Page last updated at 00:00 GMT, Wednesday, 17 December 2008

Altitude and that woozy feeling

By Jason Palmer
Science and Technology reporter, BBC News


'They sucked out the air around me.' Jason Palmer's oxygen deprived trip under the watchful eye of Qinetiq's Tim D'Oyly.

I'm in a simulated altitude chamber under the watchful eye of Des Connolly of QinetiQ's Human Performance division, and they're about to suck most of the air out.

It's all perfectly safe - for much of my simulated journey into the skies I'll be wearing a mask that provides air, much like the one fighter pilots wear. But for four minutes, I'll drop that mask and see how it feels to breathe the thin air at 7,600 metres (25,000 feet).

QinetiQ carries out such tests on fighter pilots to instruct them on the effects of hypoxia, or oxygen starvation. They also use the chamber to test life-support systems for high-flying jets such as the Typhoon, systems which are designed to whip into action in the event of sudden depressurisation.

The symptoms are mild, and they rob the mind of the ability to reason, so if it sneaks up on you, hypoxia will have you unconscious and then dead before you notice a thing. At the altitudes that a Typhoon can fly, that time could be as little as five seconds.

Luckily, I've got physiologist and pilot Tim D'Oyly at my side in the chamber to watch over the proceedings.

Last gasp

The problem is that there's less oxygen at higher altitudes (and less nitrogen and less of everything else).

At 5,500 metres, there is just half as much as at sea level.

There's another competing process too; the water vapour from moist internal tissues quickly saturates any air you breathe in. That water exerts its own pressure, regardless of altitude, and that in turn makes it even harder for the lungs to suck precious oxygen out of the thin air.

As a result, passenger planes are kept pressurised - but only to an equivalent altitude of around 2,000 metres. Building a plane that could maintain full, sea-level pressure inside with drastically decreased pressure outside at altitude would make them bulkier and more expensive.

F-16 fighters (Greek MoD)
Fighter pilots may have just seconds to fight off hypoxia

Which means that if you've flown by plane, you'll have experienced the same symptoms I did: ears popping and the need to clear them (that sort of yawning that one does to equalise the pressure across both sides of the eardrum, I learned, is called the Frenzel manoeuvre).

But as the air was sucked noisily out of the chamber and we quickly ascended to an equivalent altitude of 7,600 metres (25,000 feet), I encountered first-hand experience of the ideal gas law that relates temperature, pressure, and volume: it got cold.

Once we reached cruising altitude, I dropped the mask and got immediately to work on a number of tasks as my blood used up the oxygen floating around in my blood and the tiny amount in the air.

My time without the mask on was called out in 30-second, and then later, 15-second intervals. I wrote down my address, tried a spot-the-difference exercise, and was asked to remember a four-digit number.

When I started hyperventilating, my body's attempt to take in more air, I didn't even notice. Apparently I had taken on a sickly grey colour, as had my fingernail beds, and my blood oxygen level plummeted from 99% before the exercise to just 61%.

I, however, felt like I was doing pretty well. The worst I felt was a bit of a warm flush, as if much of me was blushing.

Near the four-minute mark, I found myself literally trying to put a square peg in a round hole; this child's toy is a popular test of cognition in hypoxic scenarios such as this.

The result? My handwriting went way downhill, I didn't spot the differences very well, and only remembered two of the four digits when I was asked later. When I put the mask back on and got my first blast of blessed pure oxygen, I had the worst case of seeing stars I've ever experienced.

I think it safe to say that until I've had much more training, no one should put me in the cockpit of a fighter plane. But I'll keep at it.

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