INQUIRE aims to protect citizen health by providing knowledge, tools, and measures to substantially improve indoor air quality (IAQ). We will conduct research and evaluate innovative actions to reduce hazardous chemical and biological determinants in homes, positively impacting the health of residents. INQUIRE will focus particularly on infants and young children (<5 years old) as highly sensitive groups that spend a substantial time in the home environment. INQUIRE will comprehensively advance our understanding of the determinants of IAQ in homes by implementing innovative, low-cost, non-invasive sampling strategies (sensors, indoor/outdoor passive sampling, urine biomonitoring) to characterize determinants of household IAQ and their importance to human exposure. To capture the breath of IAQ determinants across Europe, the study will monitor for one month over 200 homes distributed across 8 countries, covering a gradient of conditions in each country. Tiered high-resolution chemical and biological screening techniques and wide-scope holistic characterisation of hazards will provide a comprehensive assessment of the determinants of IAQ. Multifaceted data analysis techniques (including machine learning, exposure modelling, geospatial analysis), will link chemical, biological and toxicity profiles with drivers of IAQ to identify sources and prioritize pollutants. Source identification will feed directly into the testing of both novel technologies and readily deployable strategies to improve IAQ, resulting in evidence-based recommendations and a draft of policy strategy for developing IAQ standards. INQUIREs Open Science approach and generated FAIR data on hazardous determinants, their effects, risk factors and sources will endorse continuous exploitation of results. Open dissemination of generated knowledge will raise citizen awareness while exploitation by industry and policy makers will endorse a transition towards homes with zero pollution. The project INQUIRE is part of the European cluster on indoor air quality and health (name and acronym to be decided)