Diving After Flying
If there were a relationship between diving after flying and DCI, we would expect to see a great deal of decompression illness on the very first day of diving -- indeed, some data suggests that there are more accidents on the first day of a planned multiday dive trip. Of the 88 cases reviewed from the Caribbean for 1994, 33 -- or 37.5 percent -- occurred on the first day. The remainder occurred on days two through seven. Given that there are thousands of tourist divers who fly to Caribbean and Pacific dive sites, these numbers are far too small to establish a cause and effect.
([This paragraph has been revised following valuable input from a DAN Member - under Comments]: The evidence of an increased incidence of DCI on the first day of a dive trip is not sufficient to recommend a 24-hour waiting period before diving after flying. However, it does indeed support the advice of starting off much more gradually with fewer, shallower, more conservative dives right in the beginning, especially whenever there has been significant travel; the potential for more dehydration; a delay since diving previously; where the use of unfamiliar, rented gear is involved; if there is a lack of familiarity with the dive-site; etc. Taking it slow also gives divers an opportunity to rest and rehydrate, adjust to a new climate and time zone, and acquaint them selves with the new / rented dive equipment.
Why not recommend good hydration and a low deco-stress dive to checkout rented equipment as first dive. 24 hours after flying seems a bit extreme for something which has no convincing evidence one way or the other. Excessively conservative recommendations will get DAN a reputation for excessive conservatism, and people will stop taking the recommendations seriously.