Complex sentence: To prevent hypoxia and decrease the chance of decompression sickness, pilots breath 100% oxygen an hour prior to take off to remove nitrogen from the body, a portable oxygen supply is used prior to entering the aircraft.
Split into simpler sentences: To prevent hypoxia and decrease the chance of decompression sickness, pilots breathe 100% oxygen an hour prior to take off to remove nitrogen from the body. A portable oxygen supply is used prior to entering the aircraft.