Short-term exposure to ultrafine particles and respiratory infection hospital admissions in children in Copenhagen, Denmark

Introduction: Short-term exposure to ultrafine particles (UFP; <100 nm) may trigger respiratory hospitalizations, potentially even more so among children than adults, but available evidence is limited. We examined the association between short-term UFP exposure and respiratory infection hospital admissions in children in Copenhagen, Denmark. Methods: Daily concentrations of UFP were monitored at an urban background station during 2002–2018. Hospital admissions for lower and upper respiratory infections (LRTIs/URTIs), pneumonia, bronchitis, and influenza in children (0–18 years) were obtained from the Danish National Patient Register. Associations between UFP concentrations up to one week prior to admission and hospital admissions were examined using case-crossover design. Relative risks (RR) with 95 % confidence intervals (CI) were estimated per interquartile range (IQR) increase in UFP for the total population, and by sex, age (0-4/5-14/15-18 years) and socio-economic status (income, mother's education). Results: We observed 109,585 hospital admissions for respiratory infections. We found positive associations of UFP with total respiratory infections, URTIs, and pneumonia with RRs of 1.04 (95 % CI: 1.01, 1.06), 1.04 (1.01, 1.08), and 1.06 (1.01, 1.12), respectively, per IQR increase in three-day mean UFP (lag 0–2), that were robust to PM_2.5 and NO₂ adjustment. Associations were stronger in boys and children younger than 15 years, with no differences between socio-economic groups. Conclusion: Short-term exposure to UFP triggered hospital admissions for respiratory infections, especially URTIs and pneumonia, in children in Copenhagen independently from PM_2.5 and NO₂. Our findings emphasize the need for policies and regulations aimed at improving urban air quality to protect children's health.