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Iodine is a chemical element with symbol I and atomic number 53. There is currently a lot of discussions regarding radioactivity in Japan, and iodine-131.

Iodine-131 has a half-life of 9 days.

Does it mean that people drinking radioactive water (with iodine-131) will lose half of it within 9 days?
And thus the health risk would be lower and lower over time?

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The cumulative health risk will certainly not decrease with time, i.e. its still not a good idea to drink radioactive liquids. – user346 Mar 24 '11 at 4:34
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Yes, the risk-per-unit-time decreases exponentially with time. It's similar as if you fall from a skyscraper. When you hit the sidewalk, your velocity starts to decrease, and the potential for further damage decreases virtually to zero within a fraction of a second, too. Well, there's still a problem with the fraction of a second after you touched the sidewalk for the first time, and there's still also a problem with the first 8 days of decaying radioactive iodine, so your interpretation of the facts as "good news" could be a bit premature. ;-) – Luboš Motl Mar 24 '11 at 5:30
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On the other hand it means that the alarm can be lifted after a few lifetimes, so the precautionary ingestion of iodine, so that the thyroid does not use the radioactive one and the body ejects it with the urine (probably in less than a day), can be stopped. It has dangers too of over stimulation. – anna v Mar 24 '11 at 7:16
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What it means is that if you pour out two glasses of water now, then the one you drink today is twice as radiocactive as the one you store in the fridge for 9 days and then drink will be when you drink it. That is the way the health risk goes down, rather than storing radioactive material inside you body while it decays. – Henry Mar 24 '11 at 12:51
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The entire health danger to you from ingestion of a radioactive atom, is that it might decay. So a short half-life does not help you, once it is internal to you. You would rather your body excreted it before it decayed. If you let it decay outside of the body (by waiting a few weeks before you consume it), then the halflife is your friend. – Omega Centauri Mar 24 '11 at 19:37
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The Iodine-131 undergoes beta decay. This is what makes it radioactive. The faster it decays the more radiation it gives off per unit time. Yes the material disappears faster than something with a longer half-life, but its speed of decay means that it is hotter.

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Is there a way to accelerate decay? Like boiling water having some I131? – ring0 Mar 26 '11 at 0:38
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Not by manipulating the radioisotope (though there are chemical means to remove iodine). Nothing can be done on a reasonable scale to the radioisotope that will result in a safer material faster. You can bombard the iodine with neutrons and hopefully turn a bunch of it into iodine 132 with a half-life of 138 minutes. You can retard delay by accelerating the nuclei at high, almost relativistic speeds. These are things that "could" be done, but they are not things that could really be done to make iodine 131 go away faster. – AOA Mar 26 '11 at 4:38
OTOH, leave the water for a 9 days before drinking and it will be half as radioactive... – SF. Apr 6 '11 at 13:42
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Iodine-131 decays with a half-life of 8.02 days into stable xenon-131 which is not considered carcinogenic. Xenon certainly does not belong in your body, but the amount produced would be chemically insignificant. Iodine has a biological half-life of 100 days in the body so any iodine-131 will probably decay before being excreted. So the answer is YES to both questions, because the radiation is much more dangerous than the presence of xenon and this risk is incurred at the time of decay.

EDIT: The above answer assumes iodine-131 has been consumed and some time has passed, and it is incorrect unless there is some way of removing the radioactive iodine (KI therapy?). The implication is that such treatment should be started as soon as possible, but it may not be worth the cost if months have already passed. Without this possibility, the risk is incurred at the time of ingestion and the answer to the question of health risk is NO. The above answer is also incorrect if we allow for other possibilities (e.g. a cure for cancer). If as Henry suggests in the comments we have a glass of water contaminated with iodine-131, and we keep it stored in the fridge for 90 days, then more than 99.9% of the isotope will have decayed to stable and harmless xenon. But I would drink it only if there was no doubt that it was free of other radioactive or poisonous chemicals (such as caesium-137).

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""Xenon certainly does not belong in your body, but the amount produced would be chemically insignificant."" Rofl Xenon as all noble gases (including argon with about .9 % in air!) enter and leave our body as all gases do, to the equilibrium that is given by Henrys law. – Georg Mar 24 '11 at 9:43
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Yes, but Xenon has no biological role and I don't know in what amounts it is ordinarily present in e.g. the thyroid or bone. The point is that the tiny amounts produced by the decay of Iodine 131 would be harmless. If the amounts of Xenon were large enough to be harmful, a person would already be dead from the radiation. For what it's worth, I found this page home.earthlink.net/~sweetwind7/thyca/thyca.html with a similar explanation. It claims that the same would be true even in a fantasy universe where Iodine 131 decays to Arsenic. – Dan Brumleve Mar 24 '11 at 18:08
"Xenon has no biological role" Not quite true! Xenon does have use as anesthesia. Xenon may be dull at covalent and ionic chemical bonds, but it has the right size and physical properties to block neurotransmitter receptors. See en.wikipedia.org/wiki/Xenon#Anesthesia or goo.gl/NMo6t – DarenW Nov 23 '12 at 4:06
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I131 is a gamma and beta emitting isotope. As such, the beta emission is the destructive component. Beta emission is an electron ejected from the nucleus and is ionizing. This aspect makes beta electrons very destructive to thyroid tissue which concentrates iodine ~200 times more than any tissue in order to make thyroxine, an endocrine hormone which regulates the rate of metabolic processes. The important consideration is absorbed dose. This is the total dose absorbed while the I131 is present. So, the health risk is not 1/2 as much in one half life because the dose is internal and 1st half has already been absorbed.

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Perhaps you could explain why the gamma emission is not dangerous. – Carl Brannen Mar 27 '11 at 5:57
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Beta rays, a product of I131 decay,have a very short range. If concentrated in the thyroid, it will for all practical puirposes, not be biologically eliminated before it decays. This will ionize a lot of the surrounding tissue.

On the other hand, gamma rays have a low absorbtion cross-section in the body and so do very little damage. Finally, Xenon is inert and should cause no damage at all.

Accordingly, the trick is for people to minimize the uptake of I131. There is some atmospheric in dust I131 near the reactor. There is contaminated ground and sea water. There could also be uptake in marine life and vegetation.

It would seem to me, that except for people who cannot avoid being close to the plant and its environs, Iodide tablets are not called for. Avoidance would be much more practical.

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To this discussion should be added that apparently radioactive iodine-131 has what is known as an "effective half life" when in the human body....now physically, the 131 isotope has a half-life of 8.02 days and this remains precisely the same whether it is internal or external to the body---the helf-life of any radioactive isotope is a physical constant that depends strictly on the structure of the nucleus and its location within or without the human body is of no consequence whatsoever. Nonetheless, for some physiological reason, when radioactive iodine-131 is introduced into a human (usually as NaI) and a radiation meter, Geiger counter or some instrument that reads counts per minute (CPM) is placed close to the thyroid gland to which the iodine is most strongly gravitated and held, and the time for the count to drop to exactly half its original highest value is taken and averaged....the resulting figure is other than 8.02 days. This could result from migration of the "hot" iodine to other parts of the body or whatever but this new "half-life" as measured is referred to as the "effective half-life" in the body...but this term can mislead people into believing that once ingested, radioactive iodine's physical properties change---this is not so.

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