Appendix a Background Information on the Assessment of Additivity and Interactions

The approaches to assessing the joint action of components of a mixture are based in large measure on the conceptual groundwork laid by Bliss (1939) and Finney (1971), and are mathematical rather than biological in nature. The approaches commonly known as dose addition and response addition, discussed in the following sections, are non-interactive forms of joint action that assume the chemicals in the mixture do not affect the toxicity of one another, i.e., that they act independently. These assumptions are the bases for methods of risk and health assessment discussed in the Guidance Manual. In addition, the assessment of interactions depends on being able to define what constitutes non-interaction.