Flying after diving, however, increases decompression stress, since the pressure in an aircraft cabin is lower than that of ground-level atmospheric pressure. Commercial aircraft must have the capability of maintaining cabin pressure at an equivalent of 8,000 feet (2,438 meters), approximately 0.76 ATA. This does not mean that cabin pressure is always maintained at higher pressures. A recent study found that 10 percent of the commercial flights tested had cabin pressures exceeding 8,000 ft (Hampson et al. 2013). Now imagine that you have just completed a dive to 66 feet (20 meters), where you experienced an underwater pressure of 3.0 ATA. Your return to the surface, and the 1.0 ATA pressure of sea level, has already subjected your body to a threefold reduction in pressure (3.0:1.0). If you then get on a plane that has a cabin altitude of 8,000 feet, you would be subjecting yourself to a fourfold reduction (3.0:0.76) and thus to even greater decompression stress. Furthermore, should your plane suffer an unlikely but not impossible cabin depressurization, you would be subjected to a much greater decompression stress.
DAN and the Undersea and Hyperbaric Medical Society (UHMS) held a workshop in 2002 to review the available data regarding the decompression stress of flying after diving and develop consensus guidelines (Sheffield and Vann 2004). There were two important stipulations regarding the guidelines: first, adhering to them will reduce your risk but offers no guarantee that you will avoid DCS, and second, observing even longer surface intervals than the recommended minimums will reduce your DCS risk further still. Keeping in mind these caveats, these are the guidelines:
- After a single no-decompression dive, a minimum preflight surface interval of 12 hours is suggested.
- After multiple dives per day or multiple days of diving, a minimum preflight surface interval of 18 hours is suggested.
- After dives requiring decompression stops, there was little evidence on which to base a recommendation, but a preflight surface interval substantially longer than 18 hours is considered to be prudent.
- They apply to flights at altitudes of between 2,000 and 8,000 feet (610 and 2,438 meters). The effect of a flight at an altitude below 2,000 feet was considered mild enough not to warrant special consideration — giving divers the flexibility to engage in modest postdive air travel, such as a short, low-altitude, inter-island flight.
- They apply only to divers who have experienced no DCS symptoms. It is essential that a diver who is experiencing any symptoms consistent with DCS seek treatment prior to flying.