Parts per million (ppm)

Parts per million (ppm) expresses a very small concentration: one part of a substance in a million parts of the whole, which is the same as one milligram per kilogram. In food certification it usually measures how much of an unwanted substance a product contains — most often gluten. The U.S. FDA gluten-free rule sets that limit at under 20 ppm.

In depth

One ppm is one part in 1,000,000. For a solid food that works out to one milligram of the substance in one kilogram of product, so 20 ppm of gluten means fewer than 20 milligrams of gluten per kilogram of food.

The figure matters most for gluten-free certification: under the FDA rule (21 CFR 101.91), a product may carry a gluten-free claim only if it contains under 20 parts per million of gluten. Some programs test to a stricter internal limit; a V-Cert gluten-free certificate always states the exact threshold the product was tested against.

Concentrations this small are measured in a laboratory, commonly with an ELISA test that detects gluten protein. Because the number is a measured limit rather than a promise of zero, the honest claim is gluten-free to a stated threshold — see also ALARA, the principle of keeping an unavoidable contaminant as low as reasonably achievable.

Related marks

• Gluten-FreeNot built for retailers mandating GFCO specifically — if yours does, we'll tell you before you pay anything.

Related terms

• Gluten — Gluten is a group of storage proteins found in wheat, barley, rye and their crosses, which some people must avoid for medical reasons.
• ALARA (as low as reasonably achievable) — ALARA is a control principle meaning an unavoidable contaminant should be reduced to a level as low as reasonably achievable, not merely below a legal limit.
• Cross-contact — Cross-contact is the unintended transfer of an allergen or gluten from one food to another, usually via shared equipment, hands or airborne particles.

Frequently asked questions