Diving at Altitude

Excerpt

Recreational scuba diving is widely practiced, with 2.6 million active divers in the U.S. and approximately 6 million worldwide in 2025. (Source: Wallmo et al, 2021) Participation spans recreational, commercial, and military contexts. High-altitude diving introduces additional risk, increasing the likelihood of diving-related injuries and illnesses. Exposure to lower atmospheric pressure at altitude may compound inherent risks present in recreational, occupational, scuba, or surface-supplied air diving.

Diving at altitude introduces exposure to atmospheric pressures lower than those at sea level, which serve as the reference point for standard decompression tables. Upon surfacing, the reduced atmospheric pressure increases decompression stress, necessitating adjustment of standard dive tables used in basic certification programs. The relative pressure change associated with altitude correlates with an increased probability of decompression illness, proportional to the magnitude of atmospheric pressure reduction.

Under this assumption, risk approximations may correspond to that experienced by a diver surfacing from a depth greater than the one actually reached. In addition to altitude adjustments, water density must be considered, as mountain lakes often contain freshwater rather than saltwater.

Accounting for these factors enables calculation of a standardized equivalent sea depth (SESD), converting the actual lake diving depth to an equivalent sea-level dive depth. Using this equivalent depth, divers can adjust bottom time, decompression stop duration if required, surface intervals, and residual nitrogen load for repetitive dives, thereby reducing overall risk of decompression illness.

Adjustment of dive profiles at altitude assumes risk comparable to sea-level dives. However, additional environmental factors may further elevate risk. These factors include freshwater versus saltwater density, weather conditions, dehydration, acute mountain sickness (AMS), and relative hypoxia. (Source: Chimiak and Nord, 2026)