Short-term Exposure to Ultrafine Particles and Children’s Hospital Admissions in Copenhagen, Denmark

BACKGROUND AND AIM: Ultrafine particles (UFP; particulate matter 100 nm in diameter) may be more harmful to human health than larger particles, but epidemiological evidence on their health effects is still limited. Short-term exposure to UFP may trigger morbidity from respiratory diseases, potentially even more so among children. In this study, we examined the association between short-term exposure to UFP and children's hospital admissions, and possible modification of this association by socio-economic status, age and sex in Copenhagen, Denmark. METHOD: Daily levels of UFP (particle number concentration in a size range 11-700 nm) and meteorological factors were monitored at an urban background station in central Copenhagen during 2002-2018. Daily counts of hospital contacts (outpatient and inpatient) from respiratory diseases among children (14 years) in Copenhagen were obtained from Danish health registers. Hospital admissions associated with interquartile range (IQR) increases in UFP exposure for up to six days preceding the admission were examined in a case-crossover study design. Odds ratios (OR) with 95% confidence intervals (CI) were estimated after adjusting for temperature and relative humidity. Additionally, we analyzed the associations for different sex, age and disposable family income groups. RESULTS: We observed 126,156 hospital admissions between 2002 and 2018. Hospital admissions due to respiratory diseases were significantly associated with IQR increases in two-day moving averages of UFP (OR: 1.04 [95% CI: 1.00, 1.07]). Among the specific causes, the strongest associations were found for asthma hospital admissions and five-day moving averages of UFP (1.18 [1.06, 1.31]). Associations were highest among boys, younger children, and those in the lowest income group. CONCLUSIONS: We find that short-term exposure to UFP can trigger respiratory diseases in children, with strongest associations with asthma. We also find that boys, youngest children, and children from families in lowest income groups are most susceptible.