At 30,000 feet an Aircraft is pressurized, and decompression could occur in 2 of 3 ways:
Air Force and Military Aircraft: In explosive decompression, cabin air humidity immediately cools and condenses into fog upon a sudden reduction of air pressure to that equivalent to 60,000 feet altitude. Within 2 seconds, the fog boils back into vapor in the new, low-pressure environment.
Explosive decompression occurs at a rate swifter than that at which air can escape from the lungs, typically in less than 0.1 to 0.5 seconds. The risk of lung trauma is very high, as is the danger from any unsecured objects that can become projectiles because of the explosive force, which may be likened to a bomb detonation.
Immediately after an explosive decompression, a heavy fog may fill the aircraft cabin as the air cools, raising the relative humidity and causing sudden condensation. Military pilots with oxygen masks must pressure-breathe, whereby the lungs fill with air when relaxed, and effort has to be exerted to expel the air again.
This is what that aircraft would be undergoing. Rapid decompression typically takes more than 0.1 to 0.5 seconds, allowing the lungs to decompress more quickly than the cabin. The risk of lung damage is still present, but significantly reduced compared with explosive decompression. In modern aircraft there are panels that open in the cockpit to equalize the pressure between the cabin and the cockpit.
Gradual decompressionSlow, or gradual, decompression occurs slowly enough to go unnoticed and might only be detected by instruments. This type of decompression may also come about from a failure to pressurize as an aircraft climbs to altitude. An example of this is the 2005 Helios Airways Flight 522 crash, in which the pilots failed to check if the aircraft was pressurizing automatically, eventually losing consciousness (along with most of the passengers and crew) from hypoxia.