# Modelling Pollutant Dispersion in Urban Canyons to Enhance Air Quality and Urban Planning
## 4 Discussion
### Practical Application

Table 2: Characteristics of the selected streets in Seville for study. \begin{table}
\begin{tabular}{c c c} \hline
**Street Name** & **Low Traffic** & **_c_ **(mg/m\({}^{3}\))** & \\ \hline Calle Zaragoza & 0.1280 & 9.6400 \\ \hline Calle de Castelar & 0.0734 & 5.5100 \\ \hline Calle Padre Marchena & 0.1330 & 9.9500 \\ \hline Calle Gamazo & 0.1170 & 8.8100 \\ \hline Calle Doná Guiomar & 0.1370 & 25.400 \\ \hline \end{tabular}
\end{table} 
Table 3: Mean pollutant concentration in the studied streets for low and high traffic density. CO concentration levels across all streets. Notably, the increase is more pronounced in the narrowest street, where the formation of a new vortex in the lower area has been observed. This vortex hinders street ventilation, thereby favouring the accumulation of particles. The majority of the streets fall below the first limit set by the WHO, although they are near this limit. This indicates that a small change in outdoor conditions can lead to an increase in pollution levels, thereby posing a heightened risk to the health of the inhabitants. Finally, the narrowest street exhibits a CO concentration value that exceeds the first limit of 10 mg/m\({}^{3}\) established by the WHO, signifying a heightened level of danger to health when compared to the rest of the streets studied. This reduction in the maximum time a person can remain in such an environment before experiencing adverse health effects is also of significance. As can be observed, under the same external air conditions, the street with a larger AR has better ventilation performance, with the lowest pollutant concentration among them, whereas the narrowest street has the highest one. This is logical because the findings of this work demonstrate that streets with an AR higher than 2.5 exhibit heightened sensitivity to variations in external conditions, suggesting the presence of a multi-vortex regime. This phenomenon complicates the ventilation process in the street's lower layers and contributes to an increase in pollutant levels. To visualise this phenomenon more clearly, a risk scale can be established based on the limits established by the WHO. The map depicting the streets will be coloured green for those which do not exceed the first pollution limit, yellow for those which exceed the second limit, and red for those which exceed the third limit. With this information, a risk map can be produced (shown in Figure 11). In this way, urban designers can use these results to know under which circumstances the pollution accumulation in streets can trigger health issues; thus, they can apply different solutions to mitigate it. Figure 11: Map of the chosen streets with the level of health risk. Green is for risk-free streets, and yellow is for streets in risk level one. In this case study, a range of existing streets in the city centre of Seville have been analysed to assess their response to various environmental conditions associated with pollution. This study aims to classify the risk to individuals posed by exposure to specific concentrations of pollutants, in accordance with the guidelines established by the WHO. The findings of this study provide a framework for analysing existing streets and urban configurations under diverse external conditions, facilitating the anticipation of potential health limits that may be exceeded under exceptional circumstances. This approach enables proactive measures to be taken, rather than a reactive approach that is contingent on the occurrence of hazardous situations and the subsequent assessment of pollutants' concentrations in the area. Furthermore, this methodology can be employed to formulate new urban configurations that encourage the natural ventilation of streets, thereby circumventing the construction of new streets that may pose hazardous situations under external conditions conducive to the accumulation of dangerous pollutants.