If other investigations establish the need for an inert gas in manned spacecraft atmospheres, gases other than nitrogen may be considered. Compared with nitrogen, the physical properties or helium and neon offer advantages with respect to solubility in body fluids, storage weight, and thermal properties. Studies at Ohio State University in 1964, under a NASA grant, showed that helium substituted for nitrogen in a closed container causes humans to feel "cold" at a normally comfortable temperature. Studies with animals have shown that in a helium atmosphere there is greater heat loss due to the increased conducting capacity and probably greater evaporative capacity. In 6 days at 21 percent oxygen and 79 percent helium at 1-atmosphere pressure, young rats grew at the same rate as controls, but drank more water, excreted more urine, and had a higher rate of food and oxygen consumption than controls in air at 1 atmosphere. Men are being tested on a bicycle ergometer in saturated and low relative humidity helium atmospheres to study heat balance. Mice were exposed to 80 percent argon and 20 percent oxygen continuously at 1-atmosphere pressure for 35 days at Oklahoma City University. Carbon 14 studies of metabolism showed a slight slowing and a twofold to threefold increase in fat deposition. Bends Decompression, whether accidental (due to damage of the spacecraft) or intentional (as in the use of the pressure suit outside the capsule), carries the risk of bends if the inert gases dissolved in the tissues and body fluids come out of solution. The magnitude of this risk is determined to a very considerable extent by— (1) Individual susceptibility (2) The extent to which the nitrogen (or other inert gas) concentrations of tissues and body fluids have been reduced (3) The magnitude and rate of the inert-gas, partial pressure change on decompression The probability of getting bends is reduced by— (1) Selection of bends-resistant individuals (2) Thorough denitrogenation before flight (3) Limitation of decompressive pressure changes by appropriate choice of cabin atmosphere pressure and composition (4) Space-suit pressure setting In some cases, further improvements might be obtained by using, in the cabin atmosphere, an inert-gas component which has a lower solubility in tissue and body fluids or less tendency than nitrogen to form bubbles. Fire Hazard Experience indicates that fires in pure oxygen atmospheres, even at low pressures (e.g., 1/3 atm), are extremely difficult to extinguish. While this phenomenon has nothing to do with respiratory physiology, the risk on flights of long duration may be so serious as to demand special measures. Unless effective countermeasures can be devised, this risk may argue very strongly against the use of such atmospheres in the future. Further experimental investigation is required. Acceleration Effects on the Lungs and Pulmonary Circulation