Increased popularity in recreational Scuba diving adds to an ongoing category of diving-related injuries and illnesses. Whether recreational or occupational, Scuba, or surfaced supplied air, diving at higher altitudes may compound inherent risks. An individual initiating a dive at altitude is exposed to an atmospheric pressure less than sea level pressure, which is the presumed endpoint for standard decompression tables. Upon surfacing, the decreased atmospheric pressure in the environment exerts an increased “decompression stress” on the diver. This is often thought of as a relative increase in probabilistic risk for decompression illness in proportion to the diminishment in atmospheric pressure. Under this assumption, best approximations may translate risk equal to that experienced by the diver surfacing from a depth deeper than actually achieved. These calculations determine a new "equivalent depth" for a diver to record bottom time, duration, decompression stops if needed, surface intervals, and residual nitrogen load for repetitive dives to reduce his or her overall risk for decompression illness. Adjusting a dive profile at altitude to an equivalent depth assumes the diver is at risk equal to a sea level dive; however, other environmental effects inherent to the altitude may factor in for the additional risk.
Professional divers often dive at elevations greater than 1000 feet for occupational projects like bridge construction or maintenance or search and rescue situations. Recreational divers more commonly visit mountain lakes or rivers for training or sport.
In most cases, the approach to the dive site and the return route following the dive are at lower altitude, but in mountain areas, divers often traverse mountain passes that add significant pressure changes that may induce off-gassing of residual nitrogen in the bloodstream. This risk can be mitigated by applying the same warning given to divers at sea level: “do not fly for 18 hours following a single dive, or 24 hours after the last dive of a multi-dive profile.” When arriving at altitude from a lower higher pressure environment, the diver is saturated with nitrogen, and time for off-gassing should be allowed. Acclimatization to new altitude levels is important to reduce the overall risk before the event. Minimum acclimatization of 12 hours before diving is recommended; however, 3-day acclimatization may be required at elevations greater than 10,000 ft (3000 m). When diving before the recommended wait time ends, a diver should take into account the residual nitrogen and appropriate time added to bottom times when calculating the decompression obligation.
High-altitude weather may also significantly affect risk. Temperature extremes and reduction in oxygen concentrations exist with a greater rise in elevation. Colder, dry, humidified air is common at higher altitudes. This may affect respiratory function, usually in the form of bronchial irritation with effects ranging from a minor cough to asthmatic wheeze. When combined with reduced oxygen availability, increasing hypoxia may develop. Respiratory rate increases initially to blow off more carbon monoxide (CO) induced by the lower PaO2 levels. This also increases insensible fluid losses leading to dehydration. It may also result in a greater, unplanned depletion of air in the diver’s tank.
The higher altitudes (greater than 8000 to 10,000 ft) may also expose the individual to cold injuries, acute mountain sickness (AMS), high-altitude pulmonary edema (HAPE), or high-altitude cerebral edema (HACE).
Dive profiles are for fresh water in most mountain waterways. The increased density of seawater exerts a pressure greater than that of freshwater (64 pounds per cubic foot versus 62.4 pounds per cubic foot). Therefore, a diver must either convert from depth calculations in feet of seawater (fsw) to feet in freshwater (ffw) or utilize freshwater charts or a dive computer that allows for this conversion.
Specialized dive tables and procedures are required for planning all dives at elevations greater than 1000ft (305 m). These tables should be used for any elevation over 300 ft (91 m) if the planned depth of the dive is greater than 145 ft. No correction is necessary for dives between sea level and 299 ft (90 m).
Standard depth gauges are zeroed at sea level and read less than zero at altitude. Some gauges can be calibrated or re-zeroed at altitude before the dive. Alternatively, the diver will roughly correct for altitude by adding approximately 1 fsw/1000 ft in altitude, with a conversion to ffw if indicated.
Dive computers use decompression algorithms to reduce risk for decompression illnesses by continually updating depth profiles and re-computing the decompression risk variables. Most modern dive computers will automatically adjust for starting altitude and freshwater versus seawater conditions, but some require manual input. Many calculate additional risk according to temperature, but none can factor in individual risk factors such as age, body weight, and/or habitus, presence of patent foramen ovale (PFO), alcohol consumption, or dehydration. Many dive computers formally tested have significant errors and are unreliable at altitude. Consequently, meticulous manual dive profile planning is essential, even with dive computers.
Adequate periods of acclimatization should be calculated into the dive profile. Additional post-dive surface intervals (time spent at the altitude of the dive) should be planned to reduce the nitrogen load before any travel (whether by air or automobile) with altitude changes greater than 1000 feet (305 m).
Hemoconcentration may occur as a protective function of acclimatization with elevations in erythropoietin seen as early as 2 hours after arrival to altitude and a red cell mass increase within 2 to 4 days.
Prior exposure to cold immersion improves adaptation to cold and may mitigate the risk of hypothermia. Surface-level swims, with or without a wetsuit, within days of the dive may improve cold adaptation.
Having a portable one-man hyperbaric chamber or Gamow bag on site for higher risk altitude dive profiles is recommended.
Historically, Jacques Cousteau is credited for the first extreme altitude diving record in 1968 when he mounted an expedition to Bolivia and Peru’s Lake Titicaca searching for submerged Inca treasure at an altitude of 12,507 ft (3812 m). In 1982 a team led by Charles Brush and Johan Reinhard set a new record at the Lago Licanbur volcano in Chile at 19,400 ft (5900 m), and another team led by Nathalie Cabrol (SETI Institute/NASA Ames) at the same location in 2006. At the date of this publication, however, the highest recorded dive was at the Pili Volcano in Chile at 20,000 ft (6000 m) by Philippe Reuter, Claudia Henriquez, and Alain Meyes. The highest scuba dive in the continental United States was done in September 2013 by John Bali at the Pacific Tarn Lake in Colorado at 13,420 ft (4090 m).