# 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
However, formaldehyde can additionally cause various other disorders. So, it is included also in the \"non-carcinogens\" category. In the following, the analysis outcomes of each category are discussed. **Respiratory inorganics**. Natural gas shows an impact three to four times lower than biogas (see Figure 6a). To justify this discrepancy, it is interesting to analyse the impact of each substance belonging to this category (see Table A8). The most harmful emission is \(\text{NO}_{\text{x}}\). This is not related to the quantity of the compound but to the high value of the characterization factor. In conclusion, since \(\text{NO}_{\text{x}}\) concentration in the biogas engines is higher than in the natural gas one, the average damage related to the biogas is about 3.5 times the damage produced by natural gas emissions. **Respiratory organics**. For this category, Figure 6b shows that the damage from biogas and natural gas exhaust is comparable. However, again, it is important to understand the impact of each substances. As shown in Table A9, the most harmful emissions are VOCs. This is related to their huge amounts, which are about 10000 times higher than those of other substances. Being VOCs is comparable in biogas and natural gas engines emissions, it can be concluded that the average damage related to biogas is approximately equal to that from natural gas. **Carcinogens**. Due to the high exhaust gases temperature in three plants, some substances can not be measured. So, the evaluation of this damage is difficult. Nonetheless, the results obtained for BGPs 4, 5 and 6 can be compared with those for NG and, at the plots in Figure 6c, it is clear that the damage related to biogas is approximately double that from natural gas. Figure 6: Damage analysis for the categories (**a**) respiratory inorganics, (**b**) respiratory organics, (**c**) carcinogenesis and (**d**) non-carcinogens. In fact, based on the values listed in Table 10, formaldehyde produces the highest damage in plants without WHRUs such as BGPs 1, 2 and 3 due to the absence of measurements related to other harmful substances. In the case of BGP 4, 5, 6 and the NG engine, besides formaldehyde, the most impactful substance is dioxin 2,3,7,8 TCDD due to its high characterization factor, although B(a)P continues to assume significant percentages. Therefore comparisons shows that biogas produces damage that is almost twice that caused by natural gas. **Non-carcinogens**. The analysis reveals that (i) the impacts of biogas and natural gas are comparable (see Figure 6d) and (ii) formaldehyde and dioxins continue to be the most harmful substances (details are given in Table 11). Acrolein also shows high damage values. But, its concentration is always below the detection limit. Based on the category analysis, it can be concluded that the average damage from biogas is approximately 40% higher than that from natural gas. In Table 4, the total damage of each impact category is expressed as percentage of the total. The most damaging category is the \"respiratory inorganics\" for both biogas and natural gas powered ICEs. However, in the case of biogas, the impacts of this category accounts for 96.8%-98.5% while, in the NG unit, its contribution is approximately 93%. In the NG ICE, the impact on the total damage of the \"respiratory organics\" category reaches 6.7%, while in the biogas case it is always half of this value. So, it can be concluded that, for biogas and natural gas plants, the main impact is on the respiratory tract. Fortunatly, their emissions are not carcinogenic at all, unlike those of diesel engines, as stated by IARC [57]. The last two categories can not be considered a source of particular concern. It is also possible to claim that the average damage of biogas is about three times that of natural gas. Therefore, it is interesting to investigate individual emissions to identify the most dangerous one. With this purpose, in Table 5, the six emissions with the highest specific damage are combined.