Today they manage to check only 1 percent of the 3.7 million shipments of imported food that arrive each year, according to Congress’s General Accounting Office. Similarly tiny percentages of domestic produce and processed food get tested. Still, as experts assess the vulnerability of America’s food supply, from farm to fork, what is emerging is fairly reassuring.

Start in the fields. Poisoning grain or produce there is both hard to pull off and unlikely to lead to mass illness, let alone death. Unscheduled “aerial applicators” would “raise questions immediately,” says Peter Chalk of the think tank Rand. And ever since reports that the Sept. 11 hijackers had inquired about such planes, hangars have been locked and some operators have even removed parts to keep the aircraft grounded. More important, pesticides and other poisons can’t even be sprayed on crops in doses that are both lethal and surreptitious. Even if a terrorist managed to drench a field with, say, the pesticide methyl parathion or arsenic or anthrax, the farm workers would be affected before the crop reached consumers.

From the farm, grain is stored in massive silos, which also make unlikely targets. At The Andersons, which operates grain elevators in the Midwest, employees have increased security around their stored corn, soybeans and wheat, and are ensuring that the rail cars that transport the crops are completely sealed. “The opportunity and the effectiveness of adulterating the grain at this level is very low,” says general manager Joe Needham.

The best way to keep poisons out of processed food is to keep poisoners out of food-processing facilities, where tomatoes turn into ketchup, apples into juice and the rest of the harvest into groceries. Manufacturers are ramping up in-house security through “a variety of safeguards,” says spokesman Peter Cleary of the Grocery Manufacturers of America, “from the raw product through the manufacturing process to delivery.” One step: companies are monitoring closely when an employee asks for a job transfer, especially one that would give him new access to a process that could affect food’s safety. They are also securing gates, fences and transports and ensuring that no workers carry anything from the locker rooms to the factory floor. But manufacturers are not required to register with the FDA. “Almost anyone can go into business, start producing food and ship it around the country with no federal oversight,” says Caroline Smith DeWaal, head of food safety at the Center for Science in the Public Interest.

Reassuringly, however, failure to prevent contamination might not be disastrous. Four microbes, because of their availability and pathogenicity, appear to pose the greatest danger to the food supply: salmonella, E. coli O157, Clostridium botulinum and cholera. But even here the risk is limited. When a religious cult poisoned 10 salad bars and drinking glasses in an Oregon town with salmonella in 1984, they made 751 sick (mostly with nausea, diarrhea, fever and chills) but killed no one. And food poisoning already kills some 5,000 people and sickens millions more in the United States each year, suggesting that anything but the most horrific poisoning might not even be noticed. Terrorists usually try to be noticed.

Even anthrax in food is unlikely to cause many illnesses, let alone deaths. Security at processing plants would keep all but the smallest vials of the stuff out, so any particular can or box would not harbor a lethal dose. Also, processing usually means heat: canned food is sterilized, milk is pasteurized, most frozen food is either pasteurized (ice cream) or blanched (fruits and vegetables), and packaged food like bread, cereal, crackers and cookies are baked. That renders harmless even botulinum. Pathogens added after the final processing step pose a greater risk. In 1998 salmonella was accidentally sprayed on a toasted-oat cereal called Malt-O-Meal; 409 people in 21 states got sick. To prevent contamination at this point in the food chain requires more inspectors.

An in-store poisoning could take the form of something as low-tech as injecting cucumbers with cyanide. But food retailers and wholesalers already use security cameras and conduct background checks of employees. And this kind of terrorism would cause few deaths. To add a layer of security, wash produce (that gets rid of most pathogens) and cook food to 165 degrees Fahrenheit (that kills salmonella and E. coli.) Boiling for 10 minutes inactivates botulinum, says Michael Doyle of the Center for Food Safety at the University of Georgia. Should we irradiate all food and produce? Irradiation, in which powerful gamma rays from radioactive compounds zap food, kills microbes. But an irradiation machine costs at least $1 million, and could pose risks to workers exposed to radioactive compounds.

FIRST STEPS: Boost security at processing plants and in stores, wash produce and cook food to 165 degrees Fahrenheit.