 Narcosis may be completely reversed in a few minutes by ascending to a shallower depth , with no long @-@ term effects . Thus narcosis while diving in open water rarely develops into a serious problem as long as the divers are aware of its symptoms , and are able to ascend to manage it . Diving beyond 40 m ( 130 ft ) is generally considered outside the scope of recreational diving . Below these depths , as narcosis and oxygen toxicity become critical risk factors , specialist training is required in the use of various helium @-@ containing gas mixtures such as trimix or heliox . These mixtures prevent narcosis by replacing some of the breathing gas with non @-@ narcotic helium . 
