# 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  
A damage assessment analysis is conducted to properly evaluate the weight of biogas plant emissions. To do that, the Health Impact Assessment is used. This is an approach derived from the Life Cycle Impact Assessment [59; 61]. The LCIA is a method which can be used to assess the impact on human health or on the environment of emissions produced by human activities. As shown in Figure 5, the LCIA is made up of four steps. Figure 4: Carcinogenic risk evaluation for (**a**) PAHs, (**b**) dioxins and furans. Figure 5: The life cycle impact assessment structure. The first step is called the \"emissions inventory\" and it is used to build an inventory of emissions released into the environment by, e.g., the biogas plant. The second step (\"characterization\") allows the emissions to be assigned to the relevant impact categories, within which they are transformed into equivalent emissions of a substance taken as reference. The third step is named \"damage assessment\" and its aim is to count the damage caused by each equivalent issue by evaluating it in DALY units. The last step, called \"normalization\", aims to collect the outcomes of the four damage categories and, after a normalization process, it provides a single value. Since the focus of this work is only on the effects of biogas engine regulated and unregulated emissions on human health, the damage to ecosystems and climate change are left to future studies. In the case of emissions regulated by the Italian standard for biogas (NO\({}_{\mathrm{x}}\), CO, VOC and HCI) and NG engines (NO\({}_{\mathrm{x}}\), PM and SO\({}_{\mathrm{x}}\) expressed as SO\({}_{2}\)), they appear only in the first two impact categories (see Figure 5). So, the damage assessment is limited to the \"respiratory inorganics\" category and the \"respiratory organics\" category and stops at the third step which is the \"human health\" step. However, if both regulated and unregulated emissions are considered, among the six impact categories used to assess damage to human health (respiratory inorganics, respiratory organics, carcinogens, non-carcinogens, ionizing radiation, ozone layer depletion), only the first four are involved because the remaining two are not-significant, since none of the substances found in this investigation are present in them. For this reason, the analysis stops again at the third step. The quantification of damage (DA) caused by a substance is done by multiplying the quantity of the substance itself \"x\" (kgemission), by the characterization factor \"C\({}_{\mathrm{f}}\)\" (kgsubstance,eq. \(kgemission^{-1}\)) and for the specific damage \"s\({}_{\mathrm{d}}\)\" (DALY \(kg_{\mathrm{substance\ eq}}\)\({}^{-1}\)). \[DA=x\times s_{d}\times C_{f} \tag{3}\]  
The damage is expressed in DALY and represents the number of years of life lost by a population due to premature death and/or disability caused by a single harmful emission. This investigation considers a population equal to that of Europe (431 million inhabitants), since the specific damage values have been obtained through statistical evaluations carried out on the basis of the whole European population. In addition, based on data collected by the authors, the average number of operating hours of each biogas plant exceeds 8000 h per year. So, it is assumed that the BG and NG plants run for 8000 h per year. For each impact categories, the damage analysis outcomes are summarized in Figure 6. For a clearer explanation, in Appendix A.2.4, the computations performed are also listed. For each plant, the specific damage in DALY Nm\({}^{-3}\) and the damage per unit of electricity produced (expressed in DALY GJ\({}^{-1}\)) are given. It should be noted that some substances are present in more than one category, indicating that they can cause several types of damage. For example, formaldehyde can cause cancer but also respiratory damage. So, it belongs to the \"carcinogens\" category as well as the \"respiratory organics\" family.