Hormesis is an intriguing phenomenon that is characterized by low dose stimulation and high dose inhibition. Several traditional parameters, such as the concentration of the zero equivalent point (ZEP) and the maximal stimulatory effect (Y-max), have been used to characterize the zone of hormesis or the extent of the stimulatory effect. However, the characteristics of hormesis for chemicals cannot be quantified completely by one parameter, which is important to accurately compare the hormetic effects of chemicals and to describe the combined effects of chemical mixtures at low doses. In the present study, a novel parameter, termed the relative standard area of hormetic zone (Hor(Area)(R)), was developed and proposed to quantify the hormetic effects (24h exposure) of nine Metal ions (Cr3+, Cu2+, Mg2+, Cd2+, Fe3+, Ni2+, Zn2+, Co2+, ce) towards Vibrio fischeri, both individually and as binary mixtures. The results indicate that Hor(Area)(R) can be used not only to accurately assess the hormetic effects and its relationship with structural characteristics but also to conveniently describe the combined effects of interactive mixtures at low dose. Thus, the Hor(Area)(R) parameter can quantitatively assess the hormetic effects and can offer a useful approach to perform environmental risk assessments of chemicals at low doses. (C) 2016 Elsevier B.V. All rights reserved.