Relationships between Aquatic Hazard Quotients and Probabilistic Risk Estimates: What Is the Significance of a Hazard Quotient >1?

This study examined relationships between hazard quotients (HQ) and probabilistic estimates of aquatic ecological risk. Questions addressed included the magnitude at which an HQ equates to significant risk, and the factors influencing the HQ-risk relationship. The analysis was based upon predicted exposure concentrations (PEC) for copper, hypothetical predicted no effect concentrations (PNEC) distributions, and measured PNEC data for aquatic species acutely and chronically exposed to copper, ammonia, cadmium, cyanide, dieldrin, DDT, phenanthrene, silver. and zinc. The cumulative PNEC and PEC distributions differed in slopes and magnitudes. The relationship between HQ and probabilistic risk, both of which were computed using conventional techniques, depended on the slopes of the PNEC and PEC distributions. Hazard quotients equaling 1.0 affected ∼ 5% of the species because they were based on PNECs intended to protect 5% of the species. Hazard quotients greater than 1.0 depended on PNEC slope. For example, H...