Modern air travel has made distant dive locations easily accessible. Flying to a destination near sea level before diving engenders virtually no risk (outside the possibility of mild dehydration or impairment due to long periods of relative immobility). Since flights end with compression, the tissues of plane passengers will be undersaturated upon landing and subsequently accumulate inert gases to re-establish equilibrium with the ambient pressure. 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. \nDAN 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. There are two further factors of note regarding the DAN-UHMS flying after diving guidelines: - 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. It is important to remember that any postdive ascent to a higher altitude — even using ground transportation — increases your decompression stress. Taking a cautious approach in such a case, by keeping your final dive profiles more conservative and/or delaying your travel to the higher altitude, is always advisable. The U.S. Navy has generated detailed tables and procedures that allow computation of exposure limits to a greater range of altitudes and with more time flexibility than the DAN-UHMS guidelines (USN 2008). It is important to appreciate, though, that these are simply mathematical constructs based on the same data used in developing the DAN-UHMS guidelines. Furthermore, they require the computation of repetitive groups for planning, something that is done with dive tables but not dive computers. Despite these limitations, they can be useful, particularly for a regular pattern of altitude diving.