The Full Form of TEF is‍ Toxic equivalency factor.

Dioxins are generally found in mixtures containing several kinds of dioxins and dioxin-like compounds, each having its own degree of toxicity. To express the overall toxicity of such a mixture as a single number, the concept of “International Toxic Equivalents” (TEQ) has been developed.

The “Toxic Equivalent” (TEQ) scheme weighs the toxicity of the less toxic compounds as fractions of the toxicity of the most toxic TCDD. Each compound is attributed a specific “Toxic Equivalency Factor” (TEF). This factor indicates the degree of toxicity compared to 2,3,7,8-TCDD, which is given a reference value of 1.

To calculate the total TCDD toxic equivalent (TEQ) of a dioxin mixture, the amounts of each toxic compound are multiplied with their Toxic Equivalency Factor (TEF) and then added together.

The TEQ scheme refers only to adverse effects (e.g. cancer) following interactions with the cellular Ah receptors. Other toxic effects of dioxins and dioxin-like compounds are not quantified by this method. Toxic Equivalency Factor (TEF) values vary for different animal species.

Two schemes:

1) I-TEF and I-TEQ: The older International Toxic Equivalent (I-TEQ) scheme by the North Atlantic Treaty Organisation (NATO) initially set up in 1989 and later extended and updated.

2) WHO-TEF and WHO-TEQ (also referred to as TEF or TEQ): More recently, the World Health Organization (WHO) suggested modified Toxic Equivalency Factor (TEF) values (see level 3, question 5.3.1, table 3 WHO-TEFs for human risk assessment).

On average, the result of TEQ-calculations is about 10% higher when I-TEFs are used compared to when WHO-TEFs are used.

References to I-TEQ or TEQ are sometimes omitted when figures are given in a text, which makes it impossible to know which TEFs have been used.