[1] Traditionally, compressed air work was limited to maximum ambient pressures of between 3 and 4 bars (3.0 and 3.9 atm), but experience with offshore saturation diving shows that higher pressures can be managed at acceptable risk using the techniques developed in that industry, including saturation exposures and the use of breathing gases other than air.
The relatively hot environment made the usual procedure of freezing the surrounding rock with liquid nitrogen impracticable so it was decided to pressurise the tunnel and work at ambient pressure until repairs could be completed.
[1] When the pressure of the workplace is relatively high, decompression at the end of a shift can take an uneconomically long time and exposes the worker to daily risk.
[2] Compressed air work is generally considered a potentially hazardous occupational environment, and may be regulated accordingly.
In some jurisdictions legislation mag be specific to compressed air work, while in others it may be combined with diving regulations.