http://www.jerrypournelle.com/archives2/archives2mail/mail428.html#Carmack

I am not a scientist, however, I have had some experience as an architect designing flammable filling facilities, and had incidently learned a little about some of the chemicals used, because when such substances are in buildings, you need to do all kinds of special things to the building in which they are located.

There was one chemical, I recall that was still considered flammable at 97% mixture with water.

Then, there exist some chemicals, called pyrophorics, that explode or burn on contact with air. Hydrazine, for example, was used in a pharmaceutical production facility (a pilot plant actually) I’d worked on, and is one of the more common pyrophorics. It’s also used a rocket fuel. That could be the type of chemical that they are talking about.

The issue here really is this though: at cruising altitude, an airplane, due to it’s pressurized cabin, is already extremely vulnerable to explosions within it, so not very much is need to start a progressive failure of the fuselage. In fact, the air marshals will fire their guns as a last resort, because there is a possibility that a gun discharge alone could start that process. So, at cruising altitude, when the cabin is pressurized, the needed explosion is actually very small.

But it is all security theater, because some really dedicated terrorist could have some bladder filled with liquid in a body cavity, and it almost certainly wouldn’t be detected.