New evaluation of toxicity equivalents for dioxins and dioxin-like compounds

The system of toxicity equivalents (TEQ) was introduced as a means of enhancing assessment of the toxicity of dioxins and dioxin-like polychlorinated biphenyls (PCBs). The TEQ system makes it possible to express a range of different concentrations of substances that all have the same toxicological active profile, but different toxic effects, as a single, combined value. So-called "toxicity equivalent factors" (TEF) play a key role in calculation of TEQ.

Under the system, toxicity of individual substances is expressed in terms of 2,3,7,8-tetrachlorodibenzodioxin (2,3,7,8-TCDD), which is considered to be the most toxic compound in the substance group and which achieved notoriety as the "Seveso dioxin". As the reference substance for the system, 2,3,7,8-TCDD has been assigned a factor of "one". Other compounds in the dioxins / dioxin-like PCBs substance group have received correspondingly lower values in proportion to their lower toxicity. In any given case, the different compounds involved, with different toxicity equivalent factors, and with the different quantities in which they are present, account for different shares of the pertinent TEQ. The toxicity equivalent factors are determined in the framework of scientific studies on the toxicity of the various relevant compounds. 

Consequently, the factors have to be updated at regular intervals, taking account of any new findings. In 1998, the World Health Organization (WHO) compiled a list of toxicity equivalent factors (TEFs). For some time thereafter, that list was used as a basis for defining the EU's maximum permitted levels for dioxins and dioxin-like PCBs in foods. In 2005, the toxicity equivalent factors were reviewed, and new values were proposed, based on evaluations of new studies using suitable mathematical and statistical techniques. With the new values, the toxicity of dioxins and dioxin-like PCBs in foods is classified as ten to 20 percent lower, depending on the food involved, than the corresponding values obtained with the TEFs from 1998. In November 2023, the World Health Organization (WHO) published new TEFs (2022 WHO TEF) after reviewing the TEF of 2005. These new TEFs are based on a scientifically recognized evaluation approach that was applied to a comprehensive dataset and significant toxicological studies of individual compounds. Currently, the European Food Safety Authority (EFSA) is preparing a risk assessment for human health based on the new TEFs, which is expected to be available in the spring of 2026.

The maximum-level and action-level regulations applying since 1 January 2012 for dioxins and dioxin-like PCBs in foods are based on the toxicity equivalent factors from 2005.