Chemical substances—pesticides and herbicides—that are used as weed and insect control on crops and animal feed always leave some kind of residue either on the surface or within the crop itself after treatment. The amounts of residue vary, but they do remain on the crop when ready for harvest and consumption.

While these chemicals are generally registered for use on food crops, it does not necessarily mean they are "safe." As part of its program to regulate the use of pesticides, Environmental Protection Agency (EPA) sets "tolerances" which are the maximum amount of pesticides that may remain in or on food and animal feed. Tolerances are set at levels that are supposed to ensure that the public (including infants and children) is protected from unreasonable health risks posed by eating foods that have been treated.

Tolerances are initially calculated by measuring the amount of...