1. GENERAL

Numerous experiments on the nature and effects of blast have been made in the United States and Great Britain, and hundreds of thousands of soldiers and civilians have personally experienced the effects of blast by bombs, shells, and mines. The subject of blast is treated in this section mainly for the benefit of soldiers who have not experienced these effects. The facts and observations set forth below are based largely on experiments conducted in England, the results of which appeared in The Bulldozer, a British military publication.

Before the war, the No. 1 bogey of the civilian population was poison gas and the No. 2 bogey was blast. During the earlier stages of the Japanese war against the Chinese, and also during the Spanish Civil War, stories were circulated on the streets to the effect that persons in the war-torn areas were being found dead in streets and houses, without any visible wounds. We now know that these stories were largely untrue, and that blast--as far as human lives are concerned--is by no means the terror that some persons have imagined. Nevertheless, we know that blast is destructive to buildings and equipment, according to the circumstances, and that its effects--rather than the actual blast--are injurious or fatal to persons under certain conditions.

2. WHAT IS BLAST?

Generally speaking, blast is a violent disturbance of the air, and is produced by any very sudden movement. One characteristic of a blast wave is its sudden start. The air pressure reaches its peak immediately and then falls below normal. The pressure is below normal longer than it is above normal. These periods are known as the pressure and suction phases of blast. The pressure wave travels outward at a speed a little faster than that of ordinary sound, which is approximately 1,000 feet per second.

When a bomb bursts, the case swells like a balloon to half again or more of its original size; then it cracks into fragments, and the compressed gases from the explosion come out fast--up to 10,000 feet per second or faster. The surrounding air does not have time to get out of the way, and something like a solid wall of high pressure is formed. The gases, some few feet outward from the bomb, cool off and lose speed, but the push they give to the air still carries on as the pressure wave. (Such waves can actually be photographed.) As the wave moves on, its pressure spreads wider and wider and gradually decreases until it becomes an ordinary sound wave.

3. WHAT ARE THE EFFECTS OF BLAST?

As we hear it, a nearby explosion makes a cracking sound while a distant one makes a booming sound.

When a blast wave hits an object, there are two different effects: one caused by the pressure of the wave and the other caused by the immediate forward movement of the air behind the pressure wave. Therefore, an object in the path of a blast is compressed and pushed away from the explosion and then drawn toward it--by suction.

In the case of large objects near the explosion, the pressure phase is the most damaging. For example, the walls of ordinary houses are pushed in by the pressure. (It takes about 50 pounds per square inch to push in a wall and about 10 pounds to push in a window.)

Large objects some distance from the explosion suffer most from the suction phase. For example, pressure from the explosion of a large bomb would not be great enough to push in the walls of a house, but the suction immediately following the pressure wave would suddenly take away most of the normal atmospheric pressure in front of the structure. Because of this partial vacuum outside of the house, its walls would be pushed outward by the normal pressure from inside. Although the walls would be pushed out in all directions, the main force of inside pressure would be against the wall facing toward the explosion.

In the case of small objects, movement is more important than pressure. Leaves and branches are torn off trees. Clothes may be ripped off, and people flung about. Actually, far more casualties are caused by this flinging about by blast than by the direct blast effect. Therefore, it is always best to lie down when exposed to blast bombs, not to mention the protection afforded against fragments.

In an area affected by blast, the amount of protection available depends largely on the number and size of obstacles behind which personnel can seek refuge.

The blast wave can be thought of as having a high-frequency, short-wave pressure component and a low-frequency, long-wave suction component. The pressure component, like light but unlike sound, cannot get around a medium-size obstacle. The suction component, like ordinary sound, will get around most obstacles smaller than a medium-size hill.

The result is that behind an obstacle the blast wave is quite a different shape. The high-pressure part is practically obliterated; the suction part remains.

Fortunately, it is the pressure part that is dangerous to human beings, either because it knocks them about or because it damages their ears and lungs; therefore, a man behind a wall or rock is unlikely to be hurt, even though he is quite close to a bomb--that is, as long as the wall itself does not come down on top of him.

Several months ago two sailors were together when a bomb hit nearby. One just managed to get around the corner of a wall and was unhurt, while the other did not make it and was never seen again.

In the same way, blast does not easily get into closed spaces from the outside. Even comparatively fragile underground shelters can furnish complete protection against blast from air-burst bombs. Once a whole shelter full of people were uninjured by a bomb falling 10 feet away.

But the results are very different when an explosion occurs inside confined space, where reflections of blast waves come from the walls and where the internal pressure is much increased.

What is worse, blast waves from an inside explosion will run along tunnels for great distances without losing any of their strength, and they will even travel around curves.

No tunnel shelter is safe from bombs exploding just inside, unless it is provided with baffle walls, sharp turns, or blast traps. Fortunately, this lesson has been well learned by now in military establishments.

We now know, largely from experiments, what blast does to animals and men. It is really much less hazardous than people used to think.

To summarize previous statements, all the damage to buildings at close range is due to the pressure and not to the suction component.

There is no time for the suction to draw the air out of a lung, as some rumors have suggested.

The pressure phase affects chiefly those parts of the body that have air hollows behind them. The most important ones are the ears and the lungs. The effect on the ear is simply to burst the delicate membrane of the ear drum. This occurs at comparatively low pressure--although this pressure is a great deal higher than that accepted as safe for gunners. Fortunately, burst ear drums usually cause only temporary casualties. The ear drum re-forms and hearing is not permanently lost.

The lung is a more serious concern, but only when the victim is very near the explosion. A man can barely survive a pressure of between 400 and 500 pounds per square inch--the kind of thing that would completely destroy any ordinary building.

Two instances illustrate this:

a. Three men in a small brick shed, in the middle of which a 50-kilogram bomb burst, are alive and well today, although the building itself was scattered over many square yards.

b. A man, 25 feet from a bomb in the open but fortunately lying down, not only remained fully conscious but was able to get up immediately after the explosion and assist other victims.

The effect of blast on the lungs is to push in the ribs and press them against the lungs, bruising them or rupturing the small blood vessels and lung spaces and thereby leading to more or less extensive internal bleeding. The immediate effect may not be noticed, but it is similar to a severe case of pneumonia, since the amount of lung space available for breathing is reduced.

The treatment is simply rest; obviously a man taking violent exercise immediately after exposure to blast is likely to injure himself fatally.

It is surprising how very few cases of true blast deaths have occurred in all the bombing of this war--so few, in fact, that the doctors have great difficulty in getting good case histories. The reason is, of course, that you have to be very close to a bomb to get blasted, and if you are very close, you are far more likely to be killed by splinters.

Most of the so-called mysterious deaths in dugouts and shelters were certainly not due to blast, but most often due to carbon monoxide poisoning resulting from poor ventilation, and sometimes from explosive gases.

4. WHAT ARE THE EFFECTS OF SHOCK WAVES?

In combined operations, men have to face not only blast in the air, but shock waves in water.

In principle, these shock waves are the same as blast, but there are two big differences in scale. The shock wave from an underwater explosion travels faster and farther--about 6,000 feet per second instead of 1,000--and it has a much higher pressure, measured in tons per square inch instead of pounds. But it is, accordingly, of much shorter duration.

Fortunately, the effect of the shock wave in water is greatest on deeply submerged objects and least on those on the surface. This is because the free surface can yield to the wave, so on the actual surface itself the effect is practically nil unless the intensity is such that the surface is actually flung up into the air--as in the spray dome above a mine.

Experiments and trials have shown that for a man in the water, the safest position is floating on his back and that the next safest is swimming. Treading water or hanging vertically to some floating object exposes the lungs and abdomen to a more severe shock.

5. WHAT ABOUT CONCUSSION?

There is one other condition which is sometimes confused with blast--concussion. This usually refers to supposed effects on personnel in gunrooms and magazines due to large bombs or shells exploding in the ground nearby. Experiments have shown that this is not a serious danger.

Real concussion can be caused only by a hit on the head--and such a blow must be caused by a really hard object. Certainly no one in a room can be exposed to concussion except from some flying object. Nevertheless, the effect of shock is an alarming and distressing experience; but if the explosion is not so close that it actually breaks into the room and throws its contents about, no one inside will sustain a concussion.

The research on blast and concussion has done something to provide protection where protection is necessary. And perhaps its real value lies even more in debunking the many misconceptions about mysterious dangers.

There are enough real dangers in war to make worrying about the others a waste of time.