Two different things that can happen

1. The whole distinction rests on one question: has any radioactive material been moved onto or into the object?
2. In irradiation the answer is no: alpha particles, beta particles or gamma rays travel across from the source, deposit energy and are absorbed, but the unstable nuclei stay where they were.
3. In contamination the answer is yes: atoms with unstable nuclei end up on the skin, on a surface, or inside the body, and they carry on decaying there.
4. An object can be irradiated without being contaminated, contaminated without anyone realising, or both at the same time.

The effects of irradiation

1. While the radiation is arriving, it ionises molecules in whatever it passes through, so living cells can be killed and DNA can be altered.
2. The moment the source is taken away, shielded or switched off, the exposure stops immediately.
3. The irradiated object does not become radioactive, because nothing has been added to it that has an unstable nucleus.
4. A patient who has had an X-ray, a syringe that has been sterilised and a strawberry that has been irradiated are all safe to handle straight afterwards for exactly this reason.
5. The size of the hazard depends on the type of radiation, the activity of the source, the time spent near it, the distance from it and the shielding used, so all of those can be controlled.
6. From outside the body, gamma radiation is the greatest irradiation hazard because it penetrates deeply, while alpha radiation is the least because the dead outer layer of skin absorbs it.

The effects of contamination

1. Contamination happens when radioactive dust settles on skin or clothing, when a radioactive liquid is spilled, when radioactive gas is breathed in, or when radioactive material is swallowed with food or water or enters through a cut.
2. Once the material is there, the object or person carries the source with them, so moving away from the original spill changes nothing.
3. The exposure continues until the material is washed or cleaned away, is excreted by the body, or has decayed to a low activity, and none of those can be done instantly.
4. Contamination also spreads, because contaminated hands touch door handles and contaminated boots walk material out of the laboratory.
5. Contamination inside the body is the most serious case, because there is no layer of skin between the source and living cells.
6. This reverses the usual ranking: an alpha emitter is the most dangerous contaminant, because it is strongly ionising and deposits all of its energy in a very small volume of living tissue.
7. Some contaminants are worse still because the body concentrates them: radioactive iodine collects in the thyroid gland and strontium behaves chemically like calcium, so it is built into bone.
8. Contamination is controlled by keeping sources sealed, working over trays, wearing gloves and laboratory coats, banning food and drink, and monitoring hands and surfaces with a detector before leaving.

Comparing the hazards

1. Both hazards do their damage the same way, by ionising molecules in cells, so the difference is not in the kind of harm but in how long it goes on and how easily it can be stopped.
2. Duration: irradiation is over the instant the source is gone, whereas contamination keeps exposing the person for as long as the material stays put.
3. Control: irradiation is reduced by time, distance and shielding, while contamination is dealt with by removing the material and by stopping it spreading.
4. Reach: irradiation affects only what is within range of the source, while contamination travels wherever the contaminated person, object, air or water goes.
5. Radiation type: gamma is the worst for irradiation from outside, and alpha is the worst for contamination inside the body.
6. Neither is always the more dangerous, and the honest comparison depends on the numbers in the situation.
7. A few minutes beside a high-activity gamma source can give a larger exposure than a trace of contamination ever would.
8. Contamination of a large area with a long-lived isotope is the harder problem, because the exposure cannot be switched off and the land may stay unusable for decades.