I learned about shallow water hypoxia when I first took up SCUBA diving (before most of you were born :)). It occurs during ascent, for two reasons. The most obvious is that you've been holding your breath longer when you're ascending. The other is that when your body is under pressure, the partial pressure of oxygen in your lungs is higher, allowing the hemoglobin to absorb more. As you ascend and the pressure lessens, the partial pressure of oxygen decreases accordingly.
Apparently shallow water hypoxia is now a thing that life guards worry about, despite the fact that it's unlikely in a swimming pool. I swam underwater about fifty feet in my neighborhood pool, which is five feet deep, and was told by the lifeguard never to do that again. Indeed there's a sign that says "don't hold your breath underwater." Maybe you're supposed to breathe underwater?
I blame the American Red Cross, which has a history of dumb things IMO, starting with teaching the crawl stroke to non-swimmers instead of the side stroke or back stroke, both of which require far less coordination, and allow you to keep your face out of the water while swimming.
But don't get me started, or I'll tell you about "drown proofing."
A kid at my high school, who was on the swim team, died from swimming underwater. They did informal competitions to see who could swim the furthest underwater. He was practicing that by himself, and overextended himself, passing out underwater. There were other people in the pool but no one noticed until it was too late.
I'm sure that's not the only case. Between the risk of drowning, and the difficulty for a lifeguard to tell between someone holding their breath underwater vs actually drowning, I'm not surprised a lot of pools have "don't hold your breath underwater" signs.
wow, that's terrible. I was a competitive swimmer in my youth, and we did similar competitions from time to time (very rarely). I've never seen or even heard of a swimmer passing out underwater.
A more common drill we would do in practice were called "hypoxic" sets, where we would do one length of the pool breathing every 3 strokes, then the next every 5 strokes, then 7, 9 etc.. until you were going across the whole length (25 meters) without breathing. Not everyone could do it towards the longer distances without breathing, and the coaches would look out for "cheaters", but never once did anyone pass out. Maybe most swimmers, by way of the typical training and exertion in the pool, just don't develop a very good suppression of the "breath signal". I also never remember seeing anyone purposely hyperventilate so that they can stay under water longer.
I think you must have misunderstood your scuba diving instructor because you have that part almost entirely wrong. Scuba divers don't hold their breath while ascending. This is extremely dangerous due to lung over-expansion injuries (gas embolism).
Among healthy people at sea level, hemoglobin oxygen saturation is already near 100%. You can't squeeze more oxygen into the hemoglobin regardless of inspired PPO2. What actually happens at higher PPO2 levels is that extra oxygen dissolves into the bloodstream and circulates unbound from hemoglobin. (This is why recompression chambers can be used to treat patients suffering from carbon monoxide poisoning.)
Ascending back to the surface reduces PaO2 (assuming no change in breathing gas) but this presents no risk of hypoxia since the diver would just be returning to normal levels. The only exceptions would be for a technical diver breathing a hypoxic mix (like less than about 15% O2), or using a rebreather where some sort of error or failure prevents oxygen from being injected into the loop.
No, I understood perfectly well that they were talking about coming up from a deep(ish) breath holding dive (snorkeling). But I learned this in the context of a SCUBA course, for which at least at that time the first half was snorkeling. I think that was to weed out people who weren't comfortable in the water. I realize I didn't make that clear when I wrote that.
Holding your breath when ascending is the number 1 thing every scuba diver is taught not to do.
Divers may hold breaths a little longer or breath out a little earlier to make subtle changes to their buoyancy when swimming horizontally. But this is definitely now how you are trained to ascend or descend.
Also a diver here. There are generally different biological mechanisms involved for “free diving blackout” vs “hypoxic blackout” (which is related to exertion while breathholding). Freedivers train to relax muscles and stay calm, so blackouts due to pressure changes are more common rather than to exertion. Swimming horizontally under water requires a lot of effort, which burns more oxygen. Combine high exertion with untrained individuals not aware of the risk, or misuse of hyperventilation (which doesn’t increase oxygen, but decreases CO2), and suppressing urge to breath, and you can also blackout.
That's just 17 yards. I've done that far on a single flip turn. Did you just spend like 70 seconds under there or something? Some lifeguards are a bit weird.
I've always taken those "Don't hold your breath underwater." signs to mean like breath holding practice where you try and hold your breath as long as possible in one place. Not active swimming.
After watching pretty much every swimming event at the recent Olympics, there was definitely a lot of those turns spent underwater and not one of the swimmers seemed negatively impacted by it.
It's actually possible to swim faster underwater than at the surface due to differences in drag. In sanctioned swim meets such as the Olympics, competitors aren't allowed to go more than 15m underwater.
I guess the fastest stroke is the Fish Kick. Since you are kicking sideways and underwater, the pressure wave from your kick isn't being reflected from the surface or the bottom of the pool, allowing you to go way faster. It's bonkers! But I guess it's extremely difficult to pull off because of how hard it is to balance on your side while kicking underwater.
Where I'm from everyone is taught breaststroke by friends/family just to be able to stay afloat. And if one goes to real swimming classes then they are taught crawl stroke next.
Sorry, I didn't see your question before. I think there's an article in wikipedia about it, but from what I recall: there were studies that said most people drown in water just over their head, not in deep water. Drown proofing was a technique where you held your breath, and sank. I'm not sure how that was supposed to work, since most people are positively buoyant, especially if they take a deep breath--something about kicking up so your shoulders came out of the water, and then you sank down at first from the weight of the part of your body above water, and then the rest of the way from momentum. When you hit the bottom, you kicked off with your body angled toward shore. Rinse and repeat, until you were in water where you could stand up.
What's wrong with that? Lake and river bottoms (and ocean beaches) can have troughs, you could easily wind up in water too deep to do this. And if you can kick enough to get your shoulders out of the water, why not just use your arms to propel yourself toward shore? It just seemed like teaching people a simple stroke (like breast, side or back--anything but crawl) would be easier and safer.
But then I fail to understand how people drown because they get caught in rip currents. Getting out of one is so easy.
The only drown proofing I can think of is what the Navy Seals do in BUDs. They tie their hands behind their back and their legs together and then pop up for air when they need it. It's a lot harder for those guys because their body fat percentage is microscopic so they sink like rocks. They would probably have to give me a couple pounds of lead to sink properly...
> I swam underwater about fifty feet in my neighborhood pool, which is five feet deep, and was told by the lifeguard never to do that again. Indeed there's a sign that says "don't hold your breath underwater."
That's insane and not far off from a level of conservatism that dictates "Don't get in the water, because it's dangerous."
If a kid is spending all day in the pool doing 5' breath holding underwater swims... maybe I'd be concerned?
But it's crazy to say there's a >0% but <1% risk... so we're going to ban it.
> I blame the American Red Cross, which has a history of dumb things IMO,
Recommendation by committee. Initial reqs are decent, then become increasingly unreasonable as people add "What if"s.
As the quip goes, engineering is knowing what trade-offs are worth taking, not being unwilling to accept any trade-off.
PS: Did you go to GT when they still had the mandatory requirement?
If I'm in the position of that lifeguard, I A) don't want to deal with any actual or near-drownings because I generally like people, B) am going to tell people to comply with the posted rules or I'll get fired.
If I'm the pool owner, A) I don't want my pool in the news as "the one where that kid drowned", and if it is, I want 100 witnesses where the lifeguard kept shouting at him to stop doing that or he'd be kicked out. I don't want a reputation of hiring lifeguards that don't. B) I don't want to deal with any wrongful death suits.
Sure, test your limits, but if you're going to do so, do it somewhere else. Ideally with supervision, but I won't tell you how to live your life. If you look up videos for 'static apnea', WR breath-holders have one or two safety staff in the pool with them in arms' reach.
Apparently shallow water hypoxia is now a thing that life guards worry about, despite the fact that it's unlikely in a swimming pool. I swam underwater about fifty feet in my neighborhood pool, which is five feet deep, and was told by the lifeguard never to do that again. Indeed there's a sign that says "don't hold your breath underwater." Maybe you're supposed to breathe underwater?
I blame the American Red Cross, which has a history of dumb things IMO, starting with teaching the crawl stroke to non-swimmers instead of the side stroke or back stroke, both of which require far less coordination, and allow you to keep your face out of the water while swimming.
But don't get me started, or I'll tell you about "drown proofing."