# Exposure to NO\({}_{2}\) and PM\({}_{2.5}\) While Commuting: Utility of Low-Cost Sensor
## 4 Discussion
### Differences in Pollututant Concentrations  
Analysis of PM\({}_{2.5}\) and NO\({}_{2}\) concentrations measured by low-cost sensors revealed notable seasonal differences. Our analysis of each mode of transport, obtained from measurements by low-cost sensors, reveals substantial differences between heating and non-heating seasons. For PM\({}_{2.5}\), concentrations during the non-heating season were slightly higher (12.1 \(\pm\) 24.0 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) compared to the heating season (9.5 \(\pm\) 11.1 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)), whereas NO\({}_{2}\) concentrations exhibited an inverse trend, being higher in the heating season (28.1 \(\pm\) 27.7 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) than in the non-heating season (22.7 \(\pm\) 20.3 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)). This suggests that transport emissions during the heating season contribute significantly to NO\({}_{2}\) pollution, whereas PM\({}_{2.5}\) levels might be more influenced by atmospheric conditions. There is a growing body of research investigating exposure to PM\({}_{2.5}\) in relation to mode of transport; however, a clear knowledge gap remains regarding exposure to nitrogen dioxide (NO\({}_{2}\)) in this context (Table 2). For example, in Hamilton, Canada, children cycling experienced a PM\({}_{2.5}\) concentration of 15.7 \(\pm\) 19.3 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\), while studies in Thessaloniki, Greece, reported a significantly higher PM\({}_{2.5}\) concentration of 55.7 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\). Similar trends are evident in Lisbon, Portugal, where PM\({}_{2.5}\) concentrations ranged between 30.5 \(\pm\) 9.0 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\) and 85 \(\pm\) 66 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\), depending on the time of measurement. Furthermore, NO\({}_{2}\) concentrations in Montreal, Canada, were markedly higher (120.94 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)), demonstrating significant geographical discrepancies in exposure levels. During vehicle trips, PM\({}_{2.5}\) concentrations were also higher in the non-heating season (10.6 \(\pm\) 23.2 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) compared to the heating season (5.5 \(\pm\) 6.0 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)), whereas NO\({}_{2}\) concentrations were higher in the heating season (34.7 \(\pm\) 30.2 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) than in the non-heating season (24.8 \(\pm\) 21.6 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)). Comparing these findings to studies in other cities, significant differences in pollutant exposure are evident. For instance, in Thessaloniki, Greece, PM\({}_{2.5}\) concentrations in buses (84.6 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) and cars (32.9-53.1 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) were considerably higher than in Gliwice. Similarly, in Montreal, Canada, PM\({}_{2.5}\) concentrations in buses (87.62 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) and cars (114.87 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) were elevated. Lisbon, Portugal, displayed moderate values, with PM\({}_{2.5}\) levels in buses (28.4 \(\pm\) 5.3 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) and cars (33.7 \(\pm\) 8.6 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)). Milan, Italy, exhibited seasonal variations in PM\({}_{2.5}\) levels for car users, with higher winter concentrations (8.0 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)) than summer (5.3 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)), mirroring the trend observed in Gliwice.