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.
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...