In the hypoxia group: the subjects will breathe an individualized mix of nitrogen and room air for 3.5 hours using an air-tight respiratory helmet. The gas mixture will be adjusted to achieve a saturation of 80-85%. In the hyperoxia group, subjects will breathe 100% oxygen for 3.5 hours using the same respiratory helmet. In the normoxia group, subjects will breathe room air (21% oxygen, 79% nitrogen) also wearing the respiratory helmet. 1 hour after oxygen status adjustment (t=0), all subject will be administered an intravenous bolus (2ng/kg) of LPS derived from E coli O:113. 2.5 hours after LPS administration, the helmets will be removed and all subjects will breathe ambient room air.