The toxic equivalent (TEQ) is operationally defined by the sum of the products of the concentration of each congener multiplied by its TEF value. The TEQ is an estimate of the total 2,3,7,8-Cl4DD (or TCDD)–like activity of the mixture. Although the scientific basis cannot be considered as solid, the TEF approach has been adopted as an administrative tool by many agencies and allows converting quantitative analytical data for individual PCDD/PCDF congeners into a single toxic equivalent (TEQ). As TEFs are interim values and administrative tools, they are based on present state of knowledge and should be revised, as new data becomes available.

Concurrent with the development of the TEF and TEQ approach for intake situations (humans, fish, birds) has been its application to environmental matrices such as soil, sediment, industrial wastes, soot, fly ash from municipal incinerators, waste water effluents, etc. As such, the TEF approach has been and continues to be used to give a single value to complex environmental matrices.

The Stockholm Convention, according to Annex C, refers to usage of the 1998 TEF scheme that was developed by an expert group under the World Health Organisation (WHO) (van den Berg et al. 2006).