a typical halo exercise will require a pre-breathing period (30–45 minutes) prior to jump where the jumper breathes 100% oxygen in order to flush nitrogen from their bloodstream. also, a halo jumper will employ an oxygen bottle during the jump. danger can come from medical conditions affecting the jumper. for example, cigarette smoking, alcohol and drug use (including antihistamines, sedatives, and analgesics), anemia, carbon monoxide, fatigue and anxiety can all lead to a jumper being more susceptible to hypoxia. in addition, problems with the oxygen bottle and during the changeover from the pre-breather to the oxygen bottle can result in the return of nitrogen to the jumper's bloodstream and, therefore, an increased likelihood of decompression sickness. a jumper suffering from hypoxia may lose consciousness and therefore be unable to open his parachute. a jumper suffering from decompression sickness may die or become permanently disabled from nitrogen bubbles in the bloodstream, which causes inflammation of joints.