Its interesting to me as a rock climber that I've never heard of this. Its not unusual for someone to hang in their harness at an anchor for quite a while while their partner is climbing the pitch before and after. Are climbing harnesses that much more comfortable than safety harnesses that its not a problem?
As a climber who saw OSHA warning sheets about this, I had the same question and found several major differences.
1. This is almost exclusive to sustained free hangs. Bouncing against a wall or otherwise working your thigh muscles circulates blood with minimal effort, so standard climbing circumstances raise the safe time - potentially by hours. (Notably, Alpine climbing guides and specifically Alpine rescue guides do cover this syndrome in case someone falls and hangs while awaiting aid.)
2. OSHA-style harnesses are over-the-shoulder devices anchored between the shoulderblades. Many don't even have waistbands, just leg and armpit straps. That's useful for unpredictable falls, but it's the worst possible configuration for suspension trauma. Your legs hang straight down and the straps pull up at the edge of your groin, cutting off blood flow. Even hanging free, the climbing harness posture (elevated legs, pressure on hamstrings) is much safer.
3. Safety harnesses are mostly cheap and ill-fitting because you're not expected to hang in them; they're the same unpadded straps you get on the worst climbing gym rental harnesses. The time to initial risk apparently varies a lot with harness fit and padding, so comfy climbing gear is also much safer.
The ad below has a good visualization of the difference; it's from a company selling OSHA-compatible harnesses which pull you into a climber-like sitting position for exactly this reason.
Safety harnesses you mention in 3 are also often for fall prevention* not fall protection. Fall protection harnesses aren't much better but typically will at least have padded leg loops.
*: Prevention: when tied in you can't make it to a place you can fall such as a system that prevents you from getting within a few feet of a roof
Edit: the link you provide seems to be a lightweight fall protection harness and certainly one I'd like using as I prefer a lighter harness when working at height because it means less fatigue and less chance of a mistake leading to a fall.
As others have mentioned, this is mostly not a problem because climbers are usually concious, and moving about and whatnot. Hanging in a harness for too long is quite uncomfortable if you don't move at least a little bit.
Where this starts to really matter though, is in the event of an accident, particularly if you're above, and belaying a second, and the second is hit by a rock or something and falls unconcious. In this case, you often can't easily lower them, especially if you're more than 60 meters off the deck; you usually can't raise them unaided either, unless you're unhumanly strong. Setting up a z-pully in a timely manner, and hauling the second up is the best option.
Unfortunatly, these sort of skills are not often practiced in the climbing community.
Edit: also, I think it's less of a practical issue in a climbing harness, unless you also have a chest harness on, or a full body, which is pretty uncommon.
Raising an unconscious climber is potentially not viable.
1. You probably don't have time. As the article says, you have less than 14 minutes. If the climber isn't within view, you may not even realise they are unconscious for a few minutes, perhaps thinking they just can't hear you.
2. If you don't have efficient pulleys, friction will be a major issue. Hauling a person up a long pitch on a Z drag is going to be a challenge. More mechanical advantage would just add more friction and time.
3. Raising the climb could cause them injury, or make their injuries worse, and could potentially kill them. Their body or the gear attached to it is likely to jam against something or snag on the way up unless it is a completely clear path.
Another option in that scenario is going down to the injured climber after escaping the belay. If you have enough rope out of the system, you can abseil down. Otherwise, you will need to descend the taut rope down to the hanging climber.
From there, I suppose you would need to manipulate their body to get blood flowing.
...and that is only the first step of the rescue! I hope I never have to face such a scenario.
It will still happen in a climbing harness if you are hanging in space. On a hanging belay, you try to get your weight off the harness and get your weight on your feet on the wall as much as possible. This alleviates the problem.
Cavers will spend a long time hanging in space on rope when climbing out of deep pits (over 100 meters / 300 ft) with single rope technique. That doesn't lead to harness hang, because you are engaged in aerobic activity with your legs.
That is true. A free hanging belay would not be very useful! I did find some discussion on mountain project about it in the context of rescuing an incapacitated climber.
Could one possible protection mechanism be to keep an ascender clipped to the back of your climbing harness, even if you don't normally use it? Then, as long as your were conscious and not too injured, you could attach it above you, and hang from it at intervals to get the weight off your legs, (while waiting for rescue) and to help with rescue?
Many climbers keep a loop of accessory cord with them to make a prusik knot for this reason. Much lighter and cheaper than an ascender but not as efficient.
At a hanging belay, one typically does not remain motionless. People move their legs around, either just while fidgeting or balancing, or bracing their legs against the rock, or aiders.
I cannot speak to any differences between climbing harnesses and safety harnesses.
There are lots of relevant differences between climbing and safety gear and posture, but you're spot on about motionlessness as the key element.
Even for safety-gear falls, suspension trauma is somewhat overhyped for "slip and your harness catches you" accidents. The natural reactions - fidget, adjust for comfort, move your legs if they feel tingly - increase the safe time a great deal, so the main risk is falling into a hard-to-recover spot and getting tired.
That said, construction incidents are far more likely than climbing falls to be worst-case events: a substantial fall paired with injury or unconsciousness. When that happens, the impact pulls the harness tight and the victim doesn't adjust it or otherwise move around at all. That's where the stats about trauma starting in minutes enter the picture.
I think the lack of movement (especially if unconscious or barely conscious) and full suspension is key. When climbing, do you typically maintain contact with the wall, especially with you feet?
Barring injury, you wouldnt be hanging in free space for very long. I didn't realize that would make such a difference.
After my comment I did some more searching and found plenty of discussion of harness hang syndrome in the context of rescuing an incapacitated climber though.
Have you read much about prusik mistakes and accidents? I feel like this is mentioned quite a bit when talking about whether to put your third hand above or below your belay device.
I think I've only heard of a single accident when this has happened and involved death, though.
(hint: put it below or you could get stuck if you can't escape the prusik)
Interestingly - all your weight is on the harness, but the wall is still what makes the difference. Suspension trauma is basically a bloodflow issue, so resting against a wall lets you keep your feet elevated and your quads active, stopping blood from pooling. The catastrophic case for suspension trauma is hanging limp and unconscious, since it lowers your feet while stopping muscle use or even fidgeting to boost circulation.
If the belay stance is a ledgeless hanging belay and the climber above you is slow, you can be hang as much as you end up wanting. I'll end up putting a hip into the wall to give my feet a rest.
But even during super long belays, it's unlikely for the belayer to be motionless for even ten minutes.
I would assume the average rock climber is lighter and more athletic than the average tower worker, and they’re probably not also carrying heavy tools and hardware.
