Some public infrastructures do not routinely monitor air pollution, particularly in semi-enclosed transport environments such as road tunnels and metro systems. Low-cost sensors (LCS) may complement official monitoring by providing accessible exposure data.</p> <p>This study aimed to validate LCS performance and to assess PM2.5 concentrations in urban transport microenvironments in a large city with the active participation of various citizens.</p> <p>LCS measurements were compared with reference station data using Pearson correlation coefficients. The device was mounted outside a vehicle while driving through road tunnels, and additional measurements were conducted on metro platforms and inside subway carriages. All measurements were carried out by different citizens who had been previously trained.</p> <p>The correlation between LCS and the reference station was high (r = 0.9301; 95 % CI: 0.926–0.934), supporting device reliability. In road tunnels, mean PM2.5 increased from 12.62 µg/m³ (SD 11.3) in the first half of the journey to 16.6 µg/m³ (SD 15.2) in the second half (p < 0.001). On metro platforms, concentrations exceeded 10 µg/m³ (mean 20 µg/m³; range 10–32), while inside carriages levels remained above 5 µg/m³ (mean 10 µg/m³; range 5.8–17.8).</p> <p>These results have been reviewed, assessed, and discussed by all participating citizens from the signatory associations. As no safe threshold for PM2.5 exposure has been established, systematic monitoring and the integration of low-cost technologies into public health surveillance are needed to inform regulation and urban transport policies.