The "Threshold of Regulation" (TOR) rule that you refer to effectively establishes what the U.S. Food and Drug Administration (FDA) considers to be a de minimis dietary level for food-contact materials. In the preamble to the rule, FDA describes the regulation as a "process for determining when the likelihood or extent of migration to food of a substance used in a food-contact article is so trivial as not to require regulation of the substance as a food additive" (60 Fed. Reg. 36851, 36582, July, 17, 1995). Under this rule, substances that are not known to be carcinogens, and do not contain known carcinogenic constituents with a TD₅₀ value of less than 6.25 mg/kg body weight per day are eligible for exemption from regulation as food additives under the following circumstances: (1) either the dietary concentration of the substance is 0.5 parts per billion (ppb) or below, or the substance is currently regulated for direct addition to food and the dietary exposure to the substance resulting from the proposed use is less than 1 percent of the acceptable daily intake established for direct additive use; (2) the substance has no technical effect in food; and (3) the substance has no significant impact on the environment.

To obtain a letter from FDA confirming that the use of a material is exempt from regulation as a food additive pursuant to the TOR rule, a company submits a letter request to the Agency containing information identifying the material, its intended conditions of use, the estimated dietary exposure to the material resulting from that use, the results of a literature search for toxicology information, and, in some instances, an environmental assessment. If FDA agrees that the intended use of the substance of interest does not require amendment of the food additive regulations, the Agency will issue a letter stating this fact.

FDA maintains a list of substances that are the subject of TOR letters on its website. This list includes the names of the companies that made the threshold requests, the chemical names (not trade names) of the substances of interest, the specific uses that are exempted from food additive regulation, and any appropriate use limitations. A Threshold of Regulation Exemption determination is one of general applicability that is available to all and carries no proprietary rights. Therefore, if the substance of interest is already the subject of a TOR letter, you may not need to request a TOR exemption. You will only need to determine whether your intended use for the substance complies with the specifications set forth in the regulatory clearance.