Physical hazards are foreign objects such as insects, dirt, jewelry, and pieces of metal, wood, plastic, glass, etc. that inadvertently get into a food and could cause harm to someone eating that food.
FDA has established maximum levels of natural or unavoidable defects in foods for substances that present no major human health hazard. These are called Food Defect Action Levels. This is the maximum amount of unavoidable defects that might be expected to be in food when handled under good manufacturing and sanitation practices.
They are allowed because it is economically impractical, and sometimes impossible, to grow, harvest, or process raw products that are totally free of natural defects. Unavoidable defects include insect fragments, larvae, and eggs; animal hair and excreta; mold, mildew, and rot; shells, stems, and pits; sand and grit. The allowable levels of these substances are set at very specific levels deemed not to be a threat to human health. If a food contains more than these allowable levels, it is considered adulterated. While it may be unpleasant to find such substances in food, eating them at such low levels is not a health hazard and will not lead to illness.