Abstract
BACKGROUND: Ultrafine particles (UFP; <0.1 µm in diameter) are not regulated or commonly monitored but may be harmful to human health, particularly for children. In this study, we aimed to examine the association between short-term exposure to UFP and asthma hospital admissions in children. METHODS: Daily UFP concentrations (2002-2018) were monitored at an urban background station in Copenhagen, Denmark. Asthma hospital admissions, demographic and socioeconomic information of children (0-18 years) were obtained from registries. A case-crossover design was applied to estimate the association between hospital admissions and up to 6-day UFP exposure windows for all children, and stratified by age, sex, family income, mother's education, prior asthma or prior respiratory infection. RESULTS: We observed 15 903 asthma hospital admissions in total. An IQR increase in UFP was significantly associated with asthma hospital admissions, strongest at 2-day exposure windows (risk ratio (RR): 1.17 (95% CI: 1.09, 1.25)). These associations remained unchanged when adjusting for particulate matter <2.5 µm in diameter (PM(2.5)) or nitrogen dioxide (NO(2)), for which we also detected significant positive associations. Associations with UFP were stronger for school-aged children (5-14 years: RR: 1.26 (95% CI: 1.15, 1.38)) than for children younger than 5 years (1.01 (95% CI: 0.93, 1.10)). CONCLUSIONS: In this large study in a low-exposure setting, we find that short-term exposure to UFP can trigger asthma hospital admissions in children, independently of associations with PM(2.5) or NO(2). This study adds evidence calling for the regulation and improvement of UFP exposure assessment to protect children's health in urban areas.