▁ ▁Mi xt ures ▁to ▁reduce ▁nit ro gen ▁nar cos is ▁In cre ased ▁pressure ▁due ▁to ▁depth ▁causes ▁nit ro gen ▁to ▁become ▁n arc otic , ▁resulting ▁in ▁a ▁reduced ▁ability ▁to ▁react ▁or ▁think ▁clearly . ▁By ▁adding ▁hel ium ▁to ▁the ▁breath ing ▁mix , ▁these ▁effects ▁can ▁be ▁reduced , ▁as ▁hel ium ▁does ▁not ▁have ▁the ▁same ▁n arc otic ▁properties ▁at ▁depth . ▁Hel it ro x / tri ox ▁pro ponents ▁argue ▁that ▁the ▁defining ▁risk ▁for ▁air ▁and ▁nit ro x ▁div ing ▁depth ▁should ▁be ▁nit ro gen ▁nar cos is , ▁and ▁suggest ▁that ▁when ▁the ▁partial ▁pressure ▁of ▁nit ro gen ▁reaches ▁approximately ▁ 4 . 0 ▁A TA , ▁which ▁occurs ▁at ▁about ▁ ▁for ▁air , ▁hel ium ▁is ▁necessary ▁to ▁limit ▁the ▁effects ▁of ▁the ▁nar cos is . ▁ ▁Mi xt ures ▁to ▁reduce ▁o xygen ▁to x icity ▁Techn ical ▁div es ▁may ▁also ▁be ▁character ised ▁by ▁the ▁use ▁of ▁hy po xic ▁breath ing ▁gas ▁mi xt ures , ▁including ▁hy po xic ▁trim ix , ▁hel io x , ▁and ▁hel ia ir . ▁A ▁diver ▁breath ing ▁normal ▁air ▁( with ▁ 2 1 % ▁o xygen ) ▁will ▁be ▁exposed ▁to ▁increased ▁risk ▁of ▁central ▁nerv ous ▁system ▁o xygen ▁to x icity ▁at ▁depth s ▁greater ▁than ▁about ▁ ▁The ▁first ▁sign ▁of ▁o xygen ▁to x icity ▁is ▁usually ▁a ▁conv ulsion ▁without ▁warning ▁which ▁usually ▁results ▁in