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In Rhodes' The Making of the Atomic Bomb, he says that, while Pu is not that radioactive (which is surprising -- maybe he means compared with radium and some other elements), it is very toxic.

I would guess it could not be much more toxic than uranium (U) or lead (Pb) -- I believe thallium is more toxic than U and Pb and so is mercury (Hg). If it is more toxic than lead, do we understand the mechanism?

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    $\begingroup$ Since we don't have any non-radioactive plutonium it is probably impossible to determine whether it has other forms of toxicity. Rhodes may just have been (badly) describing the difference of lumps of plutonium outside the body and small amounts inside the body. $\endgroup$ – matt_black Jul 28 at 12:52
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    $\begingroup$ I suppose one could perform experiments with different Pu isotopes (with different half lives and decays), but since most folks don't encounter Pu it probably is not particularly cost effective. $\endgroup$ – Jon Custer Jul 28 at 16:00
  • $\begingroup$ I think Mr. Rhodes has just fallen for an urban myth, or to put it another way, he´s been reasoning by analogy. The statement is just completely pointless. $\endgroup$ – Karl Jul 28 at 19:37
  • $\begingroup$ Apparently, it's supposed to be less toxic than caffeine. See last paragraph in en.wikipedia.org/wiki/… $\endgroup$ – Eric Duminil Jul 28 at 20:58
  • $\begingroup$ You should read this: pubs.acs.org/doi/pdf/10.1021/acs.organomet.7b00605 - about how it is not easy to generalise the toxic properties of heavy metals $\endgroup$ – Stefan Jul 29 at 17:09
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The toxicity is primarily due to radioactivity and to absorption by the body, where that radioactivity can act internally. There is, "significant deposition of plutonium in the liver and in the "actively metabolizing" portion of bone," according to Miner and Schonfeld.

Many $\ce{Pu}$ isotopes are primarily alpha-emitters, with "high energy radiation with low penetration and thereby requires minimal shielding. A sheet of paper can be used to shield against the alpha particles emitted by plutonium-238," from Wikipedia. The risk of holding a small amount of plutonium, if it is sealed inside almost any container, is negligible, because the the heavy, positively-charged alpha particles are stopped by the container in a very short distance.

By that same token, when absorbed, the plutonium migrates to rapidly dividing liver tissue and bone's hematopoietic cells. The alpha particles are stopped in a very short distance, depositing all their energy in the few nearest cells and massively interfering with cellular reproduction. Small amounts can cause various types of cancer, such as leukemia, a cancer of the previously-mentioned blood cell producing tissues.

That said, plutonium is not quite as toxic as one might think: "The U.S. Department of Energy estimates that the lifetime cancer risk from inhaling 5,000 plutonium particles, each about 3 µm wide, to be 1% over the background U.S. average... no human being is known to have died because of inhaling or ingesting plutonium, and many people have measurable amounts of plutonium in their bodies."

Perhaps what Rhodes means by "not that radioactive" is that the radiation from plutonium is not very penetrating. The isotope used in nuclear weapons, 239Pu, has a half-life of about 24,000 years, comparable to radium, or to 231Pa. If I've done my math correctly, one mole of 239Pu, about 6e23 atoms, with half-life of 8e11 seconds, should emit about 400,000,000,000 alpha particles per second. Of course, if it were a sphere, most of those particles would be absorbed inside the mass of metal, releasing energy as heat. However, spread through a human body, each particle has a much greater chance to cause damage. And 238Pu, used in radioisotope thermoelectric generators, is far more radioactive!

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    $\begingroup$ My sense from Rhodes is toxicity not due to radioactivity just as lead is not due to radioactivity but rather, as i understand it, displacement of vital minerals in the body. It sounds like you are saying that it is only or primarily due to radioactivity so i am still puzzled about what Rhodes meant. $\endgroup$ – releseabe Jul 28 at 2:35
  • $\begingroup$ Interesting - I'm not sure I buty the 'no human ... is known to have died because of inhaling' - perhaps not directly, but plutonium-caused lung cancer leading to death certainly is a thing (similarly various leukemia varients from Pu inhalation/ingestion as well). $\endgroup$ – Jon Custer Jul 28 at 17:18
  • $\begingroup$ @JonCuster, Agreed! There were many children downwind of the US nuclear test sites who died of leukemia (the Wikipedia article, Downwinders, was cited). The statement, I believe, is trying to show that Pu alone has not been demonstrated to have caused deaths... a quibble. $\endgroup$ – DrMoishe Pippik Jul 28 at 17:26
  • $\begingroup$ Another way to do the calculation (sorry for not using SI units): The activity of 1 g Ra-226 is (by definition) 1 Ci. As Pu-239 has a half life which is a factor 24000/1600 longer than that of Ra, you'll need 15 g of Pu to give 1 Ci. $\endgroup$ – Stefan Jul 29 at 14:36
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    $\begingroup$ It would still be interesting to know about the chemical toxicity (heavy metal), e.g. have it quantified. What is the ratio of LD50s? Is it 1:10, 1:1000, or 1:1000000000000? Wouldn't acute chemical poisoning dominate at a high dose? $\endgroup$ – Peter Mortensen Jul 29 at 15:06
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Actual toxicity other than radioactivity is not, as far as I know, very well studied. Quite simply, most of the danger is the radioactivity in general, as well as the toxicity of decay products (uranium and americium).

Basic knowledge of biochemistry though suggests that it should be toxic for the same reasons that most high atomic weight metals are, namely that most biochemical processes do not differentiate well between elements beyond the ionization charge (this is why rubidium and cesium salts are toxic, the large ion size for both of them combined with the +1 ionization state causes them to get stuck in and block potassium and sodium ion transfer channels in cells). The tendency of plutonium to concentrate in the liver and areas of bone growth supports this, as both are areas where ions with similar charges tend to concentrate (note that plutonium ions can be anywhere from +2 to +7, with +4 being the predominant form and +7 being exceedingly rare).

Also, for the record, Rhodes is correct that weapons-grade plutonium is comparatively not very radioactive. There are five major isotopes of plutonium, with atomic weights from 238 to 242. Plutonium-239 is the isotope used for weapons production, and decays via alpha particle emissions, which means it's not really all that dangerous except internally, especially since it decays rather slowly (it's half-life is 24100 years). Notably, other than the spontaneous fission of Plutonium-240, all the isotopes except Plutonium-241 (which undergoes beta decay to Americium-241) only undergo alpha decay, and as such are relatively safe with minimal shielding (ignoring the high decay-heat of Plutonium-238 which is utilized to power RTGs).

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    $\begingroup$ The radioactive aspect of toxicity is not that interesting; the idea that any element (as opposed to chemical compound) can be extremely toxic is interesting. I imagine that non-radioactive effects of an element would be "swallowed up" as the element combines with cells in the body so that there is a limit on how dangerous a very small amount of anything can be although I think microgram-range amounts of something like botulism toxin can be fatal. $\endgroup$ – releseabe Jul 28 at 16:11
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    $\begingroup$ @releseabe re toxic in small quantities, see, e.g., en.wikipedia.org/wiki/Dimethylmercury $\endgroup$ – hBy2Py Jul 28 at 17:26
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    $\begingroup$ even dimethyl mercury requires more than micrograms. $\endgroup$ – releseabe Jul 28 at 17:41
  • $\begingroup$ @releseabe It kind of depends on the context of the exposure how toxic/dangerous something is. Ingesting a few mg of pure zinc can easily kill you for example (due to the formation of a zinc-chloride solution in the stomach, which will eat through your stomach lining) despite zinc being largely non-toxic. You're generally right though that pure or ionic elements tend to require non-negligible concentrations to show actual symptoms of toxicity, though they often are still toxic in very small quantities. $\endgroup$ – Austin Hemmelgarn Jul 28 at 17:47
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    $\begingroup$ The word "toxic" as applied to a radioactive element always troubled me: is molten iron "toxic" or just very hot? You would not attribute heat stroke to solar toxicity. Are acids toxic or just corrosive? (Although hydrofluoric acid is indeed also toxic -- hydrochloric acid is not to me toxic although you can die from it just as gulping hot chowder might kill you by burning your esophagus.) $\endgroup$ – releseabe Jul 28 at 17:54
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Thanks for asking this question. I'd heard before that Pu was actually more chemically toxic than its toxicity due to its radioactivity, but had never followed up by checking out this claim in detail.

tl;dr: Plutonium is very safe unless you grind it up into dust and inhale it, in which case the hazard is probably from radiation, not chemical toxicity. There is probably no way of knowing for sure, because we don't know enough about things like the dependence of harm from radiation on the dose rate as opposed to the total dose.

I'm going to assume we're talking about 239Pu, which is what is used in bombs. It has a half-life of 24,000 years, and it decays via alpha emission to 235U, which is effectively stable (half-live of almost a billion years). Note that it doesn't decay by fission (that only happens in bombs), so comparison with nuclear fallout is not relevant.

To get a comparison of chemical toxicity to toxicity due to radiation, I think all we need is a very rough order-of-magnitude estimate of chemical toxicity. Organic mercury is extremely toxic (about 10 times worse than arsenic), and it has an LD50 of about 0.1 g, so that's probably a reasonable upper limit for any other heavy metal such as plutonium.

239Pu's radioactivity is in the form of alpha particles, which are only dangerous if emitted internally, because they can't penetrate the epidermis. The hazard then seems to me like it would depend crucially on how the internal exposure occurred. If you eat an alpha emitter, then it will stay in your body until it's either excreted or decays. The half-life for excretion of organic or metallic Hg is on the order of months, so that's probably a reasonable order-of-magnitude estimate for other heavy metals. On the other hand, there are people at nuclear weapons labs who do machining of plutonium pieces, and this kicks up plutonium dust. They do this machining inside glove boxes, and I assume the chips and dust are swept up very carefully. The dust can be breathed in, and microgram particles that get into your lungs are likely to remain there for the rest of your life (Lenntech, ATSDR 2010). From the lungs, it can also migrate to the bones or liver. ("Much less than 1%" of ingested Pu would do so.)

For internal exposure to alpha emitters, we have good data on 210Po, which was used in the Litvinenko assassination. The LD50 for this substance is believed to be about 1 μg. It has a half-life of 138 days, and although I don't know its half-life for excretion, as described above I'd guess it's on the same order of magnitude, so to within a factor of order unity, we can probably pretend that all of it decays within the body. But if you eat 239Pu, then assuming it gets excreted within ~100 days, its probability of decaying within your body is only about $10^{-5}$. Therefore we would expect the LD50 for ingested 239Pu to be on the order of $10^5$ times greater than that for 210Po, or 0.1 g. This is on the same order of magnitude as the chemical toxicity. Well, this would be of interest if any human being (or lab animal) were ever going to be exposed to this much of the stuff, and then maybe we would be motivated to work out the biochemistry in more detail, do studies, and so on. But in reality, nobody has ever eaten or will ever eat this much plutonium, so it's of no interest.

For inhalation, the balance shifts. If someone lives for 50 years after exposure and carries the dust in their lungs for that whole time, then the probability of decay within the body is $\sim10^{-3}$. That means that in terms of radiation damage, the LD50 should be about 1000 times higher than for 210Po, i.e., about 1 mg. This would make it about 100 times more toxic in terms of radiation than any plausible chemical toxicity.

You could argue that plutonium might be less lethal than implied by this estimate because a radiation dose that would be lethal if suffered all at once can be less dangerous or almost harmless if spread out over a period of years. This probably reduces the radioactive toxicity of plutonium by a big factor relative to the estimate above, but I don't think it can reduce it by a factor of 100, which is what we would need in order to make the radioactive toxicity comparable to any realistic level of chemical toxicity.

So to summarize:

For external exposure: Zero hazard from radiation, some hazard from chemical toxicity but probably no worse than the (very small) risk from external exposure to substances like lead or mercury.

Ingestion: Chemical toxicity is not relevant because nobody will ever ingest enough to be in danger. On a population basis, radiation could conceivably cause some excess cancers, but I doubt that there is very much unreacted plutonium in fallout, and the population risk does not seem to have been considered sufficient to merit discussion in Simon et al.

Inhalation: The hazard from radiation is probably 1-100 times greater than the chemical toxicity is likely to be, even if the chemical toxicity is as bad as organic mercury.

References

ATSDR 2010, "Public Health Statement for Plutonium," https://www.atsdr.cdc.gov/PHS/PHS.asp?id=646&tid=119

Lenntech Water Treatment Solutions, "Health effects of plutonium," https://www.lenntech.com/periodic/elements/pu.htm

Simon et al., "Fallout from Nuclear Weapons Tests and Cancer Risks ," https://www.cancer.gov/about-cancer/causes-prevention/risk/radiation/fallout-pdf

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