5.3 not exceed the short-time allowable doses in Table 5.1 and the affected humans can be medically treated for the effects, or removed from the radiation environment altogether if their increased doses require it. Downward variations from a nominally safe radiation environment are favorable to the human inhabitants. V.2.2: Atmosphere: The significant factor in breathing mixtures for human beings is the partial pressure of oxygen. On Earth, at sea level, where the total pressure is 101325 N/m 2 , and the oxygen content 20% by volume, the partial pressure of oxygen is p = (. 2) (101325) = 20265 N/m 2 02 It is this partial pressure which pushes oxygen through the lung membranes into the bloodstream. If the partial pressure is lowered, it eventually becomes insufficient to the task and the human being suffers hypoxia (5.1). The lower the p O drops, the more severe the effects, as shown in Figure 5.1. 2 If the partial pressure of oxygen is raised above 20265 N/m 2 , the human being eventually suffers from oxygen poisoning (5.1). These effects also depend on p O , as shown in Figure 5.2. Figure 5.3 shows the tolerable range of pressures for breathing mixtures of oxygen and inert gas (such as N 2 or He). The choice of inert gas in a breathing Mixture can have effects on the human as well. For example, sound travels at a higher velocity in Helium raising the pitch of human s~eech. Although data on this issue is sparse, there may be a dependence of man's saprophytic bacteria on Nitrogen in the breathing mixture at least in trace concentrations (5.1). Human beings are also sensitive to sudden variations in breathing pressure (such as from Earth sea-level pressure to half that value in fifteen minutes). This brings on ebullism, the release of bubbles of inert gas formerly dissolved in the body fluids (also called "the bends"). Physiological damage is immediate, incapacitating, and possibly fatal (5.1) . Carbon dioxide, a waste product of breathing and hydrocarbon combustion, has ill effects on human beings even at low partial
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