What the “base on the mountain” has to say about it
The other day, a diver who travels in a small camper called me in the afternoon: she said she had done something very stupid and driven up to the Cumbrecita, to about 1000m altitude—that it was terrible, and she was already driving back down to spend the night at sea level, but was it really that bad? Did she need to worry that something would happen to her?
I was able to reassure her: No, nothing will happen to her because of that. It’s warmer down by the sea anyway, but yes, she could have stayed overnight at the higher elevation too. The dives were so gentle that it really doesn’t matter.
What bothers me about this: She was genuinely worried and probably gave up the nicer place to sleep—out of an irrational fear. And this fear is constantly fueled by rigid statements like “anything above 600m is like flying.”
Let’s take a more rational look at diving and altitude changes on land here.
Everyone remembers from their OWD that flying after diving isn’t such a good idea. And some have also heard that mountains are something similar.
Now there are different reactions to this: Some don’t think twice about visiting the observatory at 2,400 m after two heavy dives (not smart), while others don’t even dare to dive if they have to go up 700 m afterwards. Also not a good idea.
What does an altitude change after diving mean? What pressure differences are added to those during diving? And within what time frame? What could happen in the body, and what couldn’t?
What’s the deal with diving and altitude?
The models we use to calculate no-decompression limits and decompression are designed for diving at sea level—but especially in the case of the widely used Bühlmann algorithm, they’re also validated for higher altitudes. When diving at elevations above 700 m, you need to consider that air pressure is lower—approximately 0.1 bar for every 1,000 m. And that can make a difference: A dive to 27 m in a mountain lake at 1,000 m leads to supersaturation relative to the lower air pressure that you would reach at 30 m at sea level. Not insignificant. But people dive in mountain lakes, just adapted to the conditions. No problem.
If you drive up 700m after a dive, as we do here, that’s just a 0.07 bar pressure difference—equivalent to another 70cm of water column. Can that really be dangerous?
We can simulate that. I descend to 1m in 30 min (drive down), go diving, spend 30 min changing, and drive back up. This is what it looks like:
You can see it clearly here in the heatmap: even the half-hour spent packing up leads to relevant desaturation. And this is not stopped or disproportionately accelerated by the 30-minute final ascent at a speed equivalent to 3cm per minute underwater.
But what about driving uphill?
So much for the theory. Nevertheless, people report suddenly getting DCS on the drive home over mountains. Is there something more to it?
- Many people drive up mountains after diving. On La Palma, this happens after about 50% of all dives conducted island-wide.
- DCS is rare; DCS while driving over the mountain is even rarer.
- In diving, however, people like to look for a culprit when injuries occur, or the one behavior that led to the illness. Unlike other sports injuries that are simply accepted, here it must have been the PFO, dehydration, or the mountain.
- DCS often occurs up to 6 hours after diving—which is exactly the time you’re on your way home. Regardless of the altitude.
- There isn’t even a demonstrable correlation, let alone causality.
But there’s something else: The bubbles that are measurably present after the dive. The graph here shows at what point after a series of test dives what bubble formation was measured.
Clearly visible: The bubbles continue to increase slowly after surfacing, reaching their peak only 30 minutes after the dive, then are almost eliminated again after 90 minutes.
Exactly when the peak bubble formation is reached can of course vary for each person—but for many it’s within this time frame. And the timing can be different after decompression dives: With really demanding profiles, bubble formation might start already at 3 m.
On La Palma, we sometimes drive up when the bubbles are just at their peak. So it makes sense to ensure that as few bubbles as possible are present. Which isn’t a problem with our shore dives—we ascend so slowly and gently that we usually return to the surface with a gradient factor around 50, often even less. Really slow ascents, until you can stand up, also help to minimize bubbles. And when you do everything so gently, the additional 0.07 bar pressure difference really shouldn’t concern you.
If dives were more serious, if you really came out of the water with a GF of 80—then we like to stop here at sea level. Not because the 700 m would be so bad, but because after such a dive we really like to eat and drink first, and incidentally notice whether everything is OK…
To be absolutely sure
If you’re still not entirely comfortable: just set your computer to mountain lake mode (700 m). Then you could perform the dive as if you were at 700 m altitude—that covers the ascent.
And most importantly: Nothing speaks against a nice stop at one of the kiosks on the coast before driving back up. There you can satisfy thirst and hunger and off-gas some nitrogen. And since most DCS cases occur in the first hour after diving, you’re reasonably certain afterwards that you don’t have anything. Or you’ll just believe that the carbonation bubbles in your decompression drink are now responsible for the DCS….
So: dive smart with the mountain Punkfish
Learn more
The question of altitude differences after diving is especially relevant when diving in mountain lakes, and there are a few additional things to keep in mind. You’ll learn exactly what in the Altitude Diving course.
