{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,27]],"date-time":"2026-10-27T13:42:08Z","timestamp":1793108528068,"version":"4.5.0"},"reference-count":41,"publisher":"MDPI AG","issue":"4","license":[{"start":{"date-parts":[[2026,2,15]],"date-time":"2026-02-15T00:00:00Z","timestamp":1771113600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Department of the Built Environment at the Technological University of the Shannon"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Adequate ventilation is essential for maintaining indoor environmental quality in schools, where ventilation standards are often based on an indoor concentration of human-generated carbon dioxide (CO2) above ambient levels. Low-cost non-dispersive infrared (NDIR) CO2 sensors offer a practical solution for ventilation monitoring, yet variability between sensors can compromise accuracy, particularly when applications depend on the determination of precise concentration differences. This study evaluates the performance of twenty-three low-cost CO2 sensors, developing normalisation functions to improve comparability across sensors, introducing an accessible methodology for on-site sensor calibration without the need for laboratory-grade reference equipment. The sensors were co-located for three independent test periods in 2025 representing typical school internal conditions in Ireland. Pre-normalisation analysis showed strong linearity (coefficient of determination (R2) = 0.999) but notable variability, with a mean root mean square error (RMSE) of 18.3 ppm and 0.45% of measurements outside manufacturers stated accuracy. Normalisation models were trained and validated using a leave-one-period-out approach. Regression-based correction yielded the greatest improvement, reducing RMSE by 16%. When applied to the full dataset, final correction factors reduced RMSE by 27%, out-of-range measurements by 43%, and proportional bias by 31%. Corrected sensors demonstrated highly consistent performance, particularly within the CO2 ranges most relevant for classroom ventilation assessment, with an RMSE = 7.4 parts per million (ppm) at ambient concentrations and 11.9 ppm at concentrations below 1500 ppm. Field-based co-location in the deployment environment across full CO2 cycles, combined with a network-derived global reference, produced effective correction factors. Performance declined marginally above 1500 ppm and during dynamic occupancy, while overall accuracy remained strong. The study presents a practical and accessible methodology for evaluating and normalising low-cost CO2 sensors without specialised laboratory equipment, supporting reliable ventilation assessments in schools.<\/jats:p>","DOI":"10.3390\/s26041265","type":"journal-article","created":{"date-parts":[[2026,2,16]],"date-time":"2026-02-16T08:38:39Z","timestamp":1771231119000},"page":"1265","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Low-Cost CO2 Sensors: On-Site Performance Evaluation and Co-Location Correction Procedure for Reliable Ventilation Assessments in Schools"],"prefix":"10.3390","volume":"26","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-5978-2367","authenticated-orcid":false,"given":"David","family":"Honan","sequence":"first","affiliation":[{"name":"Department of Civil, Structural & Environmental Engineering, Trinity College Dublin, The University of Dublin, D02 PN40 Dublin, Ireland"},{"name":"Department of the Built Environment, Technological University of the Shannon, V94 EC5T Limerick, Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0277-1330","authenticated-orcid":false,"given":"John","family":"Garvey","sequence":"additional","affiliation":[{"name":"Kemmy Business School, University of Limerick, V94 T9PX Limerick, Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0351-9954","authenticated-orcid":false,"given":"John","family":"Littlewood","sequence":"additional","affiliation":[{"name":"The Sustainable & Resilient Built Environment Research Group, Cardiff School of Art & Design, Cardiff Metropolitan University, Cardiff CF5 2YB, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Matthew","family":"Horrigan","sequence":"additional","affiliation":[{"name":"Department of Business and Financial Services, Technological University of the Shannon, V94 EC5T Limerick, Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0848-6151","authenticated-orcid":false,"given":"John","family":"Gallagher","sequence":"additional","affiliation":[{"name":"Department of Civil, Structural & Environmental Engineering, Trinity College Dublin, The University of Dublin, D02 PN40 Dublin, Ireland"},{"name":"TrinityHaus Trinity Research Centre, Trinity College Dublin, The University of Dublin, D02 PN40 Dublin, Ireland"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,2,15]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Toyinbo, O. 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