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This year-in, year-out monitoring enables health officials to spot whether changes in food production or the environment affect food quality. In response, they can launch medical research, alter regulations or, if a problem is bad enough, recall a brand.

Bad contamination is very rare. But it happens:

• A pesticide sprayer was sent to jail after FDA discovered he used an illegal bug-killer on 19 million bushels of oats headed for top-selling breakfast cereals.
  
• Baby-food carrots were recalled because they absorbed lead while growing in an old apple orchard, where a lead-based fruit pesticide had years earlier seeped into the soil.
  
• Insecticide was found in teething biscuits that are supposed to be organic, free of conventional chemicals.
  

And ever wonder why cereal always comes in plastic bags inside the box? Because this testing once uncovered PCBs leaching into wheat cereal by contact with its package, a box made of recycled paper that contained the cancer-causing pollutant.

So far this year, FDA has discovered traces of illegal pesticides on some grapefruit, tomatoes and collard greens, not enough for a health risk but a mystery yet to be solved.

A shopping list for the lab
Now the World Health Organization is urging other countries, even poor developing ones, to adopt FDA-style testing so they can better target scarce resources to improve food safety. WHO's top priority is learning more about so-called "persistent organic pollutants" -- a class of chemicals, including the widely banned pesticide DDT, that remain in the environment for years without breaking down.

"Few countries have sufficient information on the exposure of their populations to the many chemicals that find their way, either intentionally or unintentionally, into food," says the WHO's Dr. Gerald Moy.

What started 40 years ago as checking a few foods for fallout from nuclear testing today is a $5 million canvassing of the food supply.

Four times a year, FDA employees enter grocery stores in three different cities with identical lists so long -- 9 dozen eggs, 6 pounds of bacon, gallons of soda, cases of baby food -- they dare not shop on crowded coupon days. With a few stops at fast-food restaurants to round out the menu, they can spend $3,000 per city.

They quick-ship purchases to an FDA laboratory in Lenexa, Kansas, where workers sort the food, sending ingredients that need cooking on to the nearby Belton church ladies.

Inside the lab, giant blenders grind foods into mush so scientists can test for more than 300 pesticides, cancer-causing dioxins and industrial chemicals. This year, for the first time, they're also hunting acrylamide, a possibly cancer-causing chemical formed when foods are cooked at high temperatures.

They count nutrients, too. Soon, FDA will learn how much folate, which prevents birth defects, women eat -- the first evidence of whether recent fortification of bread and cereal is working well enough.

One rule of thumb: More processing typically means less pesticide residue. A raw peach, even washed and peeled, usually has more than a canned peach, says FDA's Sack.

Second rule of thumb: The fattier a food, the more chemicals from the environment or processing can cling to it. With stick butter or margarine, for example, "you eat a bucketload of industrial compounds from that wrapper," Sack says, adding, "They're probably harmless."