Figure 2.

Theoretical models for assessing the interaction of chemicals in mixture. The combined effects of 2 chemicals in fish can be quantified by expressing the concentration of toxicants A and B in mixture either as (A) toxic units (TU) or (B) linearly, using the additive index. For the toxic unit approach (A), if 1 toxic unit (dashed horizontal line) is defined as a one‐half unit of response, then 1toxic unit A + 1toxic unit B = 1 unit of response if the interaction is strictly additive; if the interaction is less than additive (antagonistic), then 1toxic unit A + 1toxic unit B < 1 unit of response; if the interaction is greater than additive (synergistic), then 1toxic unit A + 1toxic unit B > 1 unit of response. The additive index (B) represents a linear scaling of the combined effects of chemicals A and B. If the range of the effects spans 0, then the interaction of the 2 toxicants is strictly additive; if the range <0, then the interaction is less than additive (antagonistic); and if the range of the additive index is >0, then the interaction is greater than additive (synergistic). Figure adapted from Newman (2015).