Underwater diving The display of a basic personal dive computer shows depth, dive time, and<unk>  information. To prevent the excess formation of bubbles that can lead to<unk> , divers limit their ascent rate—the recommended ascent rate used by popular<unk>  models is about per minute—and follow a<unk>  schedule as necessary. This schedule may require the diver to ascend to a particular depth, and remain at that depth until sufficient inert gas has been eliminated from the body to allow further ascent. Each of these is termed a<unk>  stop", and a schedule for a given bottom time and depth may contain one or more stops, or none at all. Dives that contain no<unk>  stops are called "no-stop dives", but divers usually schedule a short "safety stop" at , depending on the training agency or dive computer. The<unk>  schedule may be derived from<unk>  tables,<unk>  software, or from dive computers, and these are generally based upon a mathematical model of the body's uptake and release of inert gas as pressure changes. These models, such as the Bühlmann<unk>  algorithm, are modified to fit empirical data and provide a<unk>  schedule for a given depth and dive duration using a specified breathing gas mixture. Since divers on the surface after a dive may still have excess inert gas in their bodies,<unk>  from any subsequent dive before this excess is fully eliminated needs to modify the schedule to take account of the residual gas load from the previous dive. This will result in a shorter allowable time under water without obligatory<unk>  stops, or an increased<unk>  time during the subsequent dive. The total elimination of excess gas may take many hours, and tables will indicate the time at normal pressures that is required, which may be up to 18 hours.<unk>  time can be significantly shortened by breathing mixtures containing much less inert gas during the<unk>  phase of the dive (or pure oxygen at stops in of water or less). The reason is that the inert gas outgases at a rate proportional to the difference between the partial pressure of inert gas in the diver's body and its partial pressure in the breathing gas; whereas the likelihood of bubble formation depends on the difference between the inert gas partial pressure in the diver's body and the ambient pressure. Reduction in<unk>  requirements can also be gained by breathing a nitrox mix during the dive, since less nitrogen will be taken into the body than during the same dive done on air. Following a<unk>  schedule does not completely protect against<unk> . The algorithms used are designed to reduce the probability of<unk>  to a very low level, but do not reduce it to zero. The mathematical implications of all current<unk>  models are that provided that no tissue is ingassing, longer<unk>  stops will decrease<unk>  risk, or at worst not increase it. Efficient<unk>  requires the diver to ascend fast enough to establish as high a<unk>  gradient, in as many tissues, as safely possible, without provoking the development of symptomatic bubbles. This is facilitated by the highest acceptably safe oxygen partial pressure in the breathing gas, and avoiding gas changes that could cause counterdiffusion bubble formation or growth. The development of schedules that are both safe and efficient has been complicated by the large number of variables and uncertainties, including personal variation in response under varying environmental conditions and workload, attributed to variations of body type, fitness and other risk factors. Exposure to altitude One of the most significant breakthroughs in the prevention of altitude<unk>  is oxygen pre-breathing. Breathing pure oxygen significantly reduces the nitrogen loads in body tissues by reducing the partial pressure of nitrogen in the lungs, which induces diffusion of nitrogen from the blood into the breathing gas, and this effect eventually lowers the concentration of nitrogen in the other tissues of the body. If continued for long enough, and without interruption, this provides effective protection upon exposure to low-barometric pressure environments. However, breathing pure oxygen during flight alone (ascent, en route, descent) does not decrease the risk of altitude<unk>  as the time required for ascent is generally not sufficient to significantly desaturate the slower tissues. Pure aviator oxygen which has moisture removed to prevent freezing of valves at altitude is readily available and routinely used in general aviation mountain flying and at high altitudes. Most small general aviation aircraft are not pressurized, therefore oxygen use is an FAA requirement at higher altitudes. Although pure oxygen pre-breathing is an effective method to protect against altitude<unk> , it is logistically complicated and expensive for the protection of civil aviation flyers, either commercial or private. Therefore, it is currently used only by military flight crews and astronauts for protection during high-altitude and space operations. It is also used by flight test crews involved with certifying aircraft, and may also be used for high-altitude parachute jumps. Astronauts aboard the International Space Station preparing for extra-vehicular activity (EVA) "camp out" at low atmospheric pressure, , spending eight sleeping hours in the Quest airlock chamber before their spacewalk. During the EVA they breathe 100% oxygen in their spacesuits, which operate at , although research has examined the possibility of using 100% O2 at in the suits to lessen the pressure reduction, and hence the risk of<unk> .
Decompression sickness
Morning sickness
Ski sickness
Wooden chest syndrome
Acute HME syndrome
Simulator sickness
Chronic mountain sickness
Acute chest syndrome