Mice peer out from a loaf of bread which they hollow out and use the crust to live in, during an attraction for New Year visitors at the Inokashira Park Zoo in suburban Tokyo 06 January 2008, on the last day of Japan's largest holiday. This year is the Year of the Rat, according to the Chinese zodiac. AFP PHOTO / Yoshikazu TSUNO (Photo credit should read YOSHIKAZU TSUNO/AFP/Getty Images)

You've probably eaten mouse poop -- and the federal government is just fine with that. It's also fine with mold, rat hairs and insect legs.

The Food and Drug Administration, you see, has detailed guidelines on how much filth can be found in many of the foods sold in America. The FDA enumerates these guidelines in a document known as the "Defect Levels Handbook." The introduction to the handbook explains that "it is economically impractical to grow, harvest, or process raw products that are totally free of non-hazardous, naturally occurring, unavoidable defects" -- and that these so-called "defects" present no real risk to human health.

The FDA also says that many food companies take measures to ensure that their food actually contains far lower levels of these defects than their regulations require. If a food exceeds these levels, the FDA can label it "adulterated" and ask that it be taken off store shelves.

That all makes total sense, of course -- accidents happen, especially when food is produced at an industrial scale. But the Defects Levels Handbook nonetheless makes for some grisly reading. You probably already know and accept, at least on some dim level, that when you're eating delicious foods, you're also ingesting minute quantities of unrelated animal products. But to see it all spelled out is just gross.

The yuckiest of the defects is surely what the handbook calls "mammalian excreta." Not only is poop inherently disgusting, mouse droppings can spread the lethal (if exceedingly rare) Hantavirus.

The amount of excrement permitted varies from food to food. Many spices and herbs, including pepper, thyme, hot peppers, cinnamon bark and oregano, have a limit of 1 mg of excrement per pound of food. There are over 450,000 milligrams in a pound, so that's a very small fraction. Some whole spices, such as fennel seeds, ginger and mace, have a slightly higher limit of 3 mg per pound. The highest limits are on cocoa beans (10 mg per pound) and wheat (9 mg per kilogram).

The handbook also specifies acceptable limits for other potential adulterants like mold, rot, rodent hairs, insect parts and insect larvae. These, too, vary wildly. Black currant, for example, can have a "mold count" -- which refers to the percentage of samples showing any traces of fungi under a microscope -- as high as 75 percent, while pineapple juice can't exceed 15 percent.