Short-term exposure to ultrafine particles and respiratory infection hospital conctacts in children

<bold>Introduction:</bold> Short-term exposure to ultrafine particles (UFP; diameter <100 nm) may trigger morbidity from respiratory infections, potentially even more so among children. In this study, we examined the association between short-term UFP exposure and respiratory infection hospital contacts in children in Copenhagen, Denmark. <bold>Methods:</bold> Daily levels of UFP, particulate matter, nitrogen dioxide, ozone, and meteorological factors were monitored at an urban background station in Copenhagen during 2002-2018. Daily counts of hospital contacts for respiratory infections (acute lower/upper respiratory infections [ALRI/AURI], pneumonia, and bronchitis) in children (<18 years) were obtained from the Danish National Patient Register. Hospital contacts associated with interquartile range (IQR) increases in UFP were examined in a case-crossover design. Rate ratios (RR) with 95% confidence intervals (CI) were estimated after adjusting for temperature, relative humidity, and public holidays. Additionally, we analyzed the associations by sex, age, and family income. <bold>Results:</bold> We observed 109,585 hospital contacts for respiratory infections. IQR increases in UFP were associated with RRs of 1.02 (lag 0 days; 95% CI: 0.99, 1.05), 1.04 (lag 0-2; 1.01, 1.08), 1.06 (lag 0-3; 1.00, 1.12), and 1.04 (lag 0-5; 0.97, 1.12) for ALRI, AURI, pneumonia, and bronchitis hospital contacts, respectively. Associations were stronger among boys and children below 14 years, with no differences by income groups. <bold>Conclusion:</bold> We find that short-term UFP exposure can trigger respiratory infections in children, emphasizing the need for policies and regulations aimed at improving urban air quality to protect children’s health.