As of 2013, the Fukushima nuclear disaster site remains highly radioactive, with some 160,000 evacuees still living in temporary housing, and some land will be unfarmable for centuries.

The difficult cleanup job will take 40 or more years, and cost tens of billions of dollars.

The degree of hazard is determined by the concentration of the contaminants, the energy of the radiation being emitted, the type of radiation, and the proximity of the contamination to organs of the body.

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Contamination may affect a person, a place, an animal, or an object such as clothing.

Following an atmospheric nuclear weapon discharge or a nuclear reactor containment breach, the air, soil, people, plants, and animals in the vicinity will become contaminated by nuclear fuel and fission products.

Contamination does not include residual radioactive material remaining at a site after the completion of decommissioning.

Therefore, radioactive material in sealed and designated containers is not properly referred to as contamination, although the units of measurement might be the same.

The amount of radioactive material released in an accident is called the source term.

Contamination may occur from radioactive gases, liquids or particles.

Radioactive contamination, also called radiological contamination, is the deposition of, or presence of radioactive substances on surfaces or within solids, liquids or gases (including the human body), where their presence is unintended or undesirable (from the International Atomic Energy Agency - IAEA - definition).

Such contamination presents a hazard because of the radioactive decay of the contaminants, which emit harmful ionising radiation such as alpha particles or beta particles, gamma rays or neutrons.

Containment is the primary way of preventing contamination being released into the environment or coming into contact or being ingested by humans.