f sw ▁( 7 ▁m sw ). ▁NASA ▁gu idel ines ▁for ▁E AD s ▁of ▁ 2 0 – 5 0   f sw ▁( 6 – 1 5   ms w ) ▁with ▁maximum ▁d ive ▁dur ations ▁of ▁ 1 0 0 – 4 0 0 ▁minutes ▁allow ▁either ▁air ▁or ▁o xygen ▁to ▁be ▁bre athed ▁in ▁the ▁pre fl ight ▁surface ▁intervals . ▁O xygen ▁breath ing ▁during ▁surface ▁intervals ▁reduces ▁the ▁time ▁to ▁fly ▁by ▁a ▁factor ▁of ▁seven ▁to ▁nine ▁times ▁compared ▁with ▁air . ▁A ▁study ▁by ▁another ▁military ▁organization , ▁the ▁Special ▁Oper ations ▁Command ▁also ▁indicated ▁that ▁pre fl ight ▁o xygen ▁might ▁be ▁an ▁effective ▁means ▁for ▁reducing ▁D CS ▁risk . ▁ ▁Some ▁places , ▁( for ▁example , ▁the ▁Al ti pl ano ▁in ▁Peru ▁and ▁Boliv ia , ▁or ▁the ▁plate au ▁around ▁As m ara ▁( where ▁the ▁air port ▁is ) ▁in ▁E rit rea , ▁and ▁some ▁mountain ▁passes ), ▁are ▁many ▁thousand ▁feet ▁above ▁sea ▁level ▁and ▁trav elling ▁to ▁such ▁places ▁after ▁div ing ▁at ▁lower ▁alt itude ▁should ▁be ▁treated ▁as ▁flying ▁at ▁the ▁equivalent ▁alt itude ▁after ▁div ing . ▁ ▁Techn ical ▁div ing ▁Techn ical ▁div ing ▁includes ▁profiles ▁that ▁are ▁relatively ▁short ▁and ▁deep , ▁and ▁which ▁are ▁in efficient ▁in ▁terms ▁of ▁decom pression ▁time ▁for ▁a ▁given ▁bottom ▁time . ▁They ▁also ▁often ▁lie ▁outside ▁the ▁range ▁of ▁profiles ▁with ▁valid ated ▁decom pression ▁sched ules ,