Furthermore, the chemical process by which iodine is absorbed from the bloodstream by the thyroid is unable to discriminate between isotopes. So their biological relevance is actually identical.
If you want a quick and easy way to compare biological impact to the human body for a given isotope, check out the US Federal Guidance Reports 11 & 12. They have done all the heavy lifting and for a lot of the real nasties, you will actually find that the biological damage is chemical not radiological.
Interestingly, the limit for ingestion if I-129 intake is much lower (indicating a higher biological impact) than I-131 (by a factor of six), primarily BECAUSE it has a long half life. The half life is so long that the clearance rate is basically the biological half-life. Where as the I-131 is so short compared to the biological half-life so within a few months it is gone.
https://www.epa.gov/sites/production/files/2015-05/documents...
The limit for ingestion for I-129 is 5 uCi. From the formula of radioactivity:
m/m0 × NA × ln(2)/t½ = A, the amount of I-129 that is needed for 5uCi is ~28mg, which is also ~= the amount of Iodine in a normal human body. That is, if an evil wizard suddenly converted every atom of iodine in your body to I-129, you would be right about at the epa limit.
In terms of decays per second, sure. Those decays are somewhat less harmful though -- I-131 has betas and gammas in the ~500 keV range, while I-129's emissions are in the ~35 keV range.
But more importantly, 1 mol of I-129 is far less harmful than 1 mol of I-131.
the limit for ingestion if I-129 intake is much lower (indicating a higher biological impact) than I-131
The annual limit for ingestion of I-129 is 0.2 MBq, compared to 1 MBq for I-131. But given their vastly different half-lives, the annual ingestion limit for I-129 is 30 mg, compared to 220 picograms for I-131.
No one measures radioactivity in moles.
The FGR11 limit on I-129 is 5 uCi and I-131 is 30 uCi, I know it is not the latest and greatest, but the latest and greatest makes use of probabilistic risk models to quantify the increase in chances of getting cancer. this is less helpful in a discussion of personal danger rather than the collective danger.
In medical radiology, the amounts of radioactive isotopes are measured in Bq because the samples are so heavily diluted that it's the only thing you can sensibly measure. This easily hides the fact that when you are talking about roughly equal amounts, Tc-99m is roughly 29 years/6 hours ~=40000 times more dangerous than Sr-90. You just never use equal amounts, when you receive a sample of Tc-99m it's about 40000 times smaller than the Sr-90 sample.
On this I have to concede that it's not strictly true because some decay events produce more/ more dangerous radiation than others, and much more importantly, the decay of some isotopes start a long decay chain where all the successor isotopes decay much faster than the original. However, both of those effects are relatively small constant multipliers next to the 9 order of magnitude difference between decay rate of an isotope with a half-life of 16 million years compared to an isotope with a half-life of 8 days.
Of course not. But they measure amounts of Iodine in moles (or grams).
No, but people measure the amount of radioactive materials in moles and kg.
Where is this a rule of thumb. Its by no way supported by science, nor is it implemented in the regulation of radionuclides.
When it comes to biological uptakes, shorter half-life usually means less dose than longer half-life radionuclides. This is primarily because effective dose calculations integrate the absorbed dose over a persons life (well for the purpose of comparing radiotoxicity they do). So a long-live alpha or beta emitter will be far worse than a short lived gamma, even though the range and energy will likely be much higher for the short lived gamma emitter.
Iodine pills don't magically protect people from radiation. Taking them doesn't do anything to prevent nuclear decay from happening, doesn't grant people superpowers to stand in a radioactive environment, and doesn't let you lick fuel rods without consequence. In fact, taking them is to leverage the chemical benefits of diffusion.
Before I dive right into it, let me mention that a few forms of radioactive decay don't impact humans well. Alpha decays bounce off your clothing and skin, and can only travel a few cm in air. Beta decays bounce off your skin, or only make it so far into the already-dead skin cells, and don't travel very far in air either [0]. Gammas (sunlight) are a different matter, as are neutrons (from nuclear reactors). Hence, the sunscreen and the giant concrete containment units. This all changes if you inhale/ingest radioactive isotopes, and alphas suddenly become the most deadly (ask Alexander Litvinenko) and betas are up there too. Iodine 129 is a beta emitter, a pile of which will continue emitting radiation long after this generation of human beings are ash and dust. Eating this stuff is like eating glass.
Back to the iodine pills: You take this pill because $GOV says so, and it saturates your body (thyroid) with a stable isotope of iodine. It's like taking a half-empty biological sandbox, and filling the rest of it up with sand. The inner child that lives within us can continue playing safely as this iodine doesn't decay. It means no new iodine 129 will be able to be retained in your body, your body is already full of the stable stuff, so such a person will still ingest iodine 129, but won't retain it in their body for a long period of time.
However, failing to take an iodine pill beforehand will mean the sandbox is half-empty when a nuclear fission accident releases iodine 129. This means that sandbox will become (over time) quarter-sand and quarter-glass and still half empty, due to the laws of diffusion. No inner child wants to play in that box. And that means iodine 129 is sticking around inside the body, and in particular the thyroid (which is an organ more sensitive to radiation than others, for reasons I don't know since I'm an engineer not a doctor).
So, how does this stack up against ruthenium?
Your body doesn't use ruthenium in any significant way. It'll just pass it right back out. You can think of it as your body being permanently on the ruthenium pill. Yes, ruthenium 106 is a beta emitter exactly like iodine 129, but with a much shorter half-life of around a year. It decays to Rhodium 106, another beta emitter, which within a minute is already burned out into stable palladium 106. So on the whole, the human body is going to have an extremely tough time concentrating this stuff to dangerous levels inside of it.
So overall, your risk is not zero, but if you want to offset it I recommend not taking an airplane the next time you otherwise would, as flying up at cruising altitude in the atmosphere usually exposes you to more radiation than ruthenium 106 scattered around the earth ever will.
I highly recommend playing with [1] for nuclide information and decay chains.
[0] https://www.youtube.com/watch?v=Efgy1bV2aQo
[1] https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html