**4. Sustainability of CO2 Coolants**

Although the performance benefits of cryogenic cooling strategies are highly contingent upon a range of a factors, from a sustainability standpoint, their benefits are much more widespread. Whilst emulsion cooling has been able to meet a range of performance indices (i.e., long tool life, low surface roughness and cutting forces), it remains a highly imperfect solution from an ergonomic and environmental perspective. So much so that a large portion of the research into cryogenic cutting fluids is primarily motivated by the ultimate unsustainability of conventional MWF's rather than a commercial desire to reduce operating costs (although, this is clearly a significant auxiliary benefit). As such, the following section examines a range of cited sustainability issues associated with the use of conventional MWF's, and thereafter explores the ways in which these issues are entirely, or partially, mitigated by cryogenic MWF strategies. Moreover, the section will additionally outline the processing route by which CO2 is obtained, as well as address any concerns associated with the abundant release of CO2 into the atmosphere (as is associated with its use as an MWF).
