Chemicals occur normally in nature.
For example, most plants produce natural pesticides which protect them from microbes, insects, animals, and other plants. The State of the Industry Modern science now makes it possible for biologists and government inspectors to find minute amounts of potentially harmful residues in food-in parts per billion or even smaller concentrations. As testing methods become more sensitive, it becomes impossible to reduce all residues to absolute zero. But minute traces of most substances are not considered dangerous to humans. To provide some level of comparison of the levels of chemicals in our food, one part per billion equals one second in 32 years.