Figure 70-2 Ambient pressure as a function of altitude and water depth. Whereas ambient pressure increases linearly with depth, pressure and altitude are not linearly related. As air is inspired and humidified, there is a small and usually insignificant drop from atmospheric PO₂ to inspired PO₂ . At altitude, however, this decrease accounts for a greater proportion of the total ambient pressure. The O₂ partial pressure line in the water is shown for a constant FIO₂ of 21%. At increasing depth, the inspired PO₂ eventually exceeds the pulmonary toxic limit (≅14 m in depth) and the central nervous system toxic limit (≅70 m in depth). The threshold for high-pressure nervous syndrome and pressure reversal of anesthesia (observed in non-narcotic atmospheres) is around 150 to 200 m in depth. The shaded red bars represent the depth or altitude ranges over which risk progresses from low (light shading) to high (dark shading). AMS, acute mountain sickness; HACE, high-altitude cerebral edema; HAPE, high-altitude pulmonary edema.