Radioactive contamination is the unwanted presence of materials containing radioactive atoms on other materials. The hazard comes from the decay of the contaminating atoms, and the type of radiation they emit affects how dangerous it is. A contaminated object keeps giving out radiation until the atoms decay or are removed.
Irradiation is the process of exposing an object to nuclear radiation. An irradiated object does not become radioactive, because no radioactive atoms are transferred to it. The danger lasts only while the object is exposed to the source.
The type of radiation matters. An alpha source outside the body is a small hazard because alpha is absorbed by air and skin, but inside the body it is very dangerous because alpha is strongly ionising. This is why contamination, which can be breathed in or swallowed, can be more hazardous than irradiation by the same source. A gamma source outside the body is a hazard because gamma penetrates. Precautions against a source used for irradiation include shielding, handling it with tongs, keeping a distance and limiting the time of exposure.
Findings from studies into the effects of radiation on humans must be published and shared with other scientists so that they can be checked by peer review.