The Human Nonthreshold Criterion

Section 302 Section 302.651  The Human Nonthreshold Criterion The Human Nonthreshold Criterion (HNC) of a substance is the concentration or level of a substance at which humans are protected from an unreasonable risk of disease caused by a nonthreshold toxic mechanism as a result of incidental exposure to or ingestion of surface waters of the State or ingestion of aquatic organisms taken from surface waters of the State.  HNCs are derived for those toxic substances for which any exposure, regardless of extent, carries some risk of damage as specified in subsections (a) and (b). a)         For single substances, a risk level of one in one million (1 in 1,000,000) must be allowed (i.e., considered acceptable) to determine an HNC. b)         For mixtures of substances, an additive risk level of one in one hundred thousand (1 in 100,000) must be allowed (i.e., considered acceptable) to determine an HNC.