Using normal scuba equipment, breathing gas consumption is proportional to ambient pressure - so at , where the pressure is 6 bar, a diver breathes 6 times as much as on the surface (1 bar). Heavy physical exertion makes the diver breathe even more gas, and gas becomes denser requiring increased effort to breathe with depth, leading to increased risk of hypercapnia—an excess of carbon dioxide in the blood. The need to do decompression stops increases with depth. A diver at may be able to dive for many hours without needing to do decompression stops. At depths greater than , a diver may have only a few minutes at the deepest part of the dive before decompression stops are needed. In the event of an emergency, the diver cannot make an immediate ascent to the surface without risking decompression sickness. All of these considerations result in the amount of breathing gas required for deep diving being much greater than for shallow open water diving. The diver needs a disciplined approach to planning and conducting dives to minimise these additional risks.