It is easy to understand how radioactivity works if we think of how sunlight behaves.
INJURY FROM RADIOACTIVITY DEPENDS ON THE POWER OF THE RAYS AND PARTICLES, HOW LONG YOU WERE EXPOSED AND HOW MUCH OF YOUR BODY WAS HIT. In the northern part of the world, winter's slanting sun rays seldom cause sunburn, but the hotter rays of the summer sun often do. Still, just a few moments in the midsummer sun will not give you a tan or sunburn. You have to stay in its hot rays for some time before you get a burn. What's more, bad sunburn on just the face and hands may hurt, but it won't seriously harm you. On the other hand, if it covers your whole body, it can make you very sick, or sometimes even cause death.
In the same way, the harm that can come to you from radioactivity will depend on the power of the rays and particles that strike you, upon the length of time you are exposed to them, and on how much of your body is exposed.
WHAT IS "INITIAL" RADIOACTIVITY?
EXPLOSIVE RADIOACTIVITY IS THE MOST IMPORTANT KIND, BUT IT LASTS ONLY A MOMENT. Broadly speaking, atomic explosions produce two different kinds of radioactivity. First—and most important in an air burst—is an extremely powerful invisible burst of rays and particles thrown off at the time of explosion. This kind is called "initial" or explosive radioactivity. Its rays and particles fly out quickly, then promptly die. There is danger from them only for little more than a minute. The second type of radioactivity—lingering radioactivity—will be described later.
