# Insights into PCDD/Fs and PAHs in Biomass Boilers Envisaging Risks of Ash Use as Fertilizers
## 2 Literature Review  
### PCDD/Fs and TEQs; PAHs and BaP Equivalents  
Of the 210 PCDD/Fs congeners (75 PCDDs and 135 PCDFs), only 10 PCDFs and 7 PCDDs containing 2,3,7,8-Cl substitutions, have been appointed for environmental risk assessment given their high Aromatic Hydrocarbon Receptor (AhR) bioactivity (Table 1) [67]. The toxicity of a complex mixture of PCDD/Fs is usually expressed as Toxic Equivalent (TEQ). TEQ consists of an aggregate measure obtained with the sum of the individual congener concentrations multiplied by their specific Toxic Equivalent Factor (TEF). TEFs correlate each congener's toxicity potency to the most toxic 2,3,7,8-TCDD [18, 68]. The TEF concept was introduced in 1977 and was adopted internationally, in 1988, as I-TEQ for the 17 PCDD/Fs [69]. The World Health Organization and International Program on Chemical Safety (WHO/IPCS) continually reevaluates the TEF system and later on included the dioxin-like toxicity PCBs (dI-PCB) (12 non-ortho/mono-ortho-substituted polychlorinated biphenyls) in the TEQ protocol, WHO\({}_{1998}\)-TEQ [70]. In the latest revision, WHO\({}_{2005}\) TEQ [66], modified some TEF values, as shown in Table 1. The possible inclusion of other compounds with dioxin-like effects, such as brominated and mixed halogenated dioxins/furans, is currently under evaluation [66]. Unequivocally, the oldest publications use I-TEQ and subsequently WHO\({}_{1998}\)-TEQ (including or not dI-PCBs), however, they are still in use [10]. [PERSON] and [PERSON] [71] found good correlations between \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **PCDD/Fs congeners** & **I-TEF [68]** & **WHO\({}_{1998}\)-TEF \({}^{\textbf{a}}\)[69]** & **WHO\({}_{2005}\)-TEF \({}^{\textbf{a}}\)[65]** \\ \hline \multirow{8}{*}{**PCDD/F**} & & 2,3,7,8-TCDF & 0.1 & 0.1 & 0.1 \\  & & 1,2,3,7,8-PeCDF & 0.05 & 0.05 & **0.03 b** \\  & & 2,3,4,7,8-PeCDF & 0.5 & 0.5 & **0.3** \\  & & 1,2,3,4,7,8-HhCDF & 0.1 & 0.1 & 0.1 \\  & & 1,2,3,6,7,8-HhCDF & 0.1 & 0.1 & 0.1 \\  & & 2,3,4,6,7,8-HhCDF & 0.1 & 0.1 & 0.1 \\  & & 1,2,3,7,8,9-HhCDF & 0.1 & 0.1 & 0.1 \\  & & 1,2,3,4,6,7,8-HpCDF & 0.01 & 0.01 & 0.01 \\  & & 1,2,3,4,7,8,9-HpCDF & 0.01 & 0.01 & 0.01 \\  & & OCDF & **0.001** & **0.0001** & **0.0003** \\ \hline \multirow{8}{*}{**PCDD/F**} & & 2,3,7,8-TCDD & 1 & 1 & 1 \\  & & 1,2,3,7,8-PeCDD & 0.5 & 1 & 1 \\  & & 1,2,3,4,7,8-HhCDD & 0.1 & 0.1 & 0.1 \\  & & 1,2,3,6,7,8-HhCDD & 0.1 & 0.1 & 0.1 \\  & & 1,2,3,7,8,9-HhCDD & 0.1 & 0.1 & 0.1 \\  & & 1,2,3,4,6,7,8-HpCDD & 0.01 & 0.01 & 0.01 \\  & & OCDD & **0.001** & **0.0001** & **0.0003** \\ \hline \hline \end{tabular}  
* Notes: \({}^{\textbf{a}}\) dI-PCBs not mentioned in the table. \({}^{\textbf{b}}\) Changed values in bold.  
\end{table} 
Table 1: List of 17 polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) congeners and Toxic Equivalent Factors (TEFs) for Toxic Equivalents (TEQ) calculations. different TEQ protocols for stack emissions of various incineration facilities, since minor modifications were made in the toxic factors (Table 1). \[\text{(WHO}_{1998}\text{-TEQ(PCDD/F))}=1.14\times\text{(I-TEQ)} \tag{1}\]  
\[\text{(WHO}_{1998}\text{-TEQ(PCDD/F/dl-PCB))}=1.17\times\text{(I-TEQ)} \tag{2}\]  
For wood boilers, the WHO\({}_{1998}\)-TEQ values were reported to be 10-25% higher than I-TEQ, depending on the congener distribution profile, which is within the measurement uncertainty [72]. From results of [PERSON] team, two correlations are extracted; Equation (3) [73] and Equation (4) [74]. \[\text{(WHO}_{2005}\text{-TEQ(PCDD/F))}=0.88\times\text{(I-TEQ)} \tag{3}\]  
\[\text{(WHO}_{2005}\text{-TEQ(PCDD/F))}=0.99\times\text{(I-TEQ)} \tag{4}\]  
Differences in the correlations may be attributed to the variation of congeners distribution and concentration, given the different magnitude of TEFs. When reporting WHO-TEQs, reference to the inclusion of dl-PCBs should be made [75].