air Act. Consult the manufacturer for information regarding VOC in light of local, state, and federal air emissions regulations.