## 2 Materials and Methods
### Limits  
Our exploration considered occupational exposure limit values (OELVs) and short-term OELV. OELV reflects average exposure based on an 8 h workday and a 40 h workweek. Simultaneously, short-term OELV, a 15 min exposure limit, should not be surpassed at any time during a workday, even if the 8 h OELV is compliant. As stipulated by Spanish and European legislation, recommended OELVs for NO and NO\({}_{2}\) stand at 2 ppm and 0.5 ppm, respectively, with a short-term OELV of 1 ppm for NO\({}_{2}\)[47]. Table 1 presents the occupational exposure limit values (OELVs) for NO and NO2 in various countries, as compiled by the IFA [48]. These limits apply to occupational exposure for parking workers, while environmental values are designated for users.  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**NO\({}_{2}\)**} & \multicolumn{3}{c}{**NO\({}_{1}\)**} \\  & **Limit Value—8 h** & **Limit Value—8 h** & **Limit Value—8 h** & **Limit Value—8 h** & **Limit Value—Short Term** \\  & **ppm** & **mg/m\({}^{3}\)** & **ppm** & **mg/m\({}^{3}\)** & **ppm** & **mg/m\({}^{3}\)** & **ppm** & **mg/m\({}^{3}\)** \\ \hline Australia & 3 & 5.6 & 5 & 9.4 & 25 & 31 & & \\ Austria & 0.5 & 0.96 & 1 & 1.91 & 2 & 2.5 & & \\ Belgium & 0.5 & 0.96 & 1 & 1.91 & 2 & 2.5 & & \\ Canada-Ontario & 3 & & 5 & & 25 & & & \\ Canada-Quebec & 3 & 5.6 & 5 & 9.4 & 25 & 31 & & \\ Denmark & 0.5 & 0.96 & 2 & 4 & 2 & 2.5 & 4 & 5 \\ European Union & 0.5 & 0.96 & 1 & 1.91 & 2 & 2.5 & & \\ Finland & 0.5 & 0.96 & 1 & 1.91 & 2 & 2.5 & & \\ France & 0.5 & 0.96 & 1 & 1.91 & 2 & 2.5 & & \\ Germany (AGS) & 0.5 & 0.95 & 1 & 1.9 & 2 & 2.5 & 4 & 5 \\ Germany (DFG) & 0.5 & 0.95 & 0.5 & 0.95 & 0.5 & 0.63 & 1.0 & 1.26 \\ Hungary & & 0.96 & & 1.91 & 2.5 & & & \\ Ireland & 3 & 5 & 5 & 9 & 25 & 30 & 35 & 45 \\ Italy & 0.5 & 0.96 & 1 & 1.91 & 2 & 2.5 & & \\ Latvia & 0.5 & 0.96 & 1 & 1.91 & 2 & 2.5 & & \\ New Zealand & 3 & 5.6 & 5 & 9.4 & 25 & 31 & & \\ Norway & 0.5 & 0.96 & 1 & 1.91 & 2 & 2.5 & & \\ China & & 5 & & 10 & & 15 & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: NO and NO\({}_{2}\) OELVs in different countries [48].  
Figure 2: Multigas detector Drager X-am 5600 (Dragerwerk AG & Co KGaA, Luebeck, Schleswig-Holstein, Germany).  
European Environment Agency standards prescribe a maximum of 200 ug/m\({}^{3}\) (0.1 ppm) for nitrogen dioxide in one hour and 40 ug/m\({}^{3}\) (0.02 ppm) as an annual average [32; 49]. In compliance with Directive 2008/50/EC and RD 102/2011, the limit value for NOx to safeguard ecosystems is set at 30 ug/m\({}^{3}\) as an annual average, representing a new critical level for vegetation protection. This standard reflects a commitment to maintaining environmental quality and mitigating the impact of nitrogen oxides on ecosystems, aligning with established regulatory frameworks.  
Notably, certain countries, such as Japan, enforce even lower NO\({}_{2}\) environmental levels, with a limit of 113 ug/m\({}^{3}\) (0.06 ppm) in one hour. The World Health Organization (WHO) strongly advocates for an annual average of 10 \(\mu\)g/m\({}^{3}\) (0.01 ppm) and a short-term (24 h) value of 25 \(\mu\)g/m\({}^{3}\) for NO\({}_{2}\)[50] and a 1 h maximum of 200 \(\mu\)g/m\({}^{3}\) (0.1 ppm) [51].