Hacker News Reader

Comment by itishappy | Hacker News Reader

> It is a problem caused by Scuba, and the fact that Scuba does what it is supposed to, extend the time you can spend underwater, is what causes the bends.

To expand on this a bit (pun intended), freediving after SCUBA is extremely dangerous, and this even includes shallow stuff like snorkeling. SCUBA adds dissolved gas to your blood, but once it's there decompression from any source puts you at risk for the bends. Repeated dives will have additional safety limits imposed due to the residual gas, which is managed by a complicated series of dive tables or a modern dive computer.

https://www.naui.org/resources/dive-tables-review/

View on HN

To further expand upon this, quickly ascending to high altitude (flying airplane or balloon) after diving is also dangerous, for the very same reason. Pilots are trained to wait out at least 24 hours after a dive, or at least 12 hours if the dive didn't require decompression steps.

To further expound on why snorkeling and freediving after scuba can be dangerous, it is helpful to understand that the greatest gas compression occurs in the first 10 meters underwater (Boyle’s law). Scuba divers are trained to ascend slowly (~10m/minute) in order to off-gas expanding nitrogen (and other gasses) and prevent bubbles from forming in tissue (which causes decompression illness). After scuba diving the human body still has a much higher residual nitrogen saturation, so even fairly shallow (less than 10m) rapid descents/ascents in the water column can cause bubbles to form.

I was told that I shouldn't go scuba diving the day before an early-morning flight, because if the plane decompresses, I could get the bends while everyone else just gets a little hypoxic and uses their handy drop-down O2 supply.

There is also an increased risk of decompression illness simply because passenger aircraft typically maintain a lower atmospheric cabin pressure (US federal limits pressure equivalent to 8,000 ft elevation). Going from a high pressure environment (underwater) to a low pressure environment (8,000 ft equivalent elevation in aircraft cabin) too quickly can also cause the bends. Dive computers use algorithms to calculate when it is safe to fly post-dive.

I'm really surprised by that. Looking at some charts, depending on the dive you're probably spending 30 minutes or less going from 2atm to 1atm, but now you need several hours or even a day before it's safe to go to .75atm.

We'd spend more time, but it's difficult to loiter 10m underwater. The residual gas from a dive takes about a day to release, and this impacts both follow-up dives and flights. Treatment options are different as well: an airlift (or just diving again and surfacing slower) vs diverting a whole flight.

There are wide safety margins in diving. Most dive trips involve multiple dives per day (starting with deepest dive first, with subsequent dives shallower). The times I’ve gone deeper or same depth on subsequent dives my computer usually puts me into deco fairly quickly. At some point subsequent dives aren’t feasible (typically no more than 4-5/day, less is prudent). I think it is with this in mind that the recommendation against flying is made, along with an abundance of caution due to limited experimental evidence (Navy dive tables were developed based on human subjects, so the effects are better understood). It looks like there are some civilian studies done, but more is needed on flying.

https://dan.org/health-medicine/health-resources/diseases-co...

The dive shop on my last vacation made me stop 24 hours before my flight home.

Yes, there is a slight risk even without a loss of airplane cabin pressure. You can find the consensus recommendations for flying after diving here.

https://dan.org/health-medicine/health-resource/health-safet...

Reply on news.ycombinator.com