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How Altitude Affects Public Safety Diving
by Dr. Thomas Powell:
Scuba divers are a unique group of people. Often, they look for any body of water and then want to know everything that exists below the surface. A diveable location may be a lake, pond, stream, river, ocean, sinkhole, or even a large puddle if the mood is right. We dive in lots of places, and for this reason, altitude is a factor that cannot be forgotten. A body of water is not often excluded from the list of desired places to dive simply because it sits on a mountain or at a higher elevation. The human body is affected by pressure changes and altitude does present variations on atmospheric pressure.
What is altitude diving?
Altitude diving is any subsurface scuba activity performed at an altitude greater than 1000 feet above sea level. These locations could include places such as mountain lakes, streams, rivers, or even quarries. To compensate for pressure changes at altitude, the depths and bottom time durations used for planning dives are different from the figures used to plan dives at sea level. Essentially, a diver must plan a dive as if he or she is diving deeper than his or her actual depth.
When diving at altitude, a diver is operating at an atmospheric pressure less than what is found at sea level. This factor leads to an elevated risk of decompression sickness (DCS) because of a greater reduction in pressure during ascents. Similarly, a diver who has traveled to altitude from sea level must remember that he or she has been gas loading at a greater pressure. For this reason, many training bodies recommend waiting a period of at least 12 hours prior to performing a first dive.
Public Safety and Altitude Diving
Public safety divers are often called to perform arduous tasks in unusual locations. These may include searches, item recoveries, body recoveries, or a mix of tasks. In most situations, the dive conditions are poor and the water clarity is minimal. The reality is that bad guys do not throw evidence into water where items can be recovered in an easy fashion.
When a public safety dive team is called to perform tasks at altitude, altitude factors associated with dive planning and profiles must be considered. Public safety divers already perform tasks on strict time tables. Essentially, the diver often performs a search or task with one third of his or her gas. A second third is used to return to the surface or start point. The final third is considered redundant gas for emergencies or decontamination needs. This method of planning limits the bottom time for a diver using an aluminum 80 cubic foot cylinder in bad conditions. Planning a dive as if the dive was being performed at a deeper depth may reduce this bottom time to an even greater degree. Despite this concern, dives of this type must be planned with diver safety taking precedence over all other factors.
Also, many dive teams deploy to a scene during a period of emergency or concern. This factor may mean that a team travels to a scene at high altitude from a location of lesser altitude. The divers involved must remember to consider gas loading that has taken place at a location of higher pressure. If the divers have been doing things such as performing training dives in recent days, it may be smarter to allow the divers to acclimate to the scene before performing dive operations.
In addition, high altitude locations may be located off the “beaten path.” If the dive site is not easily accessible by vehicle, a team must consider how equipment, vehicles, and support will get on scene as needed. Similarly, how will any individuals needing medical support be evacuated? If a helicopter is required, has a landing zone been established and does the crew understand that a diver or victim may already be experiencing altitude-related issues? This type of planning must be recognized, considered, and implemented when any type of altitude-based operation is performed.
One method a dive team can use to increase bottom times and reduce risk when operating at altitude is through education and training. First, altitude training is essential for public safety divers performing tasks at high elevations. SDI offers an altitude diving program, so stop in at your local SDI facility for more information. Similarly, advanced training may help dive teams to accomplish objectives in a safe manner. For instance, the use of mixed gas such as normoxic trimix may allow public safety divers to safely extend bottom time while reducing dangerous gas loading at altitude. Similarly, the use of different gas configurations such as using side-mounted cylinders may allow a diver to carry more gas during operations.
Altitude diving requires divers and dive teams to recognize the need for comprehensive planning. Physiological concerns may vary at altitude and diver safety is paramount. Education, planning, and practice are critical to the success of any dive team operating 1000 feet or more above sea level. For more information or a better understanding of altitude diving and the education surrounding this type of diving, contact your local SDI/TDI/ERDI dive center.
Dr. Thomas Powell – Owner/Instructor Trainer – Air Hogs Scuba, Garner, NC