*3.1. Adverse Effects of Mineral Oil-Based MWFs*

Almost all of the available MWFs are derived from petroleum products, and the elements that are present in mineral oil-based MWFs are the major cause of the moisture and oil smoke observed during machining operation, which causes an uncomfortable environment for machine operators [31]. The health and environmental aspects are of the utmost importance in most countries, and regulations pertaining to the use of MWFs have aimed to guarantee the health of workers and to protect the environment [32,33].

There are several methods to apply MWFs onto the interface of the workpiece and the tool. This can be in the form of flooding, through a jet, or through mist in the several directions, as illustrated in Figure 4 [34]. In 1987, the International Agency for Research on Cancer (IARC) declared that the mineral oil-based MWFs, which were widely used in machining operation, were carcinogenic [31]. In an experimental investigation conducted on laboratory animals to investigate the toxicity of water-based MWFs by Bennett [35], it was reported that the specific additives and surfactants present in the MWFs caused cancer to the animals. In a review study by Park et al., the authors reported that nitrosamine and other amines in MWFs were carcinogenic, which are formed by the nitrates and are also used as corrosion inhibitors [33]. In 1984, the US Environmental Protection Agency (USEPA) fully banned the usage of nitrites that contained alkanol amines for cutting fluid, due to the detrimental effects they have on human health [31]. It was also reported in a review article that the mineral oil-based MWFs are composed of constituents which are suspected to be carcinogenic, and which favor the spread of tumors [33]. Any combination of sulfur, nitrosamines, long chain aliphatic compounds, formaldehydes, and Polycyclic Aromatic Hydrocarbons (PAHs) release biocide contaminants, which are also regarded as carcinogenic in nature, thereby posing a serious threat to machine operators [36,37]. The use of acid-refined MWFs results in the development of skin cancer. In order to decrease the PAHs present in crude oil, refining is performed. However, acid-refined MWFs contain a substantial amount of PAHs, which are a cause of skin cancer. Skin irritation is thought to be the most common health related issue resulting from the use of mineral oil-based MWFs during machining operation. These can be caused by the direct contact of the operator with MWFs [33,38]. It has been found that almost all of the mineral oil-based used in metalworking are found to have pH levels ranging from 9.5 to 11.0, where the higher acidic MWFs cause skin-related problems and, in the worst case scenario, can lead to skin diseases. Therefore, researchers around the globe are working to develop MWFs that can ensure the safety of workers and avoid any undesirable outcomes for machine operators [31].

**Figure 4.** The methods of supplying a lubricating medium to the work–tool interface, inserted from: (**a**) from the side of the rake face; (**b**) on the rake face; (**c**) on the flank and rake face [39].

Certain elements are added to enhance the properties of the mineral oil-based MWFs, e.g., sulfur, which increases the heat capacity of MWFs and also increases their ability to lubricate under extreme pressure conditions [40]. Another problem related to the health of machine operators is linked to the inhalation of MWF vapors, which has also increased since the increase in machining speed. The inhalation of mineral oil-based MWFs may lead to digestive problems and respiratory system diseases and may also lead to the development of cancers. Choi et al. [41] reported that the presence of dissolved ions of Co, Cr, and Ni in mineral oil-based MWFs are the potential source of skin disorders, and that many skins reactions occur when neat mineral oil-based MWFs are used.
