# 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
PAHs can be divided in two classes: Low Molecular Weight PAHs (LPAHs) with two and three benzenoid rings (Nap, Acy, Ace, FL, Phe and Ant), and High Molecular Weight PAHs (HPAHs) with four or more benzenoid rings (Flut, Pyr, BaA, Chry, BbFA, BkFA, BaP, DahA, BghiP and IP). LPAHs such as NaP, are usually present in larger quantities in the air phase, and considered acutely toxic to aquatic organisms since they are more water-soluble than HPAHs [93]. Other organizations selected diverse PAHs compounds, and still no worldwide consensus exists. The POPs Protocol [4] uses four PAHs (BaP, BbFA, BkFA and IP) for emissions inventory. In the EU [94], also 16 PAHs are contemplated, although less may be considered. The Directive 2004 on air quality [95] includes BaP, BaA, BbFA, BbFA, BkFa, DahA and IP. Only eight PAHs match the EU and US-EPA PAHs list; BaP, BaA, Chry, BbFA, BbFA, DahA, BghiP and IP. Excluding BghiP, the other PAHs in common are classified as carcinogenic, both by International Agency for Research on Cancer (IARC) [96] and US EPA [97] (Table 2). IARC classifies PAHs in different categories, according to carcinogenic potency (Table 2); BaP is classified in group 1- Carcinogenic to humans, and the others are 2 A and 2 B. NaP is classified as carcinogenic by IARC, but usually is not considered for risk assessment. Flut has been suspected to be carcinogenic, although not confirmed by IARC [94]. Since the eighties, correlations of PAHs toxicities with the most toxic BaP have been pursued [98; 99; 100; 94]. The equivalence factors of Nisbet and Lagoy (N&L) [98] have been broadly used. US-EPA proposes similar Relative Potency Factors (RPF) (Table 2) [101]. While US-EPA considers DahA as carcinogenic as BaP, Nisbet and Lagoy attributed a factor of five to DahA, for its carcinogenicity proved to be higher than BaP in low doses. [PERSON] et al. [102] used the factor of five for DahA for assessment of the urban air toxicity exposures of young children. Other equivalence systems are used worldwide, as those used by [PERSON] et al. [103]. Owing to the PAHs complex toxicity mechanisms, \begin{table}
\begin{tabular}{c l l c c c} \hline \hline  & **PAHs Compounds** & **Acronym** & **Number Rings** & **IARC \({}^{\text{a}}\)** & **RPF** & **RPF** \\  & & & **[96]** & **US-EPA** & **N\&L** \\  & & & & **[101]** & **[98]** \\ \hline \multirow{8}{*}{**Purchase**} & Naphthalene & NaP & 2 & 2 B & - & 0.001 \\  & Acenaphthylene & Acy & 3 & - & - & 0.001 \\  & Acenaphthene & Ace & 3 & 3 & - & 0.001 \\  & Fluorene & FL & 3 & 3 & - & 0.001 \\  & Phenanthrene & Phe & 3 & 3 & - & 0.001 \\  & Anthracene & Ant & 3 & 3 & - & 0.01 \\ \hline \multirow{8}{*}{**Purchase**} & Fluoranthene & Flut & 4 & 3 & - & 0.001 \\  & Pyrene & Pyr & 4 & 3 & - & 0.001 \\  & Benzo[a]anthracene & BaA & 4 & 2 B & 0.1 & 0.1 \\  & Chrysene & Chry & 4 & 2 B & **0.001 \({}^{\text{c}}\)** & **0.01** \\  & Benzo[b]fluoranthene \({}^{\text{b}}\) & BbFA & 5 & 2 B & 0.1 & 0.1 \\  & Benzo[k]fluoranthene \({}^{\text{b}}\) & BkFA & 5 & 2 B & **0.01** & **0.1** \\  & Benzo[a]pyrene \({}^{\text{b}}\) & BaP & 5 & 1 & 1 & 1 \\  & Dibenzo[a,h]anthracene & DahA & 5 & 2 A & **1** & **5** \\  & Benzo[b,h]perylene & BghiP & 6 & 3 & **-** & **0.01** \\  & Indeno [1,2,3-c,dl]pyrene \({}^{\text{b}}\) & IP & 6 & 2 B & 0.1 & 0.1 \\ \hline \multicolumn{6}{l}{Notes: \({}^{\text{a}}\) IARC classification: 1-Carcinogenic, 2 A-Probably carcinogenic, 2 B-Possibly carcinogenic, 3-Not classifiable as carcinogenicity. \({}^{\text{b}}\) PAHs used by Persistent Organic Pollutants (POPs) Protocol. \({}^{\text{c}}\) Different values in bold.} \\ \end{tabular}
\end{table} 
Table 2: List of Polycyclic Aromatic Hydrocarbons (PAHs), Carcinogenicity Classification and Toxicity Factors for BaP-EQ calculations. and assorted PAHs lists, the BaP toxic equivalence system is yet far from general consensus [99; 100].