2021 Yes https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8124735/ Pallarés, Susana Europe Observational Elementary (K-5) Particulate Matter (PM) Indoor airborne particles Primary schools Statistical analysis Ceramic hub Mediterranean basin The lowest concentrations of fine particulate matter, less than 2.5 µm, were registered at the rural location. The values of these particles found in industrial and urban locations were not substantially different. In the case of particulate matter between 2.5 and 10 µm, significant differences were observed between the three types of locations. The lowest concentrations of particles larger than 10 µm were registered at the rural location, and the highest concentrations were found at the industrial locations. Among the urban stations, the particle concentration of this fraction in station S2 was significantly higher than that in stations S1 and S3, which had similar concentrations. These values are also similar to those registered at school S6, which is at an industrial location.