42 SMITHSONIAN | March 2020
STEPHEN^
MORTON/^GETTY^ IMAGES^ters per second. In the lungs, sound waves
have to navigate a labyrinth of air bubbles,
and they slow down to 30 meters per sec-
ond. Therefore, a wave moving through the
body that hits the lungs is suddenly forced
to slow down by 98 percent.
If a shock wave traveling through the wa-
tery tissue of the chest wall is like an out-of-
control semi-truck speeding down a moun-
tain highway, then lung tissue is the gravel
pit of a runaway truck ramp. The truck itself
suddenly slows to less than 2 percent of its
prior speed—but its great kinetic energy
must still go somewhere. Cargo goes fl ying,
gravel fl ies everywhere. Likewise, the deli-
cate tissues that form the walls of the lungs
rupture and shred, and blood sprays into the
alveoli, the gas pockets needed for breath-
ing. This breakdown is called spalling.bered for easy reference: primary, secondary, tertiary
and quaternary. The last three injury types are logical,
meaning that they make obvious sense, and even peo-
ple with zero blast experience can predict that they
are expected possibilities.
In contrast, a primary blast injury—the kind pos-
sibly incurred by the victims in the Dresden bomb
shelters—is a strange and horrifying fl uke produced
by the bizarre physics of an explosion. It is usually
the result of a shock wave.
A shock wave is a particular kind of pressure
wave, and it can have a terrible impact on certain
human tissues. It most commonly develops during
an explosion, when molecules of air accumulated at
the wave front are shoved together by the explosive
gas urgently expanding behind them. These mole-
cules are so densely packed that they collide with
one another far more rapidly than usual, generating
a unique wave that moves faster than the normal
speed of sound.
In its purest form, as defi ned by physics, the shock
wave goes straight from zero to its maximum pressure
in an instant; the change is so abrupt that, on a graph,
it’s a vertical line. If it were a car it would go from 0
to 60 in 0 seconds. When the pressure of one of these
waves reaches a certain threshold, it can disintegrate
everything in its path. In blast physiology, we use the
term a bit more loosely: Humans are so frail that we
can die from fast-rising blast waves that don’t even
qualify through physics as proper shock waves.
Most of the human body handles fast-rising waves
surprisingly well. Such waves can move straight
through water without causing much chaos and
disruption, and human bodies are, after all, most-
ly water. It’s the gas pockets inside certain organs
that cause the real drama. In the chest wall, which
is mostly water, sound moves at roughly 1,540 me-
“NOBODY TRIED
TO ESCAPE.
THEY DIED
WHERE THEY SAT.”