Characterizing air pollution exposure levels among school children: A citizen science study in Bradford, United Kingdom

Background Children are particularly vulnerable to the harmful effects of air pollution, with those from deprived communities experiencing disproportionately higher risks. Identifying where these children face the highest exposure is critical for guiding appropriate interventions, yet this key population remains underexplored to date. Recognizing children as important agents of environmental change, we used a citizen science approach to quantify levels of exposure to fine particulate matter (PM2.5 μg/m 3 ) among primary school children across key microenvironments (indoors at home, at school, and during the school commute). Methods We recruited 209 citizen scientists aged 9–11 from 12 primary schools to measure exposure to PM2.5 using portable low-cost air pollution sensors in Bradford, England. Each child carried a sensor for 5 days (Monday-Friday) during three data collection campaigns (October–December 2021, February–April 2022 and May–July 2022). We report the mean (standard deviation) and median (interquartile range) to describe PM2.5 exposure levels across key micro-environments. Differences in exposure were assessed using Kruskal–Wallis tests with post-hoc Dunn's tests where applicable. Results Data collection coverage was 60%, ranging from 56%–73% across the monitoring rounds. Overall, mean (SD) 24-h personal exposure to PM2.5 was 7.9 (10.3). Children experienced the highest exposure to PM2.5 indoors at home (12.6 (45.2)), during commutes to (9.1 (44.6)) and from school (10.8 (44.0)), with exposure levels at least two times higher than in schools (4.4 (18.0)). Nearly 40% of children were exposed to PM2.5 greater than the World Health Organization recommended 24-h PM2.5 exposure limit of 15 μg/m 3 for median two out of 15 days (interquartile range 1–3). Exposure to PM2.5 was similar between children who drove to school (7.8 (33.5)) and those that walked or cycled to school (7.1 (33.8)). Conclusion In this study, children experienced higher exposure to air pollution at home and while commuting to school compared to exposure during school hours. Policy should target improving indoor air quality in homes, reducing traffic-related emissions, and empowering deprived communities through citizen science approaches to protect children's health and promote environmental equity.