group. Equalisation of pressure in gas spaces to avoid barotraumas. The expansion or compression of enclosed air spaces may cause discomfort or injury while diving. Critically, the lungs are susceptible to over-expansion and subsequent collapse if a diver holds their breath while ascending: during training divers are taught not to hold their breath while diving. Ear clearing is another critical equalisation procedure, usually requiring conscious intervention by the diver. Mask and regulator clearing may be needed to ensure the ability to see and breathe in case of flooding. This can easily happen, and while immediate correct response is necessary, the procedure is simple and routine and is not considered an emergency. Buoyancy control and diver trim require frequent adjustment (particularly during depth changes) to ensure safe, effective, and convenient underwater mobility during the dive. Buddy checks, breathing gas monitoring, and decompression status monitoring are carried out to ensure that the dive plan is followed and that members of the group are safe and available to help each other in an emergency. Ascent, decompression, and surfacing procedures are intended to ensure that dissolved inert gases are safely released, that barotraumas of ascent are avoided, and that it is safe to surface. Water exit procedures are intended to let the diver leave the water without injury, loss of, or damage to equipment. Underwater communication: Divers cannot talk underwater unless they are wearing a full-face mask and electronic communications equipment, but they can communicate basic and emergency information using hand signals, light signals, and rope signals, and more complex messages can be written on waterproof slates. Decompression Inert gas components of the diver's breathing gas accumulate in the tissues during exposure to elevated pressure during a dive, and must be eliminated during the ascent to avoid the formation of symptomatic bubbles in tissues where the concentration is too high for the gas to remain in solution. This process is called decompression, and occurs on all scuba dives. Decompression sickness is also known as the bends and can also include symptoms such as itching, rash, joint pain or nausea. Most recreational and professional scuba divers avoid obligatory decompression stops by following a dive profile which only requires a limited rate of ascent for decompression, but will commonly also do an optional short, shallow, decompression stop known as a safety stop to further reduce risk before surfacing. In some cases, particularly in technical diving, more