Annex 1 Toxicity Equivalency Factors

PCDD, PCDF and PCB typically occur as mixtures of many congeners in samples, including measurements at the sources (i.e., used in inventory development), in the environment or in biota including humans and wildlife. First risk assessments only focused on the most toxic congener, the 2,3,7,8-Cl4DD. Soon it was recognized, though, that all PCDD/PCDF substituted at least in position 2, 3, 7, or 8 are highly toxic and thus, major contributors to the overall toxicity of the “dioxin” mixture.

For regulatory purposes so-called toxicity equivalency factors (TEFs) have been developed for risk assessment of complex mixtures of PCDD/PCDF and later for PCB by different organizations. It should be noted that the first globally applied scheme, the International toxicity equivalency factors (I-TEFs) (NATO/CCMS 1988) did only include the 17 2,3,7,8-subsituted PCDD/PCDF congeners. Dioxin-like PCB were added later (WHO-TEFs) (van den Berg et al. 1998). The most commonly used are summarized below.

Most commonly used TEF schemes
Congener I-TEF WHO 1998-TEF WHO 2005-TEF
Polychlorinated dibenzo-p-dioxins
2378-Cl4DD 1 1 1
12378-Cl5DD 0.5 1 1
123478-Cl6DD 0.1 0.1 0.1
123678-Cl6DD 0.1 0.1 0.1
123789-Cl6DD 0.1 0.1 0.1
1234678-Cl7DD 0.01 0.01 0.01
Cl8DD 0.001 0.0001 0.0003
Polychlorinated dibenzofurans
2378-Cl4DF 0.1 0.1 0.1
12378-Cl5DF 0.05 0.05 0.03
23478-Cl5DF 0.5 0.5 0.3
123478-Cl6DF 0.1 0.1 0.1
123678-Cl6DF 0.1 0.1 0.1
123789-Cl6DF 0.1 0.1 0.1
234678-Cl6DF 0.1 0.1 0.1
1234678-Cl7DF 0.01 0.01 0.01
1234789-Cl7DF 0.01 0.01 0.01
Cl8DF 0.001 0.0001 0.0003
Non-ortho polychlorinated biphenyls
PCB 77 - 0.0001 0.0001
PCB 81 - 0.0001 0.0003
PCB 126 - 0.1 0.1
PCB 169 - 0.01 0.03
Mono-ortho polychlorinated biphenyls
PCB 105 - 0.0001 0.00003
PCB 114 - 0.0005 0.00003
PCB 118 - 0.0001 0.00003
PCB 123 - 0.0001 0.00003
PCB 156 - 0.0005 0.00003
PCB 157 - 0.0005 0.00003
PCB 167 - 0.00001 0.00003
PCB 189 - 0.0001 0.00003

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).