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.