*2.5. Prediction of Mixture Toxicity*

The quantitative structure–activity relationship (QSAR), independent action (IA), concentration addition (CA), and integrative models of the CA and IA such as two–stage prediction (TSP) are widely used models to predict the toxicity of mixtures. According to the different toxic modes of action, two conventional models are used to predict the joint effect of mixtures. One is independent action (IA, different mode of action, MOA), and the other is concentration addition (CA, similar to MOA) [56]. In this study, BPA and heavy metals indicate different toxic action modes, and both CA and IA were used to predict the mixture of toxicities. The mathematical functions of CA and IA are expressed as Equations (5) and (6), respectively:

$$EC\_{x,m} = \left(\frac{P\_A}{EC\_{x,A}} + \frac{P\_B}{EC\_{x,B}}\right)^{-1} \tag{5}$$

where *ECx*,*m*, *ECx*,*A*, and *ECx*,*<sup>B</sup>* are the concentrations of the mixtures; *A* and *B* are the decreases in the *x*% luminescence, respectively; and *PA* and *PB* correspond the concentration ratios of *A* and *B* in the mixture.

$$E(\mathbb{C}\_M) = 1 - (1 - E(\mathbb{C}\_A)) \times (1 - E(\mathbb{C}\_B)) \tag{6}$$

where *E*(*CM*) is the toxic effect of the mixture, and *E*(*CA*) and *E*(*CB*) are the toxic effects of *A* and *B* in the individual toxicity tests, respectively.
