2024 Yes https://doi.org/10.3390/ijerph21101263 Alonso-Blanco, Elisabeth Europe Experimental Observational Students Elementary (K-5) Particulate Matter (PM) Black carbon Indoor air quality Indoor/outdoor (I/O) ratio Primary schools Black carbon Ultrafine particles particulate matter Indoor and outdoor air quality measurements at a Madrid primary school assessed particulate matter, ultrafine particles, and black carbon. Traffic and indoor activity influenced IAQ, supporting targeted strategies to protect children’s health.