6 dive theory myths, and what’s actually true
In short
Six dive theory myths with safety stakes: staying within your NDL doesn’t make decompression sickness impossible; nitrox isn’t for going deeper; narcosis can start shallower than 30 m (100 ft); shallow water is where held breaths hurt most; mild symptoms after a dive need oxygen and a call; after diving, any trip to altitude adds risk.
Back on the boat after a deep dive, someone says: “We stayed inside the NDL, so we can’t get bent.” Nobody argues. Plenty of diving “facts” get repeated until they sound like rules, and most do no harm. These six can: they lead divers to take nitrox too deep, sit on symptoms after a dive or hold their breath just below the surface. For each one you get what’s true, what’s false, the source and what to do instead.
| Myth | What’s actually true | Source |
|---|---|---|
| Within your NDL, you can’t get DCS | DCS can happen even within accepted limits | DAN |
| Nitrox lets you dive deeper | Nitrox reaches its oxygen limit shallower than air | NOAA, DAN |
| Narcosis starts at 30 m (100 ft) | Usually noticed around there, but it varies by diver and by day | NOAA, DAN |
| Shallow water is safe for your lungs | Volume changes most in shallow water | DAN |
| Mild symptoms will pass | Mild, vague symptoms are typical of DCS | DAN |
| After diving, only flying matters | Any ascent to altitude adds DCS risk | DAN |
1. Myth: you can’t get DCS within your no-decompression limit
What’s true: no-decompression limits, dive computers, slow ascents and safety stops all lower your risk of decompression sickness (DCS).
What’s false: that the limit is a line between safe and bent. Divers Alert Network (DAN) says on its decompression illness page that bubbles can form even when divers follow accepted guidelines. Octo’s Divemaster lessons say the same: no decompression model draws a sharp line between safe and unsafe.
Takeaway: treat the NDL as an outer edge and end the dive well before it runs out, follow the more conservative of two computers, make your safety stop, and never dismiss symptoms because the dive “went fine.” More on decompression sickness and dive computers and the NDL.
2. Myth: nitrox lets you dive deeper
What’s true: nitrox has more oxygen and less nitrogen than air, so you absorb less nitrogen and get longer no-stop times at the same depth, as long as your computer is set for the mix.
What’s false: that it lets you go deeper. More oxygen means you reach your oxygen limit sooner. NOAA’s training material notes that a diver on nitrox reaches a ppO2 of 1.4 much shallower than a diver on air. At 1.4, EAN32 stops at 33 m (111 ft) and EAN36 at 28 m (95 ft). DAN warns that the signs of oxygen toxicity don’t always come before a convulsion, and a convulsion underwater can lead to drowning. Nitrox doesn’t remove the risk of DCS either.
Takeaway: dive nitrox only once you’re trained for it, analyze your own cylinder, set your computer for the mix and never go below your maximum operating depth. See oxygen toxicity and nitrox.
3. Myth: narcosis only starts at 30 m (100 ft)
What’s true: NOAA’s training material says nitrogen narcosis is usually noticeable around 30 m (100 ft).
What’s false: that it switches on at a fixed depth. The same NOAA material stresses differences from diver to diver and from day to day. DAN lists fatigue, alcohol before a dive and cold water among the factors that may make narcosis more likely. Octo’s lessons say most divers notice it from about 30 m (100 ft), and some shallower.
Takeaway: if you or your buddy feel giddy, slow or oddly sure of yourselves, at any depth, signal and ascend a few meters (feet) together. DAN notes that narcosis typically clears once you rise above the depth where it started. More on nitrogen narcosis.
Try a question from the app
nitrogen narcosis
At 18 m, you feel unusually giddy and laugh at nothing.
Show the answer
Answer D. Most divers notice narcosis around 30 m, but some feel it shallower. Rising a little clears it.
4. Myth: shallow water can’t hurt your lungs
What’s true: deep dives bring their own risks, such as narcosis and shorter no-stop times.
What’s false: that shallow means safe for your lungs. In an Alert Diver article for DAN, Joseph Gessert calls shallow water the easiest place to suffer an arterial gas embolism. At 3.4 m (11 ft), a held breath expands by a third on the way to the surface; even a 1.5 m (5 ft) ascent with a held breath could be enough to cause an embolism. Lung injuries can happen in a shallow swimming pool.
Takeaway: breathe continuously on every ascent, and pay most attention in the last few meters (feet). Gessert’s suggested version of diving’s golden rule: “Breathe continuously, especially in shallow water.” See barotrauma and equalizing.
Try a question from the app
breath-holding in a pool
In the deep end of the pool, a classmate takes a full breath from the regulator, takes it out and swims up holding their breath to talk to the instructor.
Show the answer
Answer C. The biggest volume changes happen near the surface. Lung injuries have occurred from pool depths.
5. Myth: mild symptoms after a dive will pass on their own
What’s true: DAN notes that most decompression sickness shows up subtly, as a minor joint ache or tingling in an arm or leg, and that mild symptoms sometimes fade by themselves.
What’s false: that you can wait and see. DAN warns that divers often blame these symptoms on overexertion, heavy lifting or a tight wetsuit, which delays treatment, and that symptoms can get worse. Going back underwater to “recompress” isn’t first aid either: in another Alert Diver article, DAN calls surface-level oxygen the preferred first aid, and notes that many authorities reserve in-water recompression for remote places with no other option and with trained staff and equipment.
Takeaway: treat it as possible DCS. Give oxygen if you’re trained to, call emergency services and a diving emergency hotline such as DAN’s, even if the symptoms seem to fade, and follow your first aid training. The Rescue Diver course covers these first steps.
6. Myth: after diving, only flying matters, and 12 hours is always enough
What’s true: DAN’s guidelines call for at least 12 hours before flying after a single no-decompression dive, and at least 18 hours after several dives a day or several days of diving.
What’s false: two things. First, these are minimums: DAN says they reduce the risk but don’t guarantee you’ll avoid DCS, and longer waits reduce it further. Second, planes aren’t the only problem. DAN explains that going up to altitude after diving raises DCS risk because the pressure drops further, and the higher you go, the greater the risk, whether you get there by plane or by car over a mountain pass.
Takeaway: after a week of diving, end your last dive at least 18 hours before your flight, longer if your computer’s no-fly time says so, and plan mountain drives with the same risk in mind; ask your dive center about the route.
Frequently asked questions
Can you get decompression sickness within no-decompression limits?
Yes. Divers Alert Network notes that bubbles can form even when divers follow accepted guidelines. No-decompression limits, computers, slow ascents and safety stops lower the risk without removing it. End dives well before your limit, follow the more conservative computer in a buddy pair, and treat any symptoms after a dive as possible decompression sickness, however well the dive went.
Does nitrox let you dive deeper?
No. Nitrox has more oxygen than air, so it reaches the oxygen limit at a shallower depth. At a ppO2 of 1.4, EAN32 has a maximum operating depth of 33 m (111 ft) and EAN36 of 28 m (95 ft). Its benefit is longer no-stop times at moderate depths, with a computer set for the mix, and it still requires nitrox training.
At what depth does nitrogen narcosis start?
NOAA’s training material says narcosis is usually noticeable around 30 m (100 ft), but it varies from diver to diver and from day to day, and some divers feel it shallower. Fatigue, alcohol before a dive and cold water may make it more likely. It typically clears once you ascend above the depth where it started.
How long should you wait to fly after diving?
Under DAN’s guidelines, wait at least 12 hours after a single no-decompression dive and at least 18 hours after several dives a day or several days of diving. These are minimums that reduce the risk without removing it, and longer waits lower it further. If your dive computer shows a longer no-fly time, follow the longer of the two.
Is it safe to drive over a mountain pass after diving?
Going up to altitude after diving raises your risk of decompression sickness, whether by plane or by car, because the surrounding pressure drops further. The higher you go, the greater the risk. Plan mountain routes and their timing with that in mind, and ask your dive center about the roads near the dive site.
Sources
- Ed Thalmann, Decompression Illness, Divers Alert Network
- Ed Thalmann, Oxygen Toxicity, Divers Alert Network
- NOAA Office of Marine and Aviation Operations, Inert Gas Narcosis and O2 Toxicity (training slides)
- Yue, Li, Abraham and Scott, Know the Risks, Alert Diver, Divers Alert Network (July 18, 2021)
- Joseph Gessert, Shallow-Water Arterial Gas Embolisms, Alert Diver, Divers Alert Network (July 19, 2021)
- Daniel A. Nord, In-Water Recompression, Alert Diver, Divers Alert Network (November 1, 2019)
- Divers Alert Network, Flying After Diving
