Want to see a shocked look on your dive leader's face? On the way back to the dive center after a day of deep diving, ask them, “How long will it take to get back to shore? My plane leaves in 3 hours”. Well on second thought you better not, too many divers simply forget the rules regarding flying after diving.
When you learned about dive tables and the risk of decompression illness, the concept of when you can fly after diving, was introduced. The statement is simple, wait xx hours after your last dive before flying.
The xx hours is different based on how many dives you have done in the last 48 hours and what authority is making the statement. Sadly, while there is some consensus, not everyone agrees with the numbers.
The generally accepted numbers are 12 hours for a single no-decompression dive and 18 hours for multiple dives or decompression dive in the last 48 hours. You will also hear the statement that you need to wait 24 hours.
The 24 hours comes from a presentation by DAN in 2002 during a conference about DCI. It was a based on a study that showed some risk reduction between 18 hours and 24 hours but insignificant changes after 24 hours.
Insurance companies started to write the restrictions into policies and many dive centers, and liveaboard started to enforce the requirement. Since then insurance and business have pulled back on their enforcement, making it clear it is the divers responsibility.
At first glance, the concept of flying causing a DCI is hard to grasp. It is not the flying; it is the pressurization of the aircraft that is the cause. Commercial aircraft are pressurized to equal the air pressure at a setting between 5,000 to 8,000 feet. That is at an ATM of .75 to .80.
If you recall your basic decompression theory, the nitrogen in the air you breath under pressure enters and is stored in your tissues.
When you return to the surface, 1 ATM, following the guidance of a No Decompression Dive, the amount of nitrogen remaining in your tissues is higher than normal but within acceptable tolerances.
DCI can happen when the amount of nitrogen still in tissue is above the recommended partial pressure. The speed that nitrogen leaves the tissue is a factor of the difference in pp in the tissue and in the blood. (blood pp and the pp in air equalizes in a matter of minutes). This rapid release causes larger bubbles that can cause the onset of decompression sickness.
The nitrogen bubbles are not the same as in the video below but it is an exaggerated example what could happen.
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When we fly, the plane rapidly climbs, and the cabin pressure is stabilized as it passes the set altitude. In a matter of a few minutes, you have changed the ATM by about 20%. Think of it as an equivalent of an uncontrolled ascent from 60 feet.
There is an easy way to see the pressure difference, just look at a bag of in-flight peanuts or a bag of chips. They appear bloated. The major point of this pressure difference is with the lower pressure the acceptable level is also lower. When you surface the remaining nitrogen was in a safe zone, that might not be the case at .8 ATM.
Also read: Why You Should Never Miss A Safety Stop?
There is no real reason to dive and fly in the same day.
The time to fly rule is really a time to altitude rule. Divers are warned not to drive higher than 2,000 feet after diving. This rule is not as stringently applied, in fact, PADI in their Altitude Adventure manual say just leave some time before going into the mountains. The main difference the rate of the climb is vastly different.
Studies have shown that divers who dive near NDL and fly within 4 hours have a 5% increase in the chance of DCS. These are the divers who have been examined and showed no clinical signs before flight. When we reach the 24-hour mark, there is no difference in the onset of DCS between those that fly and those that do not.
Plan your trip so that you have a full day of relaxation before you fly back home.
Plan that next dive trip with the no-fly rule in mind. Avoid diving to NDL on your last day of diving and have the day before your departure dive free. This gives you time to relax and buys souvenirs instead of the last minute typical airport gift shops items.
Blog written by: Rutger Thole who is an avid scuba diver and loves to travel, dive and write about scuba diving. Based in Amsterdam, he runs bookyourdive.com and at least twice a year he plans a dive trip of the beaten track.