Aflatoxin, and the twenty parts per billion figure
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What it is
Aflatoxins B1, B2, G1 and G2 are toxic metabolites of the molds Aspergillus flavus, A. nomius and A. parasiticus. They are known human carcinogens, and their growth in a crop depends on temperature, humidity and rainfall around harvest. Peanuts, corn, Brazil nuts, pistachios and some small grains are the foods most often named.
The figure
FDA may consider peanuts and peanut products containing total aflatoxins greater than 20 micrograms per kilogram, which is 20 parts per billion, to be adulterated, with an exception for raw peanuts that will be further processed to remove moldy and defective nuts. The same 20 ppb figure applies to human food generally under a separate compliance policy guide.
Roasting does not remove it
This is the important practical point and it is the opposite of the Salmonella situation. The toxin is heat stable, so a sufficient roast does nothing about it. Sorting out moldy and defective nuts is the control, and it happens before roasting rather than after.
So this one is a buying decision
Your exposure is decided by who you buy from and what they do about it. Ask suppliers directly what their aflatoxin program is and what they will document, and treat an inability to answer as information. For a small maker this is the most important supplier question after the roast.
Visible mold is a reason to reject the lot
Not to sort the good ones out and proceed. Mold on part of a delivery says something about the conditions the whole delivery came through, and the toxin is not confined to the nuts that look wrong.