The air would be so thin that even the best climber would have no choice but to use supplemental oxygen. Because of the many factors which affect atmospheric pressure. Toriccelli also went on to develop the mercury barometer, an instrument used to measure atmospheric pressure. A barometer also monitors variations in atmospheric pressure. As air becomes thinner, the density of air decreases, and so too does the pressure of air.
Many different factors affect the density of air.
Most measurably, as altitude increases, air becomes less dense, decreasing atmospheric pressure. Standard altitude-pressure tables allow mountaineers and aviators to determine their approximate height by measuring atmospheric pressure. This relationship also works inversely. The height of a mountain determines, approximately, the density of air on its summit. As air becomes less dense, it contains less gases per unit of volume, and therefore less oxygen.