## 2 Materials and Methods
### Data Processing  
The exhaled air chromatograms of the 13 volunteers, alongside the associated indoor air chromatograms, at 6 sampling points (PreSup, PreEx, PostEx, 24 h, 48 h, and 72 h) were  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Instrumentation** & **Parameter** & **Value** \\ \hline
**Thermal Desorption Unit** & Model: & UNITY series 2, Markes Int., \\  & & Bridgent, UK \\ Pre-purge & Time: & 5 min \\  & Gas: & He 99.999\% \\ Tube desorption & Desorb time: & 20 min \\  & Desorb temperature: & 200 \({}^{\circ}\)C \\  & Desorb flow: & 30 mL/min \\  & Split: & off \\  & Trap LOW temperature: & \(-\)10 \({}^{\circ}\)C \\ Cold Trap & Type: & Graphitized carbon \\  & Pre-trap fire purge: & 1 min \\  & Trap HIGH temperature: & 220 \({}^{\circ}\)C \\  & Trap hold: & 5 min \\  & Split flow: & 4 mL/min \\  & Split: & 5:1 \\  & Flow path temperature: & 150 \({}^{\circ}\)C \\  & Heating valve temperature: & 150 \({}^{\circ}\)C \\ Gas Chromatograph & Model: & 2010 GC, Shimadzu, Kyoto, Japan \\  & Capillary column: & SPB 624, 60 m-0.25 mm-1.4 \(\mu\)m \\  & Initial temperature: & 35 \({}^{\circ}\)C \\  & Hold: & 5 min \\  & Heating rate: & 4 \({}^{\circ}\)C/min \\  & End temperature: & 200 \({}^{\circ}\)C \\  & Hold: & 10 min \\ Mass Spectrometer & Model: & 5975 C, Agilent Technologies, \\  & & Santa Clara, CA, USA \\  & Ion source temperature: & 230 \({}^{\circ}\)C \\  & Mass analyzer temperature: & 150 \({}^{\circ}\)C \\  & Transfer line temperature: & 220 \({}^{\circ}\)C \\  & Mass range: & 35-230 amu \\ \hline \end{tabular}
\end{table}
Table 1: Instrumentation and analytical method parameters of TDU/GC/MS analysis.  
further processed for both the PL and SPI groups. In the current study, the focus was set on a group of straight-chain and mono-methylated alkanes C5-C14 that have been associated with oxidative stress in the literature [35]. Table 2 presents the target alkanes along with their retention times and the chromatographic curves used for quantitative analysis. Target alkanes were identified in chromatograms through library searching (WILEY138 mass spectral library) and retention times of standards.  
The alveolar gradient (AG) of the target alkanes was determined by Equation (1):  
\[\text{AG}=(\frac{A_{x}}{A_{IS}})_{Breath}-(\frac{A_{x}}{A_{IS}})_{Room} \tag{1}\]  
where \(Ax\) and \(A_{IS}\) are the Areas Under the Curve (AUC) of the specific alkane and the internal standard, respectively. AUC values were determined either by using the Total Ion Chromatogram or the Extracted Ion Chromatogram presented in Table 2 in the case of overlapping peaks. The subscripts \"Breath\" and \"Room\" in Equation (1) denote the analyzed sample. The AG has been used in many breath analysis studies [32; 35; 37] to distinguish between the endogenous and exogenous origin of an inhaled substance. In the current work, we used it to quantify the endogenous portion of an exhaled substance, excluding the portion inhaled from the environment.  
The AGs of the 15 target alkanes were determined for each of the 13 volunteers at the 6 sampling points shown in Figure 1 for both the SPI and PL groups. The AGs were determined using Excel Software (Microsoft 365 MSO, version 2410 and were used in the statistical analysis that follows.