# A Human Health Toxicity Assessment of Biogas Engines Regulated and Unregulated Emissions
## 5 Results and Discussion
### Risk Analysis  
Starting from the experimental campaign results on unregulated emissions, the toxicity for human health of two emission classes, PAHs and dioxins-furans, is computed and comparisons are made considering natural gas as the reference fuel.  
Toxicity for human health is studied by medical science for well-defined classes of agents that operate through a common mode of action for the same toxic endpoint. Examples of this approach are the \"Relative Potency Factors\" for carcinogenic compounds and the \"Toxicity Equivalence Factors\" (TEFs) for PAHs and dioxin-like compounds.  
Carcinogenic risk analysis for PAHs is carried out in accordance with TEF approach proposed by the World Health Organization [58]. PAHs are classified on a scale of relative potency with respect to benzo(a)pyrene. B(a)P is the most dangerous PAH and its potency is assumed equal to 1. Thus, the concentration (x) of each compound (i) - x\({}_{i}\) (kg Nm\({}^{-3}\)) can be expressed as an equivalent concentration of B(a)P by means of TEF (kgB(a)P kg\({}^{-1}\)), and the PAHs emissions of a fuel can be expressed in terms of equivalent B(a)P by means of the following formula:  
\[B(a)P_{eq}=\sum_{i}\left(x_{i}\cdot TEF_{i}\right) \tag{1}\]  
Several TEF scales are available in the literature [58]. However, in medical science, the most adopted is the Nisbet and LaGoy [60] scale. So, the authors use this in their computations. Note that, the weight of each compound largely depends on the assigned TEF coefficient. As an example, dibenzo(ah)pyrene and dibenzo(al)pyrene are characterised by a TEF equal to 1 like B(a)P, while dibenzo(ah)anthraces presents a TEF equal to 5. Therefore, among PAHs, the highest contribution is not limited to B(a)P.  
The B(a)P\({}_{eq}\) for the emissions of the four plants are summarised in Figure 4a while the calculations performed are given in Table A6.  
The PAHs of BGP 4 are all under the detection limit; so this conditioned the B(a)P\({}_{eq}\) value while, for the other plants, the values are more reliable. In fact, comparing the mean biogas B(a)P\({}_{eq}\) with the natural gas shows that the biogas toxicity is about 30% lower than that of natural gas. However, excluding BGP 4, comparisons do not highlight differences between biogas and natural gas. Therefore, it can be concluded that the two fuels are substantially equivalent.  
Computation of toxic equivalency of dioxins and furans are carried out using the following equation:  
\[TEQ=\sum_{i}\left(x_{i}\cdot TEF_{i}\right) \tag{2}\]  
where TEFs values are derived from [PERSON] et al. [46] while x\({}_{i}\) is the concentration of the i-th compound detected during the experimental campaign.  
Figure 4b summarises the main outcomes while Table A7 lists each computed TEQ and the total equivalent toxicity.  
Dioxins and furans which mostly contribute to the equivalent total toxicity are 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD and 2,3,4,7,8-PeCDF. They have the highest values of TEF. However, conversely to PAHs, the analysis reveals that the NG total TEQ is about 10 times lower than the average value computed for biogas.