Breathing gas, back gas, mix and air all refer to what divers breathe from their tanks. Breathing gas and back gas are general terms and can include air, nitrox, heliox or some other mixture of diving gases. Mix is a generic term for the particular mixture of gas a given diver is using. And air refers specifically to normal atmospheric gas of about 21 percent oxygen and 79 percent nitrogen (roughly the composition of surface-level air). Other gas mixes include nitrox or heliox mixes.