The diving center at altitude
- Michael Mutter
- vor 1 Tag
- 4 Min. Lesezeit
I recently got into a discussion about diving on La Palma. The dive center there is located about 700 meters above sea level in the hills above the coast. El Hierro, the neighboring island, is ranked by DAN among the “top 6 most challenging dive sites” worldwide due to its similar topography.
This raises the question for many: after a dive at sea level, isn’t there a significant risk of suffering decompression sickness during the subsequent ascent to the higher-altitude dive center?
Veronika Sievers, a local dive instructor and decompression specialist, has addressed this topic in detail. It comes as no surprise that, as a local operator, she views DAN’s assessment rather critically and sometimes perceives it as unnecessary fear-mongering.
What I found particularly interesting was the ensuing discussion. As is so often the case, it quickly drifted into vagueness: instead of hard physiology, the talk turned to “gentle dives” and “plenty of decompression.” This reveals one thing above all else—gaps in knowledge.
Yet for decades, a solid scientific concept has existed that allows one to calculate precisely whether a risk actually exists in such a situation. The key term is:
Oversaturation tolerance — the very foundation of decompression physiology
Oversaturation tolerance describes how high the inert gas pressure in a tissue may be relative to the prevailing ambient pressure without causing decompression sickness.
This concept is by no means new. It has been known since at least the 19th century and was first incorporated into systematic dive tables around the turn of the century by Haldane. Later, it was further developed by Bühlmann, among others, and remains the foundation of virtually every modern decompression model to this day.
According to Bühlmann, the maximum tolerable overpressure for each tissue can be derived directly from the prevailing ambient pressure—it is proportional to the ambient pressure and follows a linear relationship.
The practical consequence is immediately apparent: as altitude increases, the tolerance for supersaturation decreases because the ambient pressure drops. This means that tissue overpressure still tolerated at sea level may, under certain circumstances, already lead to decompression sickness at higher altitudes.
This also explains a well-known phenomenon: When diving in mountain lakes, despite lower tissue supersaturation compared to diving at sea level, no-decompression limits are shorter and decompression times are longer.
What does this mean specifically for La Palma?
What happens during a dive at sea level followed by an ascent to the dive center at 700 m above sea level?
I calculated this using the Bühlmann model for the following scenario:
2 recreational scuba dives
Depth: 40 m
Bottom time: 20 minutes each
Surface interval: 90 minutes
Gradient factors: 70/85
The result is remarkable:
Even in the unrealistic extreme case where one would ascend to 700 m immediately after the second dive without any waiting time—essentially “Scotty, beam me up!”—the supersaturation tolerance would just barely remain within limits.
However, reference tissue No. 7 would utilize 99% of the maximum tolerated inert gas overpressure—meaning it would be a hair’s breadth from the critical limit.
A one-hour wait significantly reduces this risk: In that case, the relevant reference tissue No. 10 would only reach about 80% of the maximum tolerated supersaturation.
This shows:
The risk of decompression sickness decreases particularly sharply in the first hour after the dive.
The famous deco-beer thus certainly serves a physiological purpose—and, incidentally, it also allows you to keep an eye on your dive buddies’ well-being.
Additional sensible safety measures
Safety margins can be further increased by:
Using nitrox while operating the computer in air mode
Activating mountain lake mode on the dive computer
Using appropriate decompression gases for more demanding dives
I also considered a technical scenario:
A rebreather dive to 60 m with
No-decompression limit: 20 minutes
Diluent: Trimix 18/45
Setpoint: pO₂ = 1.3
GF: 70/85
would result in 99.9% depletion in target tissue No. 8 upon an immediate ascent to 700 m—meaning the limit would be reached directly.
A one-hour wait time drastically reduces the risk here as well: In that case, depletion would be only about 71% in target tissue No. 12.
This clearly shows that the rest period before ascent is the decisive safety factor.
What do the classic recommendations say?
This insight is by no means new. It corresponds exactly to the recommendations of Bühlmann, known for decades, who advises a waiting period of at least one hour before any ascent to altitude following a dive.
For mountain passes after dives, Bühlmann recommends:
First hour: no additional ascent
After that: a steady ascent to the target altitude
The tables start at 2,500 m above sea level. This altitude may be reached no sooner than two hours after the dive (1-hour waiting period + at least 1-hour ascent).
Conclusion: Not diving folklore, but physiology
The conclusion is clear: Ocean diving followed by a return to a dive center located at 700 m above sea level is fundamentally safe—provided you act sensibly.
This means, first and foremost, taking a break at sea level after diving before traveling to higher altitudes. The safety margin can also be further increased at any time by following the well-known recommendations for safe diving.
Not surprisingly, this aligns with the practical experience of local dive centers: Despite a very large number of dives conducted, no noticeable increase in decompression sickness has been observed.
However, ascending to significantly greater altitudes is another matter. As is so often the case, the same principle applies here: Not everything that is possible is also sensible—and ultimately, it remains a question of moderation.
Or to put it another way: Haleakalā on Maui remains off-limits after a diving trip to Molokini Crater.
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