Radioactive Fallout

Particles that make up radioactive fallout can be as small as the invisible droplets produced by an aerosol spray can, or as large as ash that falls close to a wood fire. The type of radioactivity in fallout depends on the nature of the nuclear reaction that emitted the particles into the atmosphere. More than 60 different types of radioactive substances may be initially present in fallout. Some of these decay into non-radioactive products in seconds, while others take centuries or longer to become non-radioactive. It takes 28 years, for example, for a sample of strontium-90 to lose one-half of its initial radioactivity. Strontium-90 is one of the most dangerous elements in fallout because it is treated by the metabolism of humans in the same manner as calcium, an important component of bone. If animals or humans eat food contaminated with strontium-90, it will accumulate in their bodies. Other particularly harmful products in fallout include cesium-134, cesium-137, and iodine-131.

Radiation damages and kills cells in the body. Large doses of radiation can result in burns, vomiting, and damage to the nervous system, digestive system, and bone marrow. Smaller doses can cause genetic mutations and cancer years after exposure.

Fallout from a nuclear explosion can be local, tropospheric, or stratospheric. Heavy objects caught in the wind fall to Earth before lighter objects. Under the same wind conditions, a large cinder will travel less distance than a small one. The same principle applies to fallout particles.

When a nuclear weapon explodes on or near the surface of the earth, huge quantities of soil, rock, water, and other materials are injected into the atmosphere, creating the familiar shape of the "mushroom cloud." Depending on their size, particles in this cloud will fall to Earth relatively soon, or they may drift in the atmosphere for a long time. An underground nuclear explosion that does not break through the surface does not produce any fallout, and the radioactivity remains trapped below ground.

Local fallout deposits within about 10 mi (16 km) of a typical above-ground explosion. This material resembles ash or cinders that rise through a chimney and deposit nearby. Emitted particles greater than about 20 micrometers in diameter usually become local fallout. This fallout can be extremely radioactive, but only for a short time, after which its radioactivity is much less, though not zero.

Particles smaller than local fallout, as much as 200 times smaller, remain suspended in the lower atmosphere, or troposphere. Depending on the weather, these particles travel much farther than local fallout before being deposited to the surface, mostly within about one month.

Some fallout may reach the stratosphere, the high-altitude layer of atmosphere above the troposphere. To reach the stratosphere, fallout needs the force of the most powerful atomic explosions, caused by a hydrogen or thermonuclear bomb, to inject it that high. Stratospheric fallout can drift for years, and when it finally mixes with the troposphere and is deposited to the surface, it can fall-out anywhere in the world.