The partial pressure of oxygen in the air in the lungs controls the oxygen loading of blood. A critical pO2 of in the lungs will sustain consciousness when breathing is resumed after a breath-hold dive. This is about 4% oxygen in the lungs and 45% oxygen saturation of the arterial blood. At 30 msw (4 bar), 2% by volume oxygen in the lung gas gives a pO2 of . At 10 msw (2 bar), for the same 2% oxygen, the pO2 would be , i.e. marginal. At the surface the same 2% oxygen drops to , ignoring metabolic use.