The damaged nuclear reactors at the Fukushima Daiichi power plant in Japan have been releasing radioactive elements into the air and water around the plant. Those elements are dangerous because they are made up of atoms with unstable centers, or nuclei. Seeking greater stability, they constantly emit energy as radiation, in the form of waves like gamma rays or X-rays, and as particles smaller than atoms.

Such radiation can damage DNA and injure tissue, and the areas in the body most vulnerable to it are those with many dividing cells, like bone marrow and the digestive tract.

A very high dose to the whole body can be fatal. Smaller amounts can cause illness, burns and an increased risk of cancer. But the effects also depend on the type of radiation, whether it is internal or external and whether it involves the whole body or just part.

Several types of radiation from reactor accidents can have health effects.

ALPHA, BETA, GAMMA

Alpha particles are relatively large, but they cannot penetrate the skin, and a few sheets of paper will stop them, or even a few inches of air, so outside the body they are harmless. But if they get inside the body, from being swallowed or inhaled, they are extremely dangerous and can do extensive damage because they are relatively big and heavy, and can slam around inside cells like a bowling ball.

Beta particles are smaller than alpha particles. They do not pass through the body but can penetrate partway into the skin and cause burns. A piece of metal the thickness of a cookie sheet will stop beta particles. But if a beta-emitter gets inside the body, it can lodge in an organ and the particles can damage cells and organs from within.

Gamma rays are similar to X-rays, and can pass through the body and most materials. Gamma emitters can do harm whether they are inside or outside the body. It takes lead or several feet of concrete to stop them.

IODINE 131 and CESIUM 137

Most of the radioactive materials released from the Fukushima plant have been iodine 131 and cesium 137. Both substances emit beta particles and gamma rays. On the skin, they can cause burns, and their gamma emissions can also penetrate the body.

If a person ingests iodine 131 (milk from cows fed contaminated grass is the most common source), it is quickly taken up by the thyroid gland, particularly in children. The iodine then bombards the gland with radiation, and can cause thyroid cancer. If cesium is consumed — in contaminated produce or meat, for instance — it is distributed throughout the body, and acts as an internal radiation source. It can increase the risk of cancer.

PLUTONIUM

The Japanese authorities say they detected very small amounts of several forms of plutonium in soil near the plant, but they said it was unclear whether it came from the reactors or was a legacy of atmospheric atomic weapons testing. Plutonium  emits alpha and gamma radiation, and can cause lung cancer if it is inhaled.

HALF-LIVES

Radioactive elements are emitting energy all the time, and their existence is measured by half-life — how long it takes for half of a given amount to give off its radiation and decay into a different material. For iodine 131, the half life is short, only about eight days. But for cesium 137, it is about 30 years. A rule of thumb is that it takes ten half-lives for a substance to be essentially gone, which means less than three months for iodine 131, but 300 years for cesium 137.

Heavy contamination with cesium 137 is in large part what forced Soviet authorities to create a nearly 20-mile, uninhabitable exclusion zone around the Chernobyl reactor site.

Inside the body, these elements also have biological half-lives — how long it takes the body to eliminate them. For iodine and cesium in adults, the overall half-life is about three months, though it varies by organ, and children tend to clear the substances faster.

MEASUREMENTS

Government agencies in the United States commonly use a unit called the rem to measure radiation doses. The background dose that most people receive from natural sources is 0.3 rem per year; a dental X-ray, 0.0005 rem; mammogram, 0.002 rem; a CT scan of the spine, 0.6 rem. According to the Environmental Protection Agency, people can start getting sick with a whole-body dose of 50 rem, death becomes “possible” at 400 rem, and nearly certain at 1,000 rem or more. In the United States, a power plant worker is limited to 5 rem per year, and the federal government tells local officials to advise residents to take shelter or leave if the residents’ dose is anticipated to reach 1 rem to 5 rem.

In Europe and Asia, a unit called the sievert is more commonly used. One sievert equals 100 rem.