# A Human Health Toxicity Assessment of Biogas Engines Regulated and Unregulated Emissions
## 5 Results and Discussion
### Unregulated Emissions: Experimental Results and Literature Comparisons  
The \"unregulated emissions\" considered in the present study are: PAHs, aldehydes and dioxins and furans. Measured values are reported in Appendix A.2.1. In particular, Table A1 lists PAH emissions while Tables A2 and A3 report the quantity of aldehydes and dioxins and furans detected in the ICEs waste gases, respectively. Unregulated emissions are measurable only in ICEs equipped with WHRUs because, due to the technical limits of instruments, the exhaust gases temperature needs to be lower than 300 \({}^{\circ}\)C.  
To evaluate the carcinogenic hazards to humans, the authors grouped each detected substance in the proper category. Categories are defined by the International Agency for Research on Cancer [57] and are as follows:  
* Group 1: carcinogenic to humans;
* Group 2A: probably carcinogenic to humans;
* Group 2B: possibly carcinogenic to humans;
* Group 3: not classifiable as to its carcinogenicity to humans;
* Group 4: probably not carcinogenic to humans. For a clearer explanation, the group assigned to each compound is listed in Tables A1-A3 in the column named \"IARC\". The analysis reveals that:  
* Among PAHs belonging to group 1, only benzo(a)pyrene is detected in the engines' exhaust gases. However, in all production facilities, the detected values are below the detection limit. * PAHs belonging to groups 2A and 2B are mostly below the detection limit. Dibenzo(a,h) anthracene and dibenzo(a,l)pyrene are for both biogas and natual gas ICEs below instrumental limit while cyclopeta(c,d)pyrene is detected in BGP 5 and totally absent in BGP 6 and NG exhaust gases. Benzo(b)fluoranthene and naphthalene, which belong to group 2B, are detected in both BG and NG units. In particular, naphthalene is present in biogas ICEs with values double or triple those in the NG case. * Only some PAHs which belong to group 3 are above the detection limit. In particular, pyrene, fluoranthene and phenanthrene are substances detected in biogas units (BGP 5 and BGP 6) as well as in the one powered by natural gas. Comparisons between biogas and natural gas measurements do not show an appreciable difference between the two fuels. Additional analysis on the dangerousness of PAHs is given in Section 5.3 through a carcinogenic risk analysis to better underline any potential sources of risk. Aldehydes measurements show values below the detection limits except for Formaldehyde. Formaldehyde is the most dangerous substance and it is considered carcinogen to humans. Formaldehyde values show a variability of about 40 times among biogas plants. Such a large interval may depend on both the diet and the ICE calibration. However, the German standard sets a limit at 60 mg Nm\({}^{-3}\) for formaldehyde. However, in authors' measurements, the highest value is two orders of magnitude lower (0.88 mg Nm\({}^{-3}\)). So, the detection of formaldehyde in biogas ICEs' exhaust can not be considered a source of particular concern. Also in the case of dioxins and furans the risk seems limited since the detected values are very low or below the detection limit. The only dioxin ever detected is 2,3,7,8 PeCDF (2,3,4,7,8-pentachlorodibenzofuran) which, unfortunately, is a known carcinogen. However, the value is very low because it is expressed in ng Nm\({}^{-3}\). Thus, it can not be considered a source of particular concern, although the values measured in BGP 5 and BGP 6 are an order of magnitude higher than in NG ICE waste gases. For the sake of completeness, the experimental results are compared with the emission factors detected by [PERSON] et al. [41]. Computations are listed in Tables A4 and A5 ( Appendix A.2.2). The measured PAHs in biogas and NG engines are almost always lower than [PERSON] et al. 's ones except in the case of phenanthrene, fluoranthene, fluorene and pyrene, which are higher in the authors' case. However, as those compounds belong to group 3, they do not constitute a concern.