These things are perfectly safe in such small amounts, by the way. But they’re a lot of fun to think about, and a great way to gross out your friends!
The Handbook (which you can read in full here!) describes in detail the “levels of natural or unavoidable defects in foods that present no health hazards for humans.” The reason it exists is because it's impossible to mass-produce food without the occasional defect, and the FDA needs to draw the line somewhere to guarantee that people who eat that food stay safe and healthy.
(But thinking about them may just make you sick. Sorry about that!)
Before you, uh, dig in to this information, you should read what the FDA's Handbook has to say about these limits:
It is incorrect to assume that because the FDA has an established defect action level for a food commodity, the food manufacturer need only stay just below that level. The defect levels do not represent an average of the defects that occur in any of the products—the averages are actually much lower. The levels represent limits at which FDA will regard the food product “adulterated”; and subject to enforcement action under Section 402(a)(3) of the Food, Drug, and Cosmetics Act.
Got all that? Good. Now read on, and prepare to completely lose your appetite: