# A Human Health Toxicity Assessment of Biogas Engines Regulated and Unregulated Emissions
## 5 Results and Discussion
### Analysis of Damage to Human Health for Regulated and Unregulated Emissions
For the six biogas plants, an average value of each compound is computed. NO\({}_{\text{x}}\) is the most harmful compound, as it causes, for both fuels, more than 90% of human damage. Therefore, since NO\({}_{\text{x}}\) in biogas engine emissions is three times higher than in NG exhausts, biogas is about three times more harmful than natural gas. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**Category** & **BGP 1** & **BGP 2** & **BGP 3** & **BGP 4** & **BGP 5** & **BGP 6** & **NGP** \\ \hline Respiratory inorganics & 97.54 & 96.85 & 98.47 & 98.07 & 98.48 & 97.08 & 93.26 \\ Respiratory organics & 2.44 & 3.13 & 1.53 & 1.92 & 1.50 & 2.91 & 6.71 \\ Carcinogens & 0.01 & 0.00 & 0.00 & 0.00 & 0.00 & 0.00 & 0.00 \\ Non carcinogens & 0.02 & 0.01 & 0.00 & 0.01 & 0.01 & 0.01 & 0.03 \\ \hline TOTAL & 100.00 & 100.00 & 100.00 & 100.00 & 100.00 & 100.00 & 100.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary table of damage values ranked by category and expressed as percentage of the total. \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **Biogas** & \multicolumn{3}{c}{**Natural Gas**} \\  & **[DALY GJ\({}^{-1}\)]** & **[\%]** & **[DALY GJ\({}^{-1}\)]** & **[\%]** \\ \hline NO\({}_{\text{x}}\) & \(5.11\times 10^{-5}\) & 91.51 & \(1.73\times 10^{-5}\) & 93.06 \\ SO\({}_{2}\) & \(3.52\times 10^{-6}\) & 6.29 & \(1.48\times 10^{-8}\) & 0.08 \\ VOCs & \(1.22\times 10^{-6}\) & 2.18 & \(1.27\times 10^{-6}\) & 6.83 \\ Formaldehyde & \(3.60\times 10^{-9}\) & 0.01 & \(1.76\times 10^{-9}\) & 0.01 \\ Acrolein & \(1.37\times 10^{-9}\) & 0.00 & \(3.10\times 10^{-9}\) & 0.02 \\
2,3,7,8 TCDD & \(4.03\times 10^{-9}\) & 0.01 & \(5.95\times 10^{-10}\) & 0.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Ranking of the most damaging emissions for the examined fuels. The specific damage from SO\({}_{2}\) is negligible in natural gas, but not in biogas. The high sulphur content in biogas emissions can be linked to the degradation of some types of aminoacids (e.g., methionine) contained in maize and wheat. The specific damage related to VOCs is the same for the two fuels, while the damage caused by formaldehyde is double in the case of biogas with respect to natural gas. For the dioxin 2,3,7,8 TCDD, the factor is about 7. The case of acrolein is the opposite. As already observed, these two substances have impacts but the relative measures are often below the instrumental limit. So, being in trace, their values are not a source of high concern. It is also worth comparing the values obtained in the counting both regulated and unregulated emissions damage assessment with those counting only regulated emissions. The first and second row of Table 6, list the health damage computed considering the regulated emissions of biogas and natural gas, respectively. Independently from the considered substances, the health damage caused by biogas is about three times higher than that of natural gas. In the third line, the damage is assessed by considering the set of regulated emissions of both fuels. This computation allows to verify whether there are chemical species, neglected by the legislator, which contribute in a not negligible way to the damage. The comparison among rows 1 to 3 shows that the contributions of SO\({}_{2}\) and PM for the biogas damage are small but not negligible (about 7% of the total). Similarly, in the case of NG, the contribution to total damage of CO, HCl and VOCs is approximately 8%. Though, it is small but not negligible. Considering that SO\({}_{2}\) and VOCs are the second contributor to biogas and natural gas damage, respectively, it can be inferred that these two compounds play a key role. For this reason, they should be included in future standards update, among the regulated species for biogas and natural gas, respectively. Considering the current state of knowledge and the very small detected quantities, the comparison between the third and fourth line shows that the contribution to human health damage of unregulated emissions is limited.