Oxygen is a toxic gas in high enough concentrations. If the partial pressure of oxygen in a human body is more than 1.45 ATA then they could convulse heavily and if not treated die. Diving at depths in excess of 200 feet on air a diver is pushing that limit. To prevent such a catastrophic event from happening underwater divers lower the percentage of oxygen in their breathing mixture. Divers at these depths normally opt for 14/86 which is a gas mixture of 14 percent oxygen and 86 percent helium. 

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Although the Rouses did not suffer from this there was a high probability of it happening considering the depths they were diving too on air. all the more reason for them to be diving on Heliox.