Here’s a somewhat common scenario… perhaps one you have experienced yourself; or thought about at least.
Anyhow, here it is. You and your buddy are on a dive vacation someplace that requires airline travel… bummer, right!? Pack light. Hope the TSA doesn’t break anything on your way out. Hope customs at the destination doesn’t fuss over anything on the way in.
However, all those issues aside, every other piece of the planning puzzle is falling into place just fine except for one small issue. The flight home is scheduled wheels-up at O-Dark-Hundred in the morning, and there is an opportunity to dive something really, really cool the previous afternoon… late in the afternoon. The question is: Can you do that dive without getting bent like a pretzel on the flight home less than 12 hours later?
The whole issue of Pre-flight Surface Interval (PFSI) is a contentious one. The old-school guidelines were wait 24 hours after diving before jumping on a commercial flight. But that recommendation has been revisited in more recent studies and the PFSI shortened; with suggestions that various other factors such as breathing nitrox, the length of safety stops, gas breathed during safety stops, and the duration and depth of dive, can all influence by just how much the PFSI can be shortened.
A quick straw-poll of my dive buddies tells me that the definitive answer is a moving target. There is little agreement.
What we can take as read is that flying after diving has a strong potential to apply extra decompression stress on a diver and increases their risks of decompression sickness. There seems to be a direct relationship between the risk dropping and the amount of time spent out of the water increases allowing excess inert gas to be eliminated normally and harmlessly through the lungs. Some trials have estimated the PFSI necessary for a low DCS risk (read acceptable number of incidents of DCS) after relatively long single or repetitive no-decompression dive profiles sits between 11 and 16 hours.
The PFSI for dives requiring staged decompression stops, was around 22 hours. At first blush then, a 24-hour break after diving would seem in most sport-diving cases to be very conservative. But then again, what worked in a dry chamber on a couple of hundred test subjects, may not apply to the average dive tourist coming home from a week in paradise where the diving was punctuated with rum, grilled fish and late-night romps on the beach. Equally, it also may not apply to an informed technical diver who pads her/his decompression stops with extra time, and breathes pure oxygen for long periods during that PFSI!
Well worth the download and reading time is: The Influence of bottom time on preflight surface intervals before flying after diving, published by Undersea Hyperb Med. And authored by Vann RD, Pollock NW, Freiberger JJ, Natoli MJ, DeNoble PJ, Pieper CF. (2007). It is available from the ultimate diver’s research tool: http://archive.rubicon-foundation.org/xmlui/handle/123456789/7343.
The study’s conclusion suggests “that bottom time, repetitive diving, and a decompression stop may significantly influence the pre-flight surface intervals required for low DCS risk. Moreover, it highlighted the need for additional human trials to resolve the effects of exercise and immersion on DCS risk during flying after diving. Such information might assist in the calibration of dry, resting trials for the effects of immersion and exercise which would be useful as dry, resting trials are less expensive and faster to conduct because more subjects can be exposed per chamber dive. This might be of aid for improving the accuracy of existing flying after diving guidelines.”
Significant in that conclusion is the call for additional human trials to resolve the effects of exercise and immersion on DCS risk when flying after diving.
I volunteer.
However, I would be far from an average test subject since something seems to put me outside the bell-curve for DCS risk. For example, my experience with PFSI is far from what’s generally acceptable and my practices at times have been foolhardy. Furthermore, I fall outside the age category that most studies could ethically accept in any trial… but all that aside, I would love to be a guinea pig.
Doppler's Tech Diving Blog 
Put this in my vote, however this is my logic. Older aircraft have a pressurization equivalent to roughly 7000ft, rounding to safety, this is roughly 11psi or 3/4 of an atmosphere. I am OK surfacing with a GFhi of around 85 in a relaxed environment, so if I have to fly shortly after I dive, I will adjust my surfacing GF with a gradient factor equivalent of 85 at 3/4 of an atmosphere, and use the time window between diving and flying to add the safety buffer. The hour requirement is based off of a GFhi roughly equivalent to 0.95 because of the dive tables and the recreational dive computers.
So what I do is take the GFhi that I am ok with and normalize it for the surface. In this case 85 becomes 68, so I would run a GFhi of somewhere between 60 and 65 to approximate a GFhi of 85 with an immediate bounce to altitude, but remember you do also have a minimum of 2 hour surface interval prior to flying due to getting to the airport, security, boarding, then the actual takeoff. This only changes based on your comfortable GFhi, so if you don’t like a GFhi of 85, and are very conservative with say GF70, you would have to drop to around 50/55, but that is very conservative. I typically run a GF70 if I’m planning to fly that day which with an immediate flight would put my roughly around GF90/95, and with a multi hour surface interval, well within my comfort zone for DCS.