is diving using equipment supplied with breathing gas using a diver's umbilical from the surface, either from the shore or from a diving support vessel, sometimes indirectly via a diving bell. Surface oriented diving means that the dive starts and ends at surface pressure, which may involve staged decompression stops, gas switches, or surface decompression. The copper helmeted, free-flow, standard diving dress is the version which made commercial diving a viable occupation, and although still used in some regions, this heavy equipment has been superseded by lighter free-flow helmets, and to a large extent, lightweight demand helmets, band masks and full-face diving masks. Breathing gases used include air, heliox, nitrox, oxygen and trimix. Gases with raised oxygen fraction are used to reduce decompression obligation and accelerate decompression, and gases containing helium are used to reduce nitrogen narcosis. Both applications reduce the risk to the diver when applicable. The primary advantages of conventional surface-supplied diving over scuba are lower risk of drowning and considerably larger breathing gas supply than scuba, allowing longer working periods and safer decompression. Surface-supplied diving systems also improve safety by virtually eliminating the risk of a lost diver, as the diver is physically connected to the surface control point by the breathing gas supply hose, and other components of the umbilical cable system. They also significantly reduce the risk of running out of breathing gas during the dive, and allow multiple redundancy of gas supply, with main and secondary surface supply, and a scuba bailout emergency gas system. Use of helmets and full-face masks help protect the diver's airway in case of loss of consciousness. These can be considered engineering controls of the hazards. Surface supplied systems routinely include voice communication between diver and supervisor, making it relatively easy to monitor the condition of the diver from the surface. Saturation diving Decompression sickness occurs when a diver with a large amount of inert gas dissolved in the body tissues is decompressed to a pressure where the gas forms bubbles which may block blood vessels or physically damage surrounding cells. This is a risk on every decompression, and limiting the number of decompressions can reduce the risk. Saturation refers to the state where the diver's tissues have absorbed the maximum concentration of inert gas possible for that depth due to the diver being exposed to breathing gas at that pressure for periods in the order of 24 hours.