*4.2. Social Implications of CO2 Coolants*

As an aside to the environmental limitations of conventional MWFs, their usage is equally accompanied by a range of negative health repercussions, persisting during handling, use and ultimate disposal. Whilst it is true that CO2 usage as an MWF (and elsewhere) creates a non-negligible risk of asphyxiation, this health concern can largely be mitigated by reasonable precaution such as the use of proper ventilation and local O2/CO2 monitoring systems. By contrast, conventional MWFs present a much broader encompassing range of health concerns, elevated in part by their capacity for contact exposure via a range of media (primarily dermal, ocular and via inhalation). As dermal contact has been shown to account for as much as 80% of the occupational risk of MWFs [14], it is unsurprising that research into the negative health implications of cutting fluids has, historically, been heavily focused upon the deleterious effect of MWFs on skin health. One 2014 systematic review [15] notes currently established causal links between cutting fluids and the following skin conditions: irritant contact dermatitis, allergic contact dermatitis, folliculitis, oil acne, keratosis and carcinomas. Despite these concerns, much of the risk to skin health can effectively be mitigated by the combined protocol of wearing suitable gloves and adopting a thorough hand cleanliness routine. Whilst this strategy is simple, skin conditions such as those outlined previously are often extremely prevalent in the manufacturing sector. This is likely a consequence of poor adherence to the PPE guidelines, an assertion supported by a 2012 survey that found that 82% of safety professionals had observed a failure to use the requisite PPE in their workplace [16]. Clearly it is unrealistic to expect universal adherence to even the simplest of PPE guidelines, and as such, the negative dermal implications of conventional MWF's persist where they could otherwise be avoided by employing cryogenic cooling strategies.

In addition to the development of skin conditions, MWFs are linked to a range of ocular and respiratory conditions of varying severity. In general, the respiratory conditions associated with MWFs exposure are primarily a consequence of the inhalation of a fine particulate mist, which is generated as the MWF evaporates. Thereafter, either the MWF suspension or a fine atomised aerosol of non-aqueous MWF constituents (as water and water-soluble constituents are vaporised) is inhaled by individuals proximal to the machine. The consequences of which manifest as a range of negative health implications largely, but not entirely, focused upon the respiratory tract, including: airway irritation, chronic bronchitis, asthma, laryngeal cancer, alveolitis, and more abstractly, cancers of the pancreas, rectum and prostate [15]. Although these concerns could, in part, be addressed by the implementation of respirators, or even by assuring proper machine shop ventilation, this remains far from the industrial norm, with respirator use in machining typically being confined to grinding scenarios, wherein the goal is simply to filter out machined chips/dust, rather than MWF mist. Although these issues are perhaps the most severe of the negative health implications associated with MWF exposure, eye irritation, though non-permanent, is equally common amongst machinists. This ocular exposure to MWF can be extremely

painful, particularly when contact results from direct splashing of the eye (as opposed to airborne particulates) [17], and necessitates the use of safety glasses. Whilst the use of safety glasses should equally be encouraged during cryogenic machining (owing to the risk of swarf projection), it is reasonable to assume that both the respiratory and ocular risks of cryogenic MWFs are invariably reduced.
