Below this depth, extended breathing of heliox gas mixtures may cause high pressure nervous syndrome (HPNS).
Marcel Auda, R. Peilho, P. Raude, L. Schneider) of the Comex S.A., industrial deep-sea diving company performing pipe line connection exercises at a depth of 534 m (1,752 ft) of seawater (msw/fsw) in the Mediterranean Sea as part of the Hydra VIII (Hydra 8) programme.
[8] The Hydra X team Théo Mavrostomos belonged to spent 3 days at the simulated 675 metres (2,215 ft) depth.
[9][10][11] Although breathing hydreliox improves the symptoms seen in HPNS, tests have shown that hydrogen narcosis becomes a factor at depths of 500 metres (1,600 ft).
Use of hydrogen can make it possible to dive deeper, descend faster, and stay at depth longer.