May 9, 2012  -- As the Air Force searches desperately for the source of a mysterious and potentially deadly oxygen system problem in its $79 billion fleet of F-22 Raptor fighter jets, it is also investigating why the jets' pilots are coughing so often after missions that the pilots have taken to calling it the "Raptor cough."

For decades pilots in fighter jets have been contending with temporary fits of coughing after executing extreme maneuvers in the air, due to a known condition called acceleration atelectasis, but an Air Force spokesperson told ABC News that the coughing appears to be more prevalent in F-22 pilots.

And while the current thinking by the Air Force is that the F-22 pilots suffer more bouts of coughing than their counterparts is because the F-22 can fly at more extreme speeds and altitudes, Air Force spokesperson Lt. Col. Tadd Sholtis told ABC News that the service has not discounted the possibility that toxins that may have leaked in to the oxygen system could be exacerbating the coughing.

The leakage of toxins into the oxygen system is also a possible cause of F-22 pilots experiencing dangerous "hypoxia-like symptoms" while flying the Raptor in more than two dozen cases since 2008, as reported in a recent ABC News' "Nightline" investigation. Hypoxia is caused by a lack of oxygen to the brain and is characterized by dizziness, disorientation, poor judgment and, eventually, unconsciousness.

READ Exclusive: Family Demands Truth in Air Force F-22 Pilot's Death

Last year the Air Force grounded the full fleet of F-22s, which cost U.S. taxpayers more than an estimated $420 million each, for nearly five months to investigate what was causing the incidents, but never could come up with an answer and sent the planes back in the air.

Over the weekend, one of two F-22 pilots who spoke out publicly to CBS News' "60 Minutes" about their fears of flying the F-22 linked the "Raptor cough" with other symptoms associated with the "hypoxia-like" incidents, saying that in a room full of F-22 pilots, "the vast majority will be coughing a lot of the time."

Sholtis said that there are no hard numbers on how many F-22 pilots are experiencing the "Raptor cough," but said it's obviously "common enough to have developed its own moniker."

Last week the Air Force officially received the last F-22 Raptor from defense contracting giant Lockheed Martin, completing an order of 187 planes that cost U.S. taxpayers an estimated $79 billion -- meaning that including research, development and production among other costs, each plane has a price tag of more than $420 million. Despite being the most advanced fighters on the planet, none of the planes have been used on a combat mission since they went combat-ready in late 2005.

READ: McCain Says $79B Jets Still Have No Mission

F-22 Ground Crews Experience 'Hypoxia-Like Symptoms'

Sholtis also revealed another twist in the F-22's case this week when he told the Air Force Times that over a period of four months after the grounding last year, ground crews reported at least five different incidents of experiencing their own "hypoxia-like symptoms" while running engine tests with the planes on the tarmac. No similar incidents have been reported since, Sholtis told ABC News.

The F-22 Raptor is designed to feed its pilots air by pulling oxygen from the engine combustion system and filtering out any dangerous fumes or chemicals. Known as on-board oxygen generation, which differs from previous planes' systems that used air from inside the cockpit as part of the oxygen delivery, the system is meant to allow the planes to fly through noxious or poisonous air without endangering the pilots.

But in addition to the apparent possible leaks in the system, it is also designed so that if there is a catastrophic problem with the engine, the whole system can shut down -- leaving the pilot to basically suffocate until he can bring the plane down to breathable air or manually activate the emergency oxygen system tucked into a corner of the cockpit.

That's what the Air Force said happened to Capt. Jeff Haney just before he crashed his F-22 Raptor in the Alaskan wilderness while on a training mission in November 2010. Even though the plane shut down Haney's oxygen, an Air Force investigation board found that, among other factors, Haney was to blame for the crash for being too distracted by his inability to breathe.

READ Exclusive: Family Demands Truth in Air Force F-22 Pilot's Death

Top Air Force Commander to Hop in F-22 Cockpit

After the "Nightline" and "60 Minutes" reports, Air Force Air Combat Command head Gen. Mike Hostage released a statement expressing the service's "confidence" in the beleaguered jets.

"We live in a community where risk is part of our lives," he said. "If we think the risk has gone to a level where we just can't accept it, we either reduce that risk or eliminate it. But right now, we believe that risk -- although it's not as low as we would like it -- is low enough to safely operate the airplane at the current tempo."

Hostage said he would soon hop in the cockpit of a Raptor himself to help him understand what the pilots are facing and not stop flying until a solution to the mystery problem was found.

"I'm asking these guys to assume some risk that's over and above what everybody else is assuming, and I don't feel like it's right that I ask them to do it and then I'm not willing to do it myself -- that's not fair," he said.

The Air Force has always maintained that the "hypoxia-like" incidents happen in exceedingly rare circumstances -- 25 cases compared to the thousands of missions flown without incident – and they have installed a number of safety precautions to mitigate the danger.

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