LEARN will recruit schools in Denmark, Belgium, and Greece to integrate and compare children with different characteristics. Through biomonitoring surveys in cohorts, LEARN will characterize the environmental exposure to indoor and outdoor air pollutants. IAQ assessment will include volatile compounds, particulate matter, and microbiological components present on particles, together with other relevant co-exposures to characterize “indoor exposome” in schools. Particulate matter will be measured by filter methods, and further determination of the biological fraction and the volatile chemicals will be performed in the lab using validated and approved methods. Particle mass concentrations will be calculated and the identified pollutants will be fully characterized by advanced techniques before being tested in 2D conventional in vitro and advanced organ-on-a-chip models of lung and skin, and biosensor models. We´ll assess the IAQ in parallel with the newly developed sensors and in combination with IAQ purification systems. LEARN will develop and prototype innovative sensor technologies for IAQ monitoring. The prototype will include a sensor selective to individual VOCs and a sensor system for UFPs. The work will be organized in three main tasks: 1) research and innovation, where the key building blocks will be investigated and validated in the lab; 2) sensor system development, focusing on the engineering of the sensor and prototypes; 3) testing and validation phase, in the lab and in the field to inform the status of the sensor development and to shape next iterations. The prototypes will serve for the validation of the effectiveness of the air purification systems and remediation strategies LEARN will develop advanced in vitro models of lung barrier and skin on a chip. A multi-sensing platform for the detection of reactive oxygen species, cell viability, and inflammatory markers will be integrated into the SoC model. The advanced lung and skin models will be used to assess the toxicological effects of chemical and particulate matter air pollutants. Standardization of a protocol to use C. elegans as a biosensor to evaluate toxicity and neurotoxicity of the environment will be performed. A novel operation concept for air purification systems based on UFPs concentration will be defined. Real IAQ data will be used to describe the efficiency of filters and air purifiers and prototypes of air purifier devices will be optimized. With the LEARN sensor development, the operation of the air purifier will be controlled by relevant data for classrooms. LEARN will centralize all data collected and generated within LEARN according to the FAIR data principles. An API and a user interface handling all data-related functionalities will be developed. Chemical monitoring data will be shared in the IPCHEM in collaboration with the JRC-IPCHEM.