All sorts of radioactive particles, a few of which can be deadly in large quantities. These materials are known as "radioisotopes"—versions of an element that, because they have an abnormal number of neutrons, frequently decay and release radiation. They occur as a result of nuclear fission, when a Uranium atom splits into lighter elements and releases energy. Among the most common ingredients in the water steam leaking from the Japanese reactor are iodine- 131, cesium-137, xenon-133, xenon-135, and krypton-85. (Often elements have many different varieties of isotope, some more stable than others. The number after the element name identifies the specific isotope.)
Of these, the most troubling is iodine-131, which can be absorbed by the thyroid when inhaled, causing thyriod cancerand leukemia. Gases like krypton-85 and xenon-133 don't interact with bones or tissue, but since they are highly unstable they decay in bursts of radiation that can prove pernicious to other bodily systems. But the body tolerates a certain amount of radiation every day, from comsic rays when watching television, and it's only in much larger quantities that the byproducts of a nuclear power plant become dangerous. While radiation went over 1000 times its normal limits in one reactor control room, Japanese officials insist that exposure levels outside the plant are not highly hazardous. Even so, area residents have been advised to drink bottled water, stay indoors, and hold washcloths over their noses and mouths. As a precaution against iodine-131, officials have also announced plans to distribute potassium iodine pills, which saturate the thyroid with a stable form of iodine before the more dangerous isotope can be absorbed. They only work, however, if swallowed pre-emptively.