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The term ‘caisson disease’ has generally been replaced by dysbaric illness or decompression sickness (DCS) which represent the spectrum of potential problems experienced by those working under unusual pressure. The design and construction of tunnel boring machines (TBMs) are governed not only by soil conditions but now should cater for working at increased pressure and depth. In the right circumstances these machines have reduced not only the intense physical burden on the ‘miners’ but also limited pressure exposure to interventions into the excavation chamber for inspection and maintenance. Previously to limit ingress of water into the tunnel the whole underground site was kept at pressures above atmospheric. Ground conditions will determine the need for earth pressure balanced or slurry machines. Maintenance on the TBM’s cutter head and tools require hyperbaric conditions in the excavation chamber when pressures can exceed 3.5 bar. Deeper tunnelling uses tables and worker surveillance like those used for saturation diving. Breathing mixture is a non-air respirable mixture, such as oxygen and nitrogen (nitrox); oxygen and helium (heliox) or oxygen, nitrogen and helium (trimix) capable of supporting human life under appropriate hyperbaric conditions. Using examples within Egypt, North America and Europe the operational challenges will be expanded with reference to the practical aspects of safe transfer under pressure. Compressed air worker (CAW) is defined as a person certified medically fit for working in compressed air. Aspects of discussions on high pressure compressed air (HPCA) working by British Tunnelling Society Compressed Air Working Group and the International Tunnelling Association (ITA) will be highlighted. Future developments are likely to attempt to reduce worker exposures to higher pressures by mechanising cutter tool changing.