Useful consciousness at 30000 feet

News reports regarding the plane crash that killed Payne Stewart mention the possibility of explosive decompression, and usually say something about the lack of oxygen making useful consciousness at 30,000 feet being measured in seconds.

Clearly there’s something I’m not understanding. I can hold my breath, simulating a zero oxygen atmosphere, and have useful consciousness for about a minute (or however long I can hold my breath). Are they taking into account the coldness as well? Why isn’t useful consciousness as long as one can hold their breath?

I’m no expert, but I don’t think the cold has an immediate effect on consciousness. Of course it’s important long-term, or Leo DiCaprio would have married Kate Winslet and sold the necklace and lived off his wife’s earnings…but I digress.

If my supposition is correct then they are only talking about lack of oxygen. And you’re right. If you had a good breath inside you you could hold it as long at 30,000 feet as you could at sea level. A couple of problems though – if was a truly explosive decompression 1) you wouldn’t get a chance to take that good deep breath and 2) your lungs would decompress right along with everything else unless you happened to be holding your nose.

So, yes, I think useful consciousness would be in the few seconds range. But it’s always a real good idea to question what you hear on the TV news.

“Vandelay!! Say Vandelay!!”

Pulling out my 1992 copy of the skydivers information manual we find: for 30,000’ MSL both onboard and bail out oxygen is required. Estimated useful conciouness w/o oxygen is 1 minute 30 seconds,

Not quite. When you hold your breath at or near sea level, the partial pressure of oxygen inside your lungs is 760 mm Hg(Hg = mercury) minus the partial pressure of water dissoved in the air (call it 60), times the proportion of air that is oxygen (.21). This leaves a partial pressure of oxygen in inhaled air of ~ 147 mm Hg which would fall only gradually over the minute. Breathing is actually driven not by low oxygen in the blood, but by build-up of CO2.

In the lungs, though, oxygen is diffusing in, and CO2 is diffusing out, so you wind up with a partial pressure of oxygen of around 105-115. This is at sea level.

Here in El Paso, it’s more like 95-105.

At 30,000 feet, it would be drastically less, (made-up number disclaimer) maybe like 20-25 mm Hg. This means that air at 30,000 feet which is still 21% oxygen, only oxygenates your blood as effectively as sea level air which is 3% oxygen. And in the event of sudden depressurization, it would not be humanly possible to hold sea level pressure air in your lungs at 30,000 without causing massive tissue rupture. Good thing you’d exhale involuntarily…

Sue from El Paso

Mundane pointless aside I think is interesting and forgot to post above:
both the skydivers information manual and the FAA regulations specify aviators oxygen, because medical O2 is too moist and can cause regulator valves to ice over at high altitudes,

C’mon, people… Think harder…

It doesn’t matter if you hold your breath and your lungs were humongous. You have OTHER holes from where the air within your body will try to escape. What, do you think your ears pop because of the carbonated soda you drank before boarding the plane?

There is air in many little crevices within your body, and the pressure exerted by this air equals the pressure of the air that surrounds you. That is basically why we are not crushed by the atmosphere’s pressure.

When there is a de-compression in an airplane at high altitude, the air within your body tries to leave ASAP to compensate and maintain the pressures inside/outside equal.

The supplementary oxigen provided by the emergency masks is there to help increase the volume of oxygen as well as its pressure within your lungs, so it can maintain proper flow and keep you alive. The partial pressure info mentioned by Sue above is correct.

Cecil on humans in a vacuum and explosive decompression: