# A Human Health Toxicity Assessment of Biogas Engines Regulated and Unregulated Emissions
## 6 Conclusions  
The aim of the present work is to evaluate the damage to human health arising from both regulated and unregulated emissions from biogas and natural gas internal combustion engines. Particular attention is devoted to unregulated emissions, which are not usually explicitly foreseen by emission standards, but are contained within macro-groups, which in turn are required by standards, such as volatile organic compounds, unburned hydrocarbons or particulates. The unregulated emissions considered in this research are polycyclic aromatic hydrocarbons, aldehydes and dioxins and furans. Six biogas plants and one natural gas engine are involved in the analysis. All the engines entered into operation in 2012, were built by the same manufacturer, are characterised by an electric power of 999 kWel and have the same degree of technological development. The involvement of an engine \begin{table}
\begin{tabular}{l c c} \hline \hline
**Considered Substances** & **Biogas** & **Natural Gas** \\ \hline Biogas regulated emissions: NO\({}_{\text{x}}\), CO, HCl, VOCs & \(6.08\times 10^{-5}\) & \(1.87\times 10^{-5}\) \\ Natural gas regulated emissions: NO\({}_{\text{x}}\), PM, SO\({}_{2}\) & \(6.31\times 10^{-5}\) & \(1.75\times 10^{-5}\) \\ Combined regulated emissions: NO\({}_{\text{x}}\), CO, HCl, VOCs, PM, SO\({}_{2}\) & \(6.49\times 10^{-5}\) & \(1.89\times 10^{-5}\) \\ All measured emissions & \(6.50\times 10^{-5}\) & \(1.90\times 10^{-5}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Total damage computations. Values are given in DALY GJ\({}^{-1}\). fuelled by natural gas is needed to make comparisons between emissions released by biogas ICEs and their direct competitor. Measurements show that PAHs are present in biogas exhausts in small quantities, almost always below the detection limit. This is particularly true for benzo(a)pyrene, belonging to the group that is carcinogenic to humans. Also PAHs belonging to group 2A, i.e., probable carcinogens to humans, are in quantities below detectable level. The same is true also for the natural gas. The carcinogenic risk analysis of PAHs for the two fuels established their substantial equivalence. The most dangerous aldehyde, formaldehyde, is also the only one detected in significant quantities. Even if the quantity is small, the level in biogas is about five times higher than in natural gas. Also dioxins and furans are detected in small quantities in both fuels, and almost always below the detection limit. The analysis of equivalent toxicity shows that biogas is, on average, 10 times more toxic than natural gas. A more complete picture of the toxicity of both fuels is provided by the analysis of damage to human health assessed in the \"Health Impact Assessment\". The main outcomes are:  
* The damage per unit of electricity caused by biogas is about three times the damage caused by natural gas. * Among the four impact categories, \"respiratory inorganics\" is the most important because it contains \(\mathrm{NO_{x}}\) and \(\mathrm{SO_{x}}\); the two substances with the highest impact. For this reason, \"respiratory inorganics\" accounts for approximately 97% of the total damage caused by the entire set of emissions. Among other categories, only the \"respiratory organics\" contributes in a non-negligible way. * The classification of the most harmful substances shows \(\mathrm{NO_{x}}\) in the first place. It produces, on average, over 90% of the damage to human health for both biogas and natural gas. However, the damage generated by \(\mathrm{NO_{x}}\) from biogas is about three times higher than from natural gas. \(\mathrm{SO_{2}}\) is present in all investigated biogas exhausts and it accounts for about 6% of the total damage while it is practically absent in natural gas. The VOCs produce quantitatively similar damage for both fuels. Hence, the investigation highlights the role of \(\mathrm{NO_{x}}\) and \(\mathrm{SO_{x}}\). However, \(\mathrm{SO_{x}}\) is not classified as \"regulated emissions\" for biogas ICEs.