When doing a skydive from 30,000 feet, you must breathe pure oxygen on the ground for 45 minutes to purge your bloodstream of nitrogen. Nitrogen bubbles in your blood could cause the "bends" when you get up to 30K. At 30K,  most people will loose consciousness in about 30 seconds without pressurized oxygen and a mask on. We used military oxygen masks connected to an on-board oxygen system and then switched over to a personal "bailout bottle" that supplies oxygen during freefall. The bottle contains about 15 minutes of air - more than enough time to get back to earth unless you open your canopy too high. This is not a good thing.

Skydivers wishing to make a HALO jump must have a skydivers D-license (500 jumps), take a pilots flight physical, and train in a USAF high altitude chamber.  The D-license now costs about $12,000 USD (training + jumps). I got by much cheaper, since the old D-license requirement was 200 jumps.  I currently have 350 jumps in my logbook.  The rig (parachutes, container, and other gear) costs between $1800 to $5000 depending on how fancy you want to get and if you buy new or used. The flight physical costs about $100, and the altitude chamber ride was $50, not counting the hotel and travel.  The jump itself was $600, again, not counting hotel and travel.  Would I do it again?.... HELL YES!

To learn more about high altitude parachute jumps and the effects of high altitude on the human body click here.