Combination of substances with common sites of main action and no interaction between the components. The action can be additive. An example is the combined toxic action of mixtures of polychlorinated dibenzodioxins, polychlorinated dibenzofurans, and polychlorinated biphenyls (particularly congeners with planar structures) occurring in, for example, mother’s milk. The effects of these substances have been shown to be additive. Usually, the toxicity of such mixtures is expressed in terms of the concentration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) by adding the so-called TCDD toxicity equivalent concentrations of the individual components—concentration addition.