# A Human Health Toxicity Assessment of Biogas Engines Regulated and Unregulated Emissions
## 1 Introduction
The low percentage is easy to explain because, even though biomass is distributed all around the globe, biogas facilities are mainly located in Europe (12,064 MW--71.3% of the total), while the Americas and Asia accommodate only the 17.7% (2989 MW) and 6.6% (1115 MW) of the total, respectively. With an IEC of 6157 MW, Germany leads both the world and the European scoreboard. In practice, the Germany accommodates 36.4% of the world IEC and 51% of the European ones. In total, 81.3% of the total American biogas IEC (2989 MW) is installed in the USA while, in Asia, the situation is slightly different because the leading nation is Thailand with 475 MW, China follows closely with 429 MW. Italy, with 1352 MW, ranks in second place after Germany in the European scoreboard and it is considered one of the most advanced countries in the world for biogas technology (see, e.g., [33; 34; 35]). By looking deeper at the world biogas growth rates and established support policies (see, e.g., [34; 35]), it is clear that the European Union (EU) considers biogas as a key renewable energy source (RES) that can help to reduce energy related CO\({}_{2}\) emissions and to reinforce energy security. In fact, at the time of writing, the vast majority of biogas facilities are located in Europe and biogas fuel is used to generate electricity in stationary plants equipped with internal combustion engines. The conversion efficiency depends on the ICE size and technological advancement but, usually, it ranges between 31% and 43% for electricity-only plants, while it can achieve 78%-91% in CHP units. Despite ICE's high efficiency, fuel renewability and stringency in force emissions standards, biogas facilities are a source of concern for people living close to them due to the emission of ICEs into the atmosphere. To cover the lack of literature and to provide the answers required by citizens, in this work, the authors firstly analyse the biogas ICE emissions regulatory framework with the aim of identifying the commonly tracked substances (usually called \"regulated emissions\"). Then, since there is a lack of biogas ICE emission measurements, the authors conceptualize and perform an experimental campaign devoted to measure not only regulated emissions but also the so called \"unregulated emissions\" on in-operation facilities. These are points of novelty for both the literature and biogas sector. In fact, the results can complete the picture of biogas engines emissions and allow to identify the substances with the highest impact on human health. In this regard, using the data collected during the experimental campaign, the authors perform an impact analysis on human health combining the Health Impact Assessment (HIA) and the Risk Assessment (RA). The HIA is adopted for estimating the harmfulness to human health of both types of biogas engine emissions--regulated and unregulated. The procedure is then integrated with the RA for two groups of dangerous substances belonging to \"unregulated emissions\" with the aim of evaluating also their carcinogenic hazard level. In addition, to complete the analysis on emissions, the authors extended the investigation also to the biogas direct competitor: the natural gas fuel. To this end, an ICE fed by natural gas and characterised by the same nameplate data of the biogas ICEs is included in the experimental campaign. In this manner, the outcomes of the performed investigation can be used, on the one hand, to provide a clear answer to people leaving close to biogas facilities and, then, to improve their public acceptance. On the other hand, the work outcomes can also be a good starting point for the emission standards update. The rest of the work is organised as follows. In Section 2, the most relevant literature is presented and discussed in order to highlight the actual state of the art while Section 3 reports the characteristics of the biogas facilities including the ICEs' design specifications.