Originally Posted by booker

You've asked at the right time. I'm an engineer and deal with explosives.

If it's a bare explosive, there's a phenomenon called "Blast Overpressure," which is a pressure wave in air many times stronger (intensity, velocity, etc) than just noise. Depending on the intensity, it can blow out your ears, collapse your lungs and do other things to kill you. The kill mechanism isn't the air hitting you, it is that the energy is transferred directly through your body (the wave keeps going, only a small portion of it is reflected).

If it's a weapon, like a bomb or some other device that fragments, then the fragments will reach you before the pressure wave. Obviously, hot razor-sharp pieces of metal travelling many times the speed of sound will kill you.

If you are outside a building and the blast is contained until the structure breaks, then it could be either pressure or fragments.

If you are inside a building, it depends on the weapon. Some are intended to kill people from the pressure generated, some are intended to break up the structure and use the building's debris as the kill mechanism.

In terms of the heat, the heat from a high-energy explosion is very high, but short-lived, especially if vented to the atmosphere. It'll burn, char and mangle, but unless you are fairly close, the heat isn't a major issue.

The "concussion" grenades that are often used in urban warfare produce a pressure wave to incapacitate, and lots of noise/light. The noise and light are at a level that causes an information overload, leaving the victim unable to really act or respond.

Underwater... pressure waves from explosions are very strong, and there's some addition effects underwater that make it particularily violent. Divers would be affected.

Nuclear explosions are another beast. There's incredibly intense heat, and an enormous series of shock and pressure waves.

The "vacuum" created as air rushes back to fill the void, is called rarifaction. Not a kill mechanism.