ABSTRACT
Although safety in compressed air tunnelling is far from being only the management of decompression sickness risks, this is often considered as a critical issue. It was understood very long ago that breathing pure oxygen during decompression results in either shorter or better quality decompressions. This is extensively used in diving as well as in space during Extra Vehicular Activity preparation. A review of the various effects of hyperbaric oxygen breathing in decompression is made: nitrogen washout is accelerated due to low nitrogen level in alveoli; the body dissolved oxygen content is increased, which prevents hypoxia that could eventually be induced by microbubbles; nitrogen content of bubbles is reduced and bubble collapse is enhanced; inflammation due to hypoxia is prevented. Safety aspects are then considered: management of oxygen breathing to prevent oxygen poisoning by proper limitations of PO2 and duration of exposures; management of fire risk associated with the use of pure oxygen in compression chambers during tunnelling operations; oxygen systems in air-locks. An example of an air/oxygen tunnelling decompression schedule is presented and discussed in view of time saving, safety records and practicability in tunnelling operations.
