Read the following text about fallout and match the headlines given in the box to the appropriate paragraph

Fallout is radioactive material that settles over the earth's surface following a nuclear explosion in the atmosphere. It consists of atoms known as radioactive isotopes or radioisotopes. These isotopes form from the fission (splitting) of uranium or plutonium in a nuclear weapon. Radioisotopes also form when radiation that results from the explosion causes other atoms nearby to become radioactive.

After the explosion, the radioisotopes in the air, on the ground, and in the bodies of human beings and other organisms decay (break down) into more stable isotopes. They do so by emitting radiation in the form of alpha particles, beta particles, and gamma rays. Exposure to large amounts of radiation can result in immediate sickness and even death. Exposure to radiation over longer periods can cause cancer and damage genes.

The testing of nuclear weapons in the atmosphere once produced large amounts of fallout. Today, fallout has been almost eliminated by underground testing. However, a serious accident in a nuclear reactor can release the same radioisotopes that occur in fallout. In 1986, an explosion and fire at the Chernobyl nuclear power plant in Ukraine released radioisotopes that scattered across the Western Hemisphere.

How fallout is produced. All nuclear explosions produce a giant fireball of intensely hot gases. Everything inside the fireball or in contact with it is vaporized (turned into a gas). When an explosion occurs close to the earth's surface, the fireball vaporizes soil, vegetation, and buildings. It then begins to rise, carrying the vaporized material with it. As the fireball rises, a low-pressure area forms beneath it. Air rushes in to fill this partial vacuum carrying along with it dust, dirt, and other small particles. Much of this debris may be lifted up through the atmosphere along with the fireball.

As the vaporized materials rise and cool, some of them condense into solid particles. Atoms of the various radioactive elements produced by the explosion cling to these particles. These radioisotopes eventually return to the earth as fallout. Fallout particles range in size from fine invisible dust to ash of snowflake size.

The fallout hazard. Fallout can be dangerous to plants, animals, and people because of the radioactive elements it contains. These elements include about 200 isotopes of more than 30 chemical elements produced by a nuclear explosion.

The radioisotopes in fallout give off radiation for varying periods of time. Most fallout radioactivity dies off in a matter of hours or days. As a result, the radioactivity at the end of two weeks is only one-thousandth as strong as the radioactivity one hour after the nuclear explosion. But even at the end of two weeks, local fallout can be so intense that it remains a serious hazard. A few of the fallout elements continue to give off radiation over a long period. For example, the radioisotope strontium 90 loses half its radioactive strength every 28 years, and the radioisotope cesium 137 loses half its strength every 30 years. The possibility of nuclear war has caused people to think about the danger of local fallout. This type of fallout involves a twofold problem. First, there is the danger of radiation that is emitted by the radioactive debris on the ground. People can best protect themselves from this radiation by taking refuge in underground fallout shelters. For example, 3 feet (91 centimeters) of earth overhead will reduce the radiation's intensity to one-thousandth of its original intensity.

Second, there is the danger that certain fallout elements may enter the human body through breathing and eating. For example, milk is a route by which the radioisotopes iodine 131 and strontium 90 enter the body. First, fallout descends on grass. Next, cows eat the grass. Some of the radioisotopes are then transferred to the cows' milk. Anyone who drinks the contaminated milk takes in iodine 131, which collects in the thyroid, and strontium 90, which is absorbed by the bones.

Foods are contaminated by the direct deposit of fallout on plants and by the slow uptake of radioisotopes in soil by the plants' roots. The behavior of radioisotopes in the environment depends partly on the chemical properties of the element. For example, bones in the body can absorb strontium-90 because strontium and calcium have certain similar chemical properties.

History. From the mid-1940's to the early 1960/s, the United States, the Soviet Union, and a few other nations exploded many experimental nuclear weapons. As a result, distant fallout increased to alarming levels. In 1963, more than 100 nations, including the United States and the Soviet Union, signed a treaty that banned the testing of nuclear weapons everywhere but underground. Fallout then decreased greatly. China and France did not sign the treaty. They later stopped testing nuclear weapons aboveground.

Fallout shelter is a building or an underground area that protects people from nuclear fallout. A nuclear explosion scatters bits of radioactive material into the air. Within a few hours, the fallout particles settle over hundreds of square miles or square kilometers. Fallout gives off radiation that can cause burns, illness, or even death.

People can protect themselves from fallout by taking shelter in a building made of such heavy materials as brick, concrete, or stone. Any such building blocks most of the radiation and can serve as a fallout shelter. Underground areas, including mines and tunnels, also provide protection from fallout.

The United States government has designated many buildings and underground areas as public fallout shelters. Many public shelters have been set up in the basements of apartment and office buildings, factories, schools, and other large structures, or in windowless central areas aboveground. Other public fallout shelters are in subway-train tunnels or other underground areas. In the United States, public fallout shelters are marked by a black-and-yellow sign.

The U.S. public fallout shelter program began in 1961. This program is now the responsibility of the Federal Emergency Management Agency (FEMA), an independent agency established in 1979. Public fallout shelters have also been established by the governments of many other countries, including Denmark, Finland, Sweden, and Switzerland.

A family may build its own fallout shelter in a basement or outside the home. Underground shelters provide the most protection from radiation. Plans for home fallout shelters may be obtained from the Federal Emergency Management Agency, Washington, DC 20472.