Argonne National Lab has some handy fact sheets that are a good starting place for figuring out what a given radionuclide does in the body (I just Google "Argonne fact sheet <element>" to find them).
From the plutonium fact sheet:
When plutonium is inhaled, a significant fraction can move from the lungs through the blood to other organs, depending on the solubility of the compound.... After leaving the intestine or lung, about 10% clears the body. The rest of what enters the bloodstream deposits about equally in the liver and skeleton where it remains for long periods of time, with biological retention half-lives of about 20 and 50 years, respectively, per simplified models that do not reflect intermediate redistribution. The amount deposited in the liver and skeleton depends on the age of the individual, with fractional uptake in the liver increasing with age. Plutonium in the skeleton deposits on the cortical and trabecular surfaces of bones and slowly redistributes throughout the volume of mineral bone with time.
So about 90% of inhaled plutonium typically remains in the body for decades. Estimating health risks requires knowing the amount inhaled; the fact sheet gives two different ways of looking at the risk. They report a lifetime excess cancer mortality risk of about 3 x 10^(-8) per picocurie for inhalation exposure. They also say, "...for inhalation (the exposure of highest risk), breathing in 5,000 respirable plutonium particles of about 3 microns each is estimated to increase an individual’s risk of incurring a fatal cancer about 1% above the U.S. average."
From the americium fact sheet:
After ingestion or inhalation, most americium is excreted from the body within a few days and never enters the bloodstream; only about 0.05% of the amount taken into the body by ingestion is absorbed into the blood. After leaving the intestine or lung, about 10% clears the body. The rest of what enters the bloodstream deposits about equally in the liver and skeleton where it remains for long periods of time, with biological retention half-lives of about 20 and 50 years, respectively (per simplified models that do not reflect intermediate redistribution). The amount deposited in the liver and skeleton depends on the age of the individual, with fractional uptake in the liver increasing with age. Americium in the skeleton is deposited uniformly on cortical and trabecular surfaces of bones and slowly redistributes throughout the volume of mineral bone over time.
So very little americium is absorbed; but what does remain stays for a long time, very much the way plutonium does. The excess cancer mortality risk per picocurie is comparable to, but smaller than, the values for plutonium. But it appears that, per atom, plutonum is a bigger concern because a larger fraction of it stays in the body.
It's worth noting that these biological profiles are very different from the contaminants most prevalent in reactor accidents like Cs-137, which has a biological half-life of a few months rather than decades.
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